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Effects of Cross-Connecting Battery, or "Electricity: You're doing it wrong"

Hey folks,

First time posting, just wanted to pass on a recent amateur DIY experience.

Woke up for work, walked out and tried to crank my rav4 - slight clicking sounds, flickering dash lights, but no crank. Classic 'dead battery' symptoms, which makes sense since this battery was at least 4 years old at this point. Run to the auto parts store, look up the battery replacement via the finder kiosk, can't find the exact model listed in the finder but I get one that differs by a trailing 'J' in the model code. Look at red/black post covers (instead of the markings AT the posts), the orientation matches my old battery. Buy it, bring it home and get ready to rush-install so I can get to work.

Put it in place, connect positive terminal first - this should've been my first clue that something was wrong. The positive terminal connector for the positive post did not fit on the post very well, but I figured 'maybe this brand (duralast) is just a little different.' With the positive cable attached to (what I thought was) the positive post of the battery, I proceeded to do the same with the negative cable.

The car's horn IMMEDIATELY goes off with a solid-tone blare that hits me right in the face, and I almost ruin my pants as I remove the negative cable. Weird...Then I think "Oh yeah, maybe that's just the alarm system freaking out - I bet if I try to unlock the doors right after I connect the battery again the horn will stop, so I try that - no such dice. Horn just continues to blare, so I disconnect the battery again. Note that the overall amount of time connected in this configuration was very short - less than 5 seconds, certainly. Turns out that's more than enough...

Looking around, I see the problem - the colored post covers on the new battery had been swapped out, and I just didn't notice. Biggest bonehead mistake you can make, and I've never done it before, but was in a hurry and got careless.

Anyway, I figure I'll just flip the battery around and use it just long enough to get to work and then exchange it at auto parts store, so I do that. Hop back behind the wheel to drive off - car won't even attempt to crank. Turning the key to IGN produces no clicking or any movement from the starter - just some dash lights. Headlamps, dome lights, radio, etc. all work fine, whereas before the new battery these were all inoperable. Multimeter confirms battery voltage is sitting right at 12.6V with engine off, so I know the battery is good.

AFAIK, the next most obvious thing to check once you've done something stupid like this is fuses. I opened the fuse box on the right side of the engine bay, grabbed my fuse tester, and started testing. Almost immediately, I see that the 120A fuse is blown, and that this fuse is listed as an ALT fuse on the fuse box cover. "Ahhh...blew 'the' alternator fuse" I thought to myself. "No biggie - fuses are easy." As many of you know already, some of these higher-amperage fuses are NOT easy to replace (at least not as easy as the common/smaller 5/10/20A fuses). I also find a blown fuse labeled EFI-MAIN (20A), another labeled “ETCS (10A), and another labeled ALT-S (7.5A) in the left-side fuse box in the engine bay. Back to the parts store, get fuses, replace them in the car - time to test again.

First crank attempt started the car, but then immediately died...Figuring the computer might have needed to clear itself or something, I tried cranking again - success!! But, the charging system dash indicator immediately illuminated. Also, with a voltmeter across the battery terminals with engine running, I was reading less than 12.4 V, and even lower with headlights/radio/accessories turned on, which means there was no charge flowing from alternator to battery. I could see that the alternator pulley was spinning, not making any strange sounds as far as I could tell, and the serpentine belt looked to be in good condition and under good tension, so I was hesitant to blame the alternator as the root cause.

Given my struggles to get the right-side engine fuse box apart initially, I started thinking maybe I'd pulled a wire loose from a I set about re-disassembling the fuse box and re-checking all the connections. Not even 15 seconds in, I notice that the large nut near the bottom-left side of this fuse box is not tightened down - yet something else I missed. It turns out this is a ground connection. So, I tighten it back down, and decide to try cranking the car before I get back into the fuse box. Everything works, no dash lights, voltmeter at battery terminals is showing 13.8-14.2 V from idle to ~2k RPMs.

TLDR - Lessons learned:

  1. Always double-check your battery terminals and replacement battery model is correct. Be sure first, then start working.
  2. If the first battery terminal connection doesn't fit very well, re-read lesson #1...there's a good reason it might not fit.
  3. If you cross-connect a car battery, you may end up blowing SEVERAL fuses. Make sure that you check/test ALL fuses in the vehicle and replace all that are blown, NOT just the ones related to the malfunctioning system.
  4. The high-amperage fuses don't 'just pull out' like the smaller ones - you have to pull the fuse box apart to expose the hex bolts securing the fuse leads.
  5. Disassembling the fuse boxes can be difficult due to tight clearance around some corners of the box. Take your time, and get some help from a friend to keep lifting the top section as you work to separate the remaining plastic clips, if you can.
  6. Make ABSOLUTELY SURE you tighten everything back down when re-assembling the fuse box. A poor ground connection can make a charging system (or any electrical system) failure very difficult to troubleshoot.
  7. More of a general observation about automotive work - 9/10 times, it’s far easier to remove the component that’s in your way instead of trying to work around it. Give yourself some room - your hands and sanity will thank you.
Anyway, hope this helps someone save a little time.

PS: will shrink/clarify this when I get home from work, and will try to add some pictures as well.

7,899 Posts
Welcome! There has been a number of previous posts about reverse polarity connecting of batteries, but hopefully your well-reported experience will also serve to help prevent others from making the same mistake. Good that you got the problem sorted out, apparently without damage! :smile

Your Humble Administrator
16,418 Posts
I think this is worthy of a sticky. What say you James?
I totally agree! Excellent first post mbrownatx, and if you need help uploading pictures let me know!

Super Moderator
9,042 Posts
I totally agree! Excellent first post mbrownatx, and if you need help uploading pictures let me know!
Agreed here too! Excellent write-up from one who knows what he's doing - well, except for one mistake. :doh:

And some pictures will top it off quite well.

At first I thought we should list his year & model in the text but then it's really applicable across many years & models.
Maybe it belongs in RAV4 Lounge?
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