Batteries take maintenance as refilling the wet cells with distilled water every few months. When the wet cell gets low on water the lead compound is high and dry. Over time the battery loses its capacity, say from 400Ahr, (Amp Hour) to 200 Ahr and can't have enough energy in store to crank the starter especially on cold days.
My 2011 Rav3 exhibits the very same battery downfall. Every 3 months or so the cells need replenishing of water. Pure water, distilled water, not much but enough to cover the cell's plates. Without replacing the water that is lost through chemical reaction the battery will over heat, the plates will warp and with use and time become no good as a source of power or a storage tank.
Although the charging system is 140A when the battery loses capacity, the battery won't take a charge no matter the charging size. Battery, lead acid types are very fussy, very self destructive especially when they get low in their electrolyte levels. As these batteries charge and deliver amps they suffer a loss of lead that goes to the bottom of the battery. These blackened lead deposits over time will short out the battery. This is their lives untill dead.
If looked after and if the environment is not too harsh a wet cell battery can last about 8 years, but it takes constant preventive work as: cleaning terminals, tightening connections, verifying wires and cables, up-keeping battery fluid, washing the battery and its enclosure, no excess vibration.
Many people find it easier to replace their battery every 2-3 years or even replace their car every 2 years and let another owner look after it while they drive a new or under warranty car.
If money is not tight, I agree but when money is an issue learning what to do, when to do it does save you money but your work load is increased. The battery used in today's cars is more important than ever because it is always under a load. A load from your alarm, radio, computer or ECU, there is never a time when your battery can rest become fully charged. So doing a full recharge every so often will bring that battery to a float charge then you can test it.
Testing a battery you need a multi-meter and a hydrometer, but the battery hydrometer which is about $10 and looks like a turkey baster is a must. It tests the battery acid and compares that to pure water. A fully charged battery when tested this way indicates for each cell whether each cell is good, not so good and bad by the resultant number. Pure Water = 1.0, A good wet cell = 1.25-1.3 and as the number drops to 1.0 the deader that cell.
Good cells have a higher voltage around 2.1 VDC, 6 cells are in series and all add up to 12.6 VDC. But as cells are good bad or in the middle the total voltage changes.
Add to this the ampacity of the battery, ie. how many amperes will the battery deliver? Even a dead battery can read 12V but will deliver minimum amps. This often happens when you notice the lights are bright but the starter clicks or moans. Light take far less amps than the starter.
The starter motor is designed to work at a large load, starting a cold engine and needs a good battery delivering 100's of amps if not 1000.s. Then when the engine is running the alternator can begin giving these amps back to the battery but only at a maximum of 140 A, so it is going to take time to shove all these amps back into that battery.
As long as the battery is in good shape the transfer of amps back and forth works well, but if anything hinders this exchange the battery suffers, the wires or cables and connections suffer and then it all falls apart and the car stops DEAD.