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Discussion Starter · #1 · (Edited)
I have updated the posts below, on the basis that the best information is more valuable than a historical record. The analysis will not be perfect, but it is the best I can do. Thanks to CyprusCorner for challenging some unwise methods (now removed) and whether the whole idea is feasible or worthwhile. In summary: I think it is both feasible and worthwhile for a few owners (those who drive in mild climates, use power-consuming accessories such as dashcams while parked, and are willing to ensure that battery state of charge remains high enough for safe recharge currents). The capability might be improved by interposing an LFP-optimised charger and return power diode in the rav4 12v circuit, but this would be expensive, and a job for a professional automotive electrician. For most current rav4 hybrid owners, feasibility requires some help from Toyota, in the absence of which it is best to stay with a lead-acid auxiliary battery. Those who drive at ambient temperatures beyond the 0-40ºC range, and/or do not want deep-cycle ability from a 12v battery, should certainly stay with lead acid.

Has anyone replaced the stock 12v (auxiliary) battery in a gen5 rav4 hybrid with a LiFePO4 (LFP) battery for improved deep-cycle performance? Can anyone provide information on the 12v battery-charging profile used in a gen5 rav4 hybrid? This would help to determine whether a separate DC-DC charger is needed for LFP, which would complicate the installation and might cause other problems (like errors from sensors in the rav4 charging system).

By way of background, the stock 12v battery in gen5 rav4 hybrids is a Yuasa 345LN1 or 355LN2. These are small (45-60 Ah depending on model and vintage) flooded-cell, lead(/calcium)-acid, ‘cranking’ batteries. They are “maintenance free” (MF) but not sealed or AGM. They have electrolyte level lines and cell fill caps (even if these are under a Toyota clear plastic cover). They seem to have been chosen to meet EU regulations, rather than matched to hybrid vehicle use.

As the vehicle is hybrid-electric it does not use the 12v battery to crank the engine, just to boot up the vehicle electronics into ‘READY’ mode (where the much larger high-voltage NIMH battery pack can start the engine whenever required by the computer system). Then all the vehicle electricals (except the aircon, the traction motors and the HV converter) run as usual from the 12v system (and the 12v battery is charged when the vehicle is in ‘READY’ mode, by a step-down from the hybrid vehicle's high-voltage system). So I am thinking that with ongoing slow drain into the vehicle electronics (<1.2W) and a dashcam (~2.9W) while parked, the deep-cycle characteristics of LFP may be much more suitable as the primary 12v battery for this hybrid vehicle. This can be seen in the performance chart, modified from a previous post (Front and Rear Dashcam Installation in Rav4 Hybrid...):

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The Renogy 50Ah 12v LFP battery is 197L*166W*171H mm. Power-Sonic has LFP batteries are about the same size. Any of these would fit in the rav4 battery holder (LN2 = 242L*175W*190H mm). They weigh only 6.3-6.7kg and are rated at 2000 cycles with 80% discharge (at 0.5C and 25ºC)! The attraction is obvious, but LFP needs a suitable charge profile for safety and performance. Hence the questions above. Can anyone help with answers?
 

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I suspect you have answered this question with the above table. Its probably a reasonable guess that Toyota will have used a fairly sophisticated battery management system in order to minimise unnecessary energy consumption. Interpolating your table the BMS will see 12.6 volts as a battery which is 80-90% charged whereas the Lithium battery will actually only be 40% charged. So the system probably won't be optimally managing the lithium battery. I realize that this is a crude interpretation but I am sure you know that charging requirements are quite different unless you can interpose an extra black box which simulates the lead acid battery well enough to fool the electronics.
 

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Discussion Starter · #3 ·
Thanks roadster-GB. But we are just guessing. Do we know if the rav4 even has a BMS that senses voltage and adjusts charge accordingly? If it does, can it be programmed to meet the needs of different battery types? Even the stock flooded-cell vs AGM would be different. There is nothing on the topic in the owner's manual. I doubt that the stock battery itself has any BMS. LiFePO4 batteries do, but not enough to provide optimal charge and cycle life without the right applied charging profile (which is not complex, just a bit different from that used on lead acid batteries by a 'smart charger'). Hopefully someone in the forum can answer the original questions with the facts from specs not provided to owners, or from careful measurement and direct experience (not just guess). It is common in a big (ICE) 4x4 to add deep-cycle batteries and controllers as you say (keeping the cranking battery which is needed .... to crank). But it would be messy in a rav4 hybrid to add another battery. I am interested in replacing the stock battery, which is not used for cranking anyway - hence the original questions. I will also try asking Toyota, and report back if that helps.
 

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Thanks roadster-GB. But we are just guessing. Do we know if the rav4 even has a BMS that senses voltage and adjusts charge accordingly? If it does, can it be programmed to meet the needs of different battery types? Even the stock flooded-cell vs AGM would be different. There is nothing on the topic in the owner's manual. I doubt that the stock battery itself has any BMS. LiFePO4 batteries do, but not enough to provide optimal charge and cycle life without the right applied charging profile (which is not complex, just a bit different from that used on lead acid batteries by a 'smart charger'). Hopefully someone in the forum can answer the original questions with the facts from specs not provided to owners, or from careful measurement and direct experience (not just guess). It is common in a big (ICE) 4x4 to add deep-cycle batteries and controllers as you say (keeping the cranking battery which is needed .... to crank). But it would be messy in a rav4 hybrid to add another battery. I am interested in replacing the stock battery, which is not used for cranking anyway - hence the original questions. I will also try asking Toyota, and report back if that helps.
There is some feedback circuit from the battery. Check out 6:03 of this video.

 

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Discussion Starter · #5 ·
Thanks Variman. That is a great video about Prius 12V design, as is
by the same presenter (which mentions variable and constant voltage modes of output from the DC-DC converter, but no more relevant details). Prius changed from AGM to flooded-cell lead-acid over generations, but we have no details about whether the charging profiles changed. The gen5 rav4 does have sensor wires on the 12v negative battery post, but that does not tell us anything about the charging profile or whether it is suitable for LiFePO4, or whether it can be modified to be suitable. So it does not answer the original questions. From other forum posts, some Prius owners seem to have substituted LiFePO4 12v batteries, but the threads I have seen are all vague about long-term success. I have now asked Toyota (in AU and JP), but I expect the same advice given by Prof Kelly in his video: "Stick to a replacement that is identical to the original and you can not go wrong." That is true of course, but the converse may not apply; so it does not add any knowledge. If anyone has knowledge from specs not provided to rav4 owners, or from careful measurement and direct experience, I remain very interested to hear .
 

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Discussion Starter · #6 · (Edited)
No responses yet to emails seeking advice from Toyota (AU or JP), Renogy or Prof Kelly.

So, using the method described in another post (Front and Rear Dashcam Installation in Rav4 Hybrid...) I measured 12v battery charging voltage and current provided by the gen5 rav4 while it is in ‘READY’ mode.

My model has parking lights that stay on in this mode (in my garage), but I think this (along with the dash display and other electronic drain) should not affect the 12v battery charging, which is well below the system’s rated current flow. No doubt it increases the frequency with which the ICE automatically starts to recharge the NiMh battery bank. In my case, this was for about 3min every 50min (during which 12v battery charging was unaffected).

When I started the test (at 23ºC) the 345LN1 12v battery measured 12.4v with the usual <10mA ‘deep sleep’ drain. Some people call this ‘parasitic’ drain, but it is useful for the vehicle; so to a biologist it is better called ‘commensal’; but I digress. The 12v battery was below full capacity of 12.7v, so I expected it to activate recharging from the NIMH bank in ‘READY’ mode.

Here is the result: it looks like the initial charge at 14.24v and 3.5A (CC / bulk mode?) quickly dropped to 13.6v with current dropping to below 1A (CV / absorption or float mode?) over 6 hours (when I terminated the test, and the battery showed 12.7v). Bulk mode may be longer with lower initial state of charge. The results comply partly with the Yuasa guidelines (in Japanese, for the 50Ah 355LN1: https://gyb.gs-yuasa.com/assets/data/manual_enj-3.pdf) of a normal charging current of 2.5A for 5-10 hrs, to terminate when gas is produced in any cell. No charging voltage is specified.

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On these numbers, there seems to be no danger of damage to an LFP battery through over-voltage charging from the vehicle, but it would not exploit either the fast-charging nor the high-capacity of LFP chemistry; and an external charger would be needed to bring the LFP battery to full capacity.

The possibilities of vehicle error codes, failure to charge through excess sensed voltage, and/or altered charging into a lower-resistance LFP battery are still open questions.
 

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Discussion Starter · #7 · (Edited)
Power-Sonic provides good advice on charging LFP batteries (https://www.power-sonic.com/wp-content/uploads/2019/04/How-to-charge-LiFePO4-batteries.pdf) and good discharge profiles (eg https://www.power-sonic.com/wp-content/uploads/2020/07/PSL-12500-Technical-Document.pdf). I emailed them for further advice and received the same response (none) provided by other experts mentioned above.

Therefore I experimented with the Renogy 12v 50Ah LFP battery (which I bought on sale for less than either a 45Ah 345LN1 lead-acid battery or a 6.4Ah Cellink Neo LFP dashcam battery). There may be room for dual batteries in a larger ‘truck’, but not in a rav4. Therefore I replaced the 345LN1 (using a jump starter to retain ECU settings, after the vehicle had been off and the battery ventilated for a long while, being very careful to avoid sparks or short circuits during the whole process). The 50Ah LFP battery is a bit smaller than LN1, so I used some of the polystyrene packing from the Renogy shipment for a snug fit. Toyota also uses polystyrene to make the rav4 holder fit LN1. I used M8-SAE post adapters from Fullriver (which fit the Toyota battery post clamps and also provide M8 threads into the brass SAE posts). Sealed LFP batteries do not vent hydrogen or toxic fumes ( see-https://gwl-power.tumblr.com/post/141674260966/ ), so they do not need the rav4 vent tube, which I secured aside using a cable tie. I live in a mild climate, so the LFP temperature constraints (0~45ºC for charge; -20~60ºC for discharge), and the usual temperature compensations in lead acid (LA)-optimised chargers (-16 to -32mv / ºC), are fine for me.

To 70% discharge (about 13v at 25ºC with the expected low current drain) this battery should power my A129 plus duo in 1fps time-lapse parking mode (2.9W) for more than 6d. My 256 Gb SanDisk Max Endurance micro-SD card will hold about 18d of 1fps parking-mode recordings before overwriting. (Viofo claims that this unit does not overwrite any recording that includes an event that triggers its impact monitor, but that is not my experience in time-lapse parking mode). The “sweet spot” may be 2fps. The remaining 30% battery charge should hold the rav4 (at up to 120mW average commensal drain) for several months without jump starting. I have not experimentally tested these calculated times. They will vary with the efficiency of voltage converters, regulators etc. Cigarette lighter-USB converters that I tested varied from 2.8-20mA current draw (36-260mW) with no load, and 72-95 % efficiency with loads of 0.4-1.8 A. My one test with dashcam load gave battery voltage drop from 13.20v to 12.95v (~35% of battery charge at 23ºC) at 2fps parking mode (with lots of door opening = vehicle commensal drain) over 48hr.

I decided against the Viofo HK3 after reading about its poor efficiency and voltage regulation at https://dashcamtalk.com/forum/threads/hk3-hard-wire-kit-terrible-voltage-regulation.44244/. With no load, HK3 draws 86mW before and 35mW after cut-off. It does not have suitable voltage options for LFP, I prefer thicker wires, and I don’t want to stand on my head to find the right fuse sockets then leave the fuse box cover off because of piggy-back fuse connectors. So I decided on a Victron Smart BatteryProtect, which has the advantage of more adjustable cut-off and restore voltages. It is wired after a 10A fuse near the battery, then double-insulated 1.5mm2 (AWG16) wires run to a cigarette lighter-USB converter in the console box. My dascham will not draw more than 1A through this 7m circuit, so there should be no problem with voltage drop or heating of the cable. BatteryProtect power use is very low (16mW before and 9mW after cut-off, plus 3mW if Bluetooth is enabled). Cheaper units using a continuous relay, instead of periodic sampling and a MOFSET transistor, have much higher drain. However, MOFSETS can be stressed by voltage spikes from inductive loads, and if they fail (which may be silent) they normally keep power to the controlled circuit.

Wires were fairly easily routed under the trim: forward from the auxiliary battery to the side of the NiMH traction battery, along the front base of the NiMH battery to the centre, under the carpet and felt beside the tunnel, and under the console trim to enter the console box with the dashcam cable. (Pop off the rear seat and a few trim panels, but there are no screws or bolts to undo. The tunnel was trickiest because of cross-braces, but no big deal. I used a packing strap from the Renogy battery to find the route. An electrician’s plastic guide would also work, just twist and push gently to get past any obstructions). This allowed me to use a USB Y cable for automatic parking mode as described at Front and Rear Dashcam Installation in Rav4 Hybrid....

After the battery change, I conducted tests like the one described above, at several starting voltages:

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Starting at 12.95v battery voltage (estimated 25% SOC), it was a bit scary to watch 52.3A at 14.14v flow into the battery. While many LFP batteries (including the Renogy 50Ah) are specified for charging up to 1C (50A in this case), and the battery got barely warm to touch at the side after 30min, I would not like the charging current to go any higher. I assume the fast charge is simply a result of the low resistance presented by the LFP battery. The rav4 does have a (Hall effect) current sensor on the battery, but how this is used does not seem to be made public. Some car alternators might struggle with 50A continuously for 30 min, but the rav4 hybrid DC-DC converter seems fine with it. Toyota hybrid vehicle DC-DC converters are typically ≥100A (Please explain 12V system to me--how to keep battery charged while using accessories?), and the rav4 12v circuit fusible link is 120-140A (pure sinewave inverter for AC power), so there should be enough headroom for all other current draws. The ICE ran more frequently while the current into the auxiliary battery was high (for about 3min every 15min while current was 40-50A). The charge voltage dropped quite suddenly to 13.6v when the current was below 1A (at 50-60min in the charts above). This is almost no charge into the LFP battery (and the recommended current for charge termination), so I turned off the vehicle at 60-65min, at which time the battery voltage was 13.5v.

Then I kept watching battery voltage and current flow. In the first experiment, the commensal current draw by the rav4 dropped to about 100mA within 10min, about 15mA within 20min, and below my measurement limit of 5-10mA within 3hr (when battery voltage was 13.34v). In the second experiment, the commensal current draw by the rav4 was about 450mA after 10min, about 100mA after 20min, and below my measurement limit of 5-10mA within 30min (when battery voltage was 13.38v). The times to reduced commensal drain varied somewhat between experiments, for reasons I do not understand. But the stepped levels of commensal drain from the LFP battery were the same as those from the OEM LA battery, within my measurement errors. In essence, commensal drain takes the top off the fully-charged battery voltage when the car is stopped, then drops to a very low level until a door is opened.

On these results, the rav4 is fine with an LFP battery that is kept above 25% SOC. At 14.2v and ≤1C during the bulk/CC phase, there is no danger of damage to a properly top-balanced battery through overcharging, and the vehicle does exploit the fast-charging capability of LFP chemistry. I might be happier at 0.2C for longest cycle life (by reducing the possibility of irreversible plating of lithium metal onto the anode), but dascham users who need to recharge the battery during short drives will appreciate the faster charge near 1C (if the battery manufacturer allows that). The ‘automatic’ drop in charging voltage at 50-60min (presumably triggered by the charging current sensor reporting ~1A) is reassuring. Any accessories that drain current while the vehicle is running should be wired with return/ground connection downstream of this charging current sensor, so as not to interfere with it. Trickle charging of LFP batteries should be avoided, but the rav4 ‘float/trickle’ voltage of 13.6v does not raise any concerns for durations of a few hours. For long drives, a lower float voltage (≤13.2v), or no charge, would be better for LFP life. There were no problems with fusible links, sensors, relays or fuses. No errors have been displayed. The vehicle entered ‘READY’ mode as usual from the LFP battery at 25-95% SOC. If anything, fuel economy should improve due to the light weight and efficient charging of LFP, but this may be hard to see among other driving variations. The rav4 at 14.2v charged LFP to an estimated 90-95% SOC at 23ºC, which is fine for most purposes and good for battery life. Those who want 100% SOC (or more ‘absorption/CV’ time at >14v for cell balancing or erasing any memory effects) will have to use something supplementary, like a good LFP wall charger.

I have a Victron Blue-Smart IP65 12v/10A wall charger that can be programmed for the optimal charging specs from Renogy, except that it lacks the ability to set absorption charge termination based on charge current (try Enerdrive chargers for this feature). For charge termination based on current, a Victron user would need to watch charge current by Bluetooth (reported to 0.1A), and turn off the charger at the wall when it reached the desired level (1A = 0.02C for 50Ah). Charge termination based on cell current additionally requires a BMS with this output (which is not in the Renogy 50Ah) and an interface to the charger, which is lacking in everything I own. The Victron charger has split leads that can permanently attach to the LFP battery (as in the photo) and it includes the ability to report charging status and history via a smartphone. In default Lithium mode (14.2v charge, 2h absorption) it added only 0.2+0.4 = 0.6Ah to the charge from the rav4. At 14.6v (with 1 hr absorption for full charging and cell balancing) it added 10.3+0.2 = 10.5Ah (much more than I expected). The Blue-Smart IP65 can also maintain an LA battery (now my spare 345LN1, which at 14.4v charge it reported as 3.7+11.1 = 14.8Ah ie 33% undercharged by the rav4, perhaps a reflection of the low charging efficiency of LA chemistry). At the 14.7v charge recommended for lead-calcium batteries, it added another 0.0+0.2 = 0.2Ah.
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The caveat is that the initial charging current in the rav4 rose to over 73A when the LFP battery was discharged further, to around 12.7v (~20% SOC at 23ºC). The unmodified rav4 is not suited for this task, as the applied current at >1C is above the level that will quickly damage the battery. Toyota implies that using other power-consuming parts of the vehicle (headlights, aircon and blower, rear window defogger, radio) can reduce charging current to the battery (Do I have to take special steps to protect the battery of my hybrid vehicle if I plan to store the vehicle for an extended period?). In my experience the maximum effect is a few Amps, which is not enough to avoid battery damage in this situation.

As the rav4 has a current sensor on the auxiliary battery, it would be ideal if Toyota could program the vehicle for a maximum auxiliary battery charging current (10-50A for 0.2-1C charging of a 50Ah LFP battery), and at 1A either terminate charging or ‘float’ at 13.2v (which will stop any current flow into the LFP battery). Maintaining supply to the vehicle 12v system; while accurately sensing, controlling and terminating 12v battery charge current; may require separation of 12v battery charging and load circuits. This is simple enough to design, and already at least partly implemented, but the available circuit diagrams are somewhat ambiguous (Please explain 12V system to me--how to keep battery charged while using accessories?). It would not be trivial to implement after manufacture. The “float” approach seems easiest, as it only requires appropriate programming, and 13.2v is fine for both the LFP battery and the remainder of the nominal 12v electrical system. But it leaves the challenge of providing an optimal LFP charge current below the potential demand of other 12v electrical components while the vehicle is running.

The rav4 current sensor is also said to include a temperature sensor, but I can not establish how it is used in the hybrid model. Charge voltage to an LFP battery should not be altered according to temperature, but at the usual compensation of ~-27mv/ºC for a 12v battery it is not a big deal at normal cabin temperatures. Most ‘smart’ battery chargers simply disable this feature for LFP charging. Maybe LFP compatibility will be included by design in future rav4s?

In the meantime, if an LFP battery might be drained below 25% SOC, the best approach seems to be to limit charge current by interposing a DC-DC charger. This can ensure that the charge profile is optimal for LFP. But these are one-way devices, so they cannot provide power from the battery through the charging cable to the vehicle electrical system, as in the OEM rav4 wiring. At least an added cable with a power diode will be needed. This will need input from an automotive electrician. On preliminary advice, it is feasible, but expensive.
 

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Discussion Starter · #8 · (Edited)
An informative site about LFP battery technology is Lithium battery systems | Nordkyn Design and a good academic review is ScholarSpace at University of Hawaii at Manoa: Adaptive state of charge estimation for battery packs. At 1 cycle/day with a 2000 cycle life there may be a better technology before a well-installed LFP battery needs replacement. Reported calendar life for properly managed LFP batteries is >10y. Although I have been convinced about the benefits of LFP auxiliary batteries for hybrid vehicles, anyone considering a change should note:
  • LA is a more robust technology; because it is more temperature-tolerant, and because lead is more stable than lithium. But the strong acid electrolyte and hydrogen gas production from LA batteries are dangerous. Experts disagree on which battery technology is safer overall (and both require careful use for safety).
  • LFP is more stable than Lithium-Cobalt (used in most Li-Po powerbanks), but it still needs at least a dedicated battery controller (called a PCM or BMS) for safety. This may be built into the battery or wired in. It must connect to each cell within the bank that forms the battery. It is essential as the ‘protection of last resort’ to avoid over-charging and over-heating (which can be dangerous as well as damaging to the battery) and over-discharge (which can quickly destroy the battery).
  • For safety, no lithium battery chemistry other than LFP should currently be considered for in-vehicle use. Although unlikely with properly installed LFP batteries, thermal runaway followed by a lithium fire is such a dangerous scenario that redundant protections should be used against it. Protections should be redundant in the battery and charger, in case either fails in a way that keeps the circuit active.
  • Most external battery (charging) management systems use some form of battery voltage sensing. LFP batteries present higher charged voltage (~13.5v) so they have used at least 50% of capacity (>80% at low discharge rates) by the time they drop to the LA maximum around 12.7v. At 0.1-0.3C current and 20-30ºC, voltage drops steadily to 12.5v at 80-90% discharge, then rapidly to 100% discharge (typically defined by the internal BMS as 10v). See the charts below (clarified from power-sonic). True ‘resting voltages’ (with no commensal drain) would be a little higher. The slope is small from 5% to 80% discharge (and the relationship depends on both temperature and discharge rate). So a small error in specified or measured voltage, or estimation at an incorrect temperature or discharge rate, can give a large error in estimated battery state of charge (SOC). Some experts recommend Amp-hour or Coulomb counting or more complicated algorithms to estimate SOC; others find them unreliable.
  • A dumb bulk charger for LA (or even a Li-Po charger) may push out more than 14.6v and dangerously overcharge LFP. A smart (multi-stage) charger for LA may not provide any current at LFP battery voltage. Or it may work just fine if it uses suitable voltage and current cut-offs. As a conservative charger setting for LFP, a regulated 13.8-14.4v at 0.2- 0.5C should be fine for the bulk/CC stage (but check your battery specifications). As battery resistance rises with state of charge, current at this regulated charging voltage will drop (absorption/CV stage). Even ‘ripple’ above the regulated voltage can be damaging if it goes over the safe voltage limit for LFP charging. Below 13.4v there will be no charging (or very slow charging and greater risk of memory effects from partial charging).
  • Some experts recommend charging well-matched cells to only 13.8v to avoid challenges of balancing at higher voltages. Others use at least 14v and absorption to 0.05C to avoid memory effects. Some BMS units only commence cell balancing above 14v. In any case, a prolonged absorption phase (> 4 hr) should be avoided.
  • Absorption/CV charging should stop before charging current drops to 0.02C (1A for a 50Ah battery); but not all chargers (even those with ‘lithium’ profiles) have this feature, and it is complicated by any substantial ‘parasitic load’. Some chargers use a higher cut-off current, some use timed cut-off of the absorption phase, and others go on until stopped manually. If you are happy with 90% SOC (and longer cycle life) you do not need any absorption/CV charging; just a bulk/CC stage to 14.4v. To get 100% capacity (and possibly for cell balancing and avoidance of memory effects), you will have to check specifications and/or measure your particular systems (vehicle and external chargers and battery). Most likely, battery specifications will call for absorption charging to terminate at some fraction of C, in the absence of any ‘parasitic load’. The fraction may vary with charge voltage: some would say 0.03C (1.6 A for a 50Ah battery) when charged to 14.2v. It is sometimes recommended to terminate charging based on BMS readings from individual cells, which are unavailable in many LFP batteries.
  • Memory effects occur in LFP after partial charge combined with partial discharge, and become more difficult to erase after multiple partial charge cycles to the same level. In practice, to clear memory effects it is usually only necessary to charge fully (terminating by absorption current) a few times a year.
  • LFP batteries are safest, and show highest capacity and cycle life, at moderate temperatures (around 23ºC). They must never be charged at freezing temperatures, or discharged at elevated (engine bay) temperatures. Vehicle cabin temperatures are generally OK (humans function best in the same range), but can reach unacceptable levels if parked outdoors in hot or freezing conditions. Temperatures over 40ºC can greatly shorten the life of LFP batteries.
  • LFP batteries are generally not designed to be cranking batteries. If you want to be able to jumpstart another (non-hybrid) vehicle, you should use a battery designed for cranking.
  • High-quality LFP batteries can be recharged quickly (0.2-1C at a charge voltage up to 14.6v, depending on the brand and model). There are compromises between highest charge rate and capacity vs highest cycle life. For safety they should never receive a higher-voltage ‘desulfation’ or ‘equalisation’ charge, and for long life they should not be trickle/float charged for a long period. Short-term float at 13.8v or below should not be damaging. Unlike LA, their stored life is longest with a partial charge, ~50% SOC (and at < 25ºC). When fully discharged, they should be recharged (at least partially) as soon as possible. Algorithms for LFP batteries in active use typically trigger ‘recharge’ at a battery voltage of 13.1-13.2v (aimed to keep >50% SOC).
  • The vehicle charging system must be checked for safety with LFP. It may be necessary to modify the vehicle system for safety, and/or use a smart wall charger and/or manual intervention for highest capacity and cycle life. You may also need to consider whether your typical and extended drive durations would replace used battery capacity, though LFP will typically charge much faster than lead acid (if the vehicle provides sufficient current, at charging voltages of 14.0-14.6v). Cycle life will be greater with 0.2C than with 1C charging; and with current-based charge termination.
  • Ideally, the charging system should ensure that the LFP battery has discharged substantially before initiating a current-limited and current-terminated recharge. Vehicle 12v battery charging systems are not yet designed this way.
  • LFP batteries vary in quality. There are important differences in BMS features (temperature sensing, cell balancing, communication), wire size, vibration resistance, electrolyte composition, cell type, cell matching, true capacity, life-span, warranty and recommended charge or discharge profiles. It may not be possible for a buyer to check these features, so it is safest to use a reputable brand. Fortunately, prices are dropping.
  • Vehicle warranty may be affected if a case can be made that use of a non-OEM LFP battery was relevant to damage to the vehicle. Insurance is a potential minefield.
Although widely used as light, fast-charging, deep-cycle batteries in hybrid vehicle traction systems and recreational vehicles (where they may have separate cooling and charge/discharge management systems), they are not necessarily the best choice for all hybrid vehicle owners as replacements for the vehicle 12v auxiliary battery. LFP batteries work best for regular cycling. LA batteries are better for prolonged standby at full capacity. Most current mobile systems with LFP use it in parallel with at least one LA battery. There is no such thing as a simple LFP ‘drop-in’ replacement for an LA vehicle battery. It is at least essential to consider the extremes of cabin temperature in your location, and to check the suitability of the vehicle battery charging system, especially looking for excessive charging current at low battery SOC. If in doubt, stay with the OEM-supplied battery. Before even thinking about DIY installation, read LiFePO4 Batteries On Boats - Marine How To and look at Lithium battery systems | Nordkyn Design (vehicle use shares many potential pitfalls).

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Discussion Starter · #9 · (Edited)
Toyota (USA) has responded: “Toyota does not recommend or assist with modifying our vehicles from the original factory specifications.” Fair enough, though I asked about my rav4 not theirs; and such advice ignores the millions of Toyotas out there with non-OEM modifications such as dual-battery systems, or even dual dashcams or battery chargers. Perhaps compatibility with LFP auxiliary batteries will feature in original factory specifications of Toyota hybrids in the future.

In the meantime, anyone thinking about DIY installation of an LFP battery should first read LiFePO4 Batteries On Boats - Marine How To (vehicle use shares many potential pitfalls). An important point from this article is that if the BMS cuts off during charging (even if this is from a single faulty cell), it may cause a voltage spike through the circuit that risks damage to other vehicle electrical components. Such spikes can arise from a charging alternator or from any inductive load (anything with a coil, such as a motor or solenoid; especially with high current). The same applies to cut-off through a battery isolation switch, circuit breaker or fusible link while charging.

The rav4 hybrid does not have an alternator, and the 12v DC circuit is effectively isolated through the DC-DC converter from major inductive sources like the high-voltage generators. If the BMS cuts off the battery during charging, any inductive loads should continue to receive power from the DC-DC converter. But a connected car battery acts as a buffer (capacitive load) for the vehicle electrical system. Although the 12v output in a hybrid with DC converter may be more stable than a conventional vehicle with an alternator, the interactions between control circuits for the 12v and high-voltage (traction) systems are unknown (to me). So it is preferable not to run any vehicle without a connected battery. With these things in mind, we wish to ensure that the LFP is never charged above 1C, but we do not want to disconnect an LFP battery at a low SOC after the vehicle (and thus the charger) is on. Identification of any other risks requires analysis of the entire vehicle 12v system by a qualified electrical engineer familiar with LFP battery issues (and to my knowledge this has not been done).

If any technically-adept owner goes ahead with an LFP battery at their own risk, it seems best to use a high-quality ‘battery protector’ and set the cut-off voltage for any added circuit (such as dashcam power) to 13v (~30% SOC at 25ºC?) so that subsequent charging current should not go above 50A (1C). It would be wise to check battery voltage before charging from the vehicle, any time substantial battery discharge is suspected. This is simple with a cigarette lighter-USB converter that incorporates a voltage readout. If the LFP battery is below 12.95v at 25ºC, it should be disconnected from the vehicle charging system before starting, to avoid excessive charging current. That would leave problems of how to start and run the vehicle, if such a situation ever arose. To ensure that this could never arise, it would be necessary (in the absence of advice or assistance from Toyota) to seek professional advice on the DC-DC charger plus power diode approach mentioned above. With such an approach, charger current could be chosen based on preferred balance between LFP battery cycle life and charge speed. One could also choose a DC-DC charger with current-based charge termination and adjustable recharge voltage threshold, as desired for optimal life of LFP batteries.

Without these features in the charging system, life of an LFP battery kept above 25% SOC in a rav4 hybrid will be less than 2000 cycles (specified under ideal conditions at 0.2C charge, no float and no delay before discharge). But if there is substantial cycling, LFP life should be far greater than any LA battery of similar capacity, if a well-installed and high-quality LFP battery is used at moderate temperatures.
 

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some Prius owners seem to have substituted LiFePO4 12v batteries, but the threads I have seen are all vague about long-term success.
Have you seen this thread on the Prius forum? He says "all my Prius have been running 12V Lithium for years." The poster is the developer of the Dr. Prius app and a replacement Lifepo4 Prius hybrid battery.

Prius DIY 12V Lithium
 

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Discussion Starter · #11 ·
Thanks pikeviewer. I had not seen that site. It is very interesting from a hobby perspective: the poster even makes his own terminal rings! Although he started with a project using old battery cells (so he probably cares little if they die), I was amazed by the lack of mention in that entire thread of any measured charging current at any battery SOC; and the potential dangers that might pose for the vehicle and the battery cells. It is one of several potentially dangerous oversights and I strongly recommend that anyone considering the approach should first read first read LiFePO4 Batteries On Boats - Marine How To.

Also I had a response from Toyota AU: "Regrettably we cannot assist with any technical questions of this nature. As we are not privy to any part or accessory information we would advise contacting the Parts department. We suggest that you do contact a Toyota Dealership and they will certainly be able to assist with your query." I doubt that a local dealership will know or be willing to say more than Toyota at national level. On past experience they did not even know how to do a requested 5-tyre rotation (explained in the owner manual), but I will ask.
 

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Honestly, this all seems like a lot of work, and I'm not sure what your end goal is with improved deep cycle performance. What are you running that you need improved deep cycle performance of your 12v battery? If you're looking to run a 12v load for a long period of time, you have 2 easy/cheap/off the shelf options to pursue - 1) leave the car in ready mode, the Traction battery will power the 12V sockets indefinitely, and fire up the engine everyonce in a while to keep the traction battery charged until you run out of fuel. or 2) if you don't want to leave the car in ready mode, hook up a Jackery 250/500/etc. to your 12V port, then hook up whatever device your powering to the Jackery. This way when the car is on, it will charge the jackery and whatever device you're using, then when it's off, the device will just run off of the Jackery.
 

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Discussion Starter · #13 · (Edited)
A good example would be parking at an airport "park and ride" station for a few weeks while on holiday. Ideally one might like to keep a dashcam running in parking mode, without damaging the battery or returning to a flat battery. Forget about leaving the vehicle in "ready" mode, or using a wall or solar trickle charger in those circumstances. Many power banks use less safe LCO chemistry (and it is not cheap to get the required capacity). I think Jackery brand uses Li-NMC which is fairly safe. But you need to find a big space in your vehicle to safely carry even the smallest Jackery, and it will take 6.5 hours to recharge from a cigarette lighter socket. A Jackery 1000 to provide 46.4Ah weighs 10kg and needs 14h to recharge. The cellink neo system is another (LFP) alternative, with drawbacks in cost, capacity and recharge time. Each to their own preference of course. The best system for you depends on your individual needs.
 

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I'm pretty sure a small solar cell on a dash, coupled with a battery (all independent from the 12V system), could power a dash cam for an extended period of time. This way you're not tweaking the BMS of the vehicle, and any other intent the engineers had for the LA battery in that car. Alternatively, if you take these long trips and are away from the vehicle for extended periods of times and want to still integrate the LFP, maybe you could just implement a temporary hardwired solution to piggyback power the 12V system once you park the car, and you just unhook it when you return, then charge it independently at home. This way, once again, you can bypass any tweaking of the BMS/charger.

Lead acid batteries are also great for cars (even if not as a starter), as they act as giant capacitors for the entire electrical system. Not sure what the initial charge/discharge rate of LFP is, but it's probably not as good as LA. Not sure how that would impact the electrical system when heavy load items like headlights and cooling fans are turning on and off.

I've only ever heard of one car manufacturer using lithium as a 12V source, and that's hyundai in their new hybrids, maybe you could learn more of how they implemented it? I've seen/heard of people building capacitors for 12V replacements, but I can't imagine they work as well. There's a good reason the Lead Acid battery has stuck around, even in hybrid/electric cars, for as long as it has.
 

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Discussion Starter · #15 · (Edited)
A solar cell will contribute no useful current in a covered parking structure. I have not advocated that an owner should tweak their vehicle BMS, though we can be sure that Toyota will continue to do so as they improve their designs and adopt superior technologies.

Your idea of a LA battery as a giant capacitor is interesting (and updated in my post above after some more research). I can only advise for a fact that LFP safe charge and discharge rates are much greater than LA.

One reason LA has stuck around is low price (though not for the OEM battery sold by Toyota, presumably to those who believe that it is the only one that can do what the design engineers intended in that car). A Jackery 1000 "to piggyback" costs US$899 (almost as much as the Toyota-supplied Yuasa 345LN1, and much more than a high-quality LFP battery of the same capacity). Installing a Jackery 1000 safely in a rav4 would be a challenge. There are other reasons to use LA, as I have detailed above. As I said: "If in doubt, stay with the OEM-supplied battery".

You are absolutely entitled to your own doubts and speculations. I have some too. The best system for anyone depends on their individual needs.

If you actually test any of your ideas, please report on your results (in a new thread). I hope you will document it carefully, even if some forum users find it too much work to read carefully.
 

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Thanks for the great writeup, @RGeB !

This is a genuine problem for dashcam users, and I really appreciate your professional approach to finding a solution.

Two tiny things to add:

For more accurate control of cut-off voltage for a dashcam, this kind of accessory may come in handy: 2.28US $ 22% OFF|XH M609 DC 12V 36V 24V Voltage Protection Module Low voltage OverDischarge Battery Disconnect Protection Output 6 60V 6V 60V|Battery Testers| - AliExpress. It has digitally configurable cut-off voltage and hysteresis level. While I haven't used it on a car, I have found it very useful in other applications. You can also build your own using a NE555 chip - there are tutorials online.

A solar panel on the dashboard would provide very little power, even in bright sunlight. Automotive engineers go to great lenghts to add solar-repelling coatings to windshields, and it shows. Speaking from experience. :)
 

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Discussion Starter · #17 ·
Thanks for the insights ddd324. I actually bought an AliExpress voltage cut-off (and a Viofo unit), then did not use either. One reason I paid a bit more for a Victron BatteryProtect (apart from ability to use LFP-friendly voltages) is the Victron unit's very low standby current drain (specified at about 9mW, which is also what I measured). The XH-M609 unit specifies "Power consumption: Less than 1.5W". I don't think it can be that high, but I would want to measure it before use. Those digital displays are power-hungry if they stay on, and some units that use a relay actually draw more current after cut-off (because then they open a NC relay), which seems crazy. Once it enters 'deep sleep' mode, my rav4 draws less than 100mW of parasitic drain (probably more like 10mW, but I have only tested it with a clamp meter, which has limited DC current resolution). Another 1.5W after cut-off by a 'battery protector' would kill a 50Ah car battery within a few weeks. Every mW counts after voltage cut-off if you plan to be away for long.
 

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Thanks for the insights ddd324. I actually bought an AliExpress voltage cut-off (and a Viofo unit), then did not use either. One reason I paid a bit more for a Victron BatteryProtect (apart from ability to use LFP-friendly voltages) is the Victron unit's very low standby current drain (specified at about 9mW, which is also what I measured).


FWIW: I am using the same model Victron in my RAV4 installed when I upgraded to group48 AGM...in order to prevent my aftermarket stereo components from possibly running the AGM flat(y)

BTW: Your LiFePO4 install looks very clean!
 

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Without getting into great detail, what would be the average expected life of the 12v battery in a standard/stock Hybrid model?

I’m going to start shopping for one in a month or so and also wondering what the dealer would charge for a trailer hitch? I’m guessing it would cost somewhere around $500?? Thanks
 

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This all seems like a lot of work, and I'm not sure what your end goal is with improved deep cycle performance. What are you running that you need improved deep cycle performance of your 12v battery?
 
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