My 12v battery is 5 years old=Still working fine=How do I test the 'health' of the 12v battery?

Sometimes a single test isn't good enough.

If....you charge the battery like you should before any test....and it passes a load test applied soon after the charge, then you should let it sit with no load for about 24 hours and apply the test again, after checking the resting voltage.

Sometimes it will pass with flying colors immediately after charging but go dead again after sitting for a while.

 

That is true only for the cro-magnon type simple restive testers, any modern tester will perform an accurate battery evaluation. The problem is, knowing which was used.

As an example, our '05 had what I believed to be a weak 12v, so I charged it as the vehicle had been unused for over a month. The initial Open Circuit (OC) voltage reading was 12.2v, after charging with a good smart charger the OC voltage was 12.65v. I retested the next day and the OC voltage had dipped to 12.4x, which is not unexpected so I *thought* it might be OK. I applied a small load using the same light bulb set-up I use for loading the HV modules for just a few seconds and rechecked the OC voltage and it was 12.2x, again it seemed that the battery was servicable. I again recharged it using the smart charger on the lowest setting, knowing that leaving ANY battery sit unused is a really awesomely bad thing. The smart tester immediately condemned the 12v as being bad, despite "normal" tests suggesting it was at least "ok".

 

So.......how do you figure that a "modern tester" will be able to look into the future and "test" what the battery will be capable of hours down the road ?? While it might do a better job, they still aren't perfect and a "GOOD" test result doesn't always pan out.

A BAD result, however, is almost always right.

 

Not sure where that supposition came from.... No one EVER claimed that a modern smart tester could predict the future.

Generally true, but not *always* so. You have to test a battery appropriately for it's chemistry and intended use. If you "load test" a golf-cart battery, with anything but the most sophisticated modern tester, it will most likely fail as one example. The main reason is the test type is not at all appropriate for the battery type. Just like using a dumb tester cannot account for the performance difference between a FLA, AGM, or Gel battery.

 

Well, I said it needs to be tested again after sitting for 24 hours.

Then YOU implied that a "modern tester" should somehow account for that........didn't you ??

 

Not sure why you conflated your assertion that a battery needs to be tested 24 hours in the future with a modern tester can accurately predict the future, but whatever.

Let's be clear, no battery OEM that I am aware of states that one "must" (re)test after sitting 24 hours (or any other arbitrary timeframe). It's acknowledged that after recharging, the "surface charge" must be removed prior to a voltage measurement for an accurate result and further that sitting OC for 24 hours will accomplish that, sitting is not the ONLY method to remove the surface charge and not the current recccomended industry practice.

 

The electronic load testers (Solar BA5 etc) are not thrown off by surface charge. They'll report the higher voltage, but the CCA they measure, and the verdict, won't change.

 

DO-IT-YOURSELF TEST PROCEDURE (thanks to jdenenberg):
- Without brake pedal, press the Power button once and release to enter ACC mode
- Press and hold the MFD Info button, then turn the headlights on and off three times to enter Maintenance mode; release Info button
- Press “Menu” (on screen)
- Press “Display Check”
- Press “Vehicle Signal Check” - the battery voltage is shown and should be about 12.4 to 12.8 Volts (normal for an unloaded battery)
- Again without brake pedal, press Power button and release to put a current load on the battery - the voltage should stay above 12.0V (if less than 12.0V the battery is not well, or there is a fault or unusual load somewhere)
- Press brake pedal and press Power button once to enter "Ready" mode - the battery is now charging at about 14V (if less than 13.6V or more than 14.4 there may be a problem with the charging circuit)
- Turn car OFF to leave Maintenance mode

 

Easy.....
1.)...Wait two weeks.

2.)...Drive the car back to VT. Not being presumptuous, but I'm thinking that if you're a dual citizen, and you're intelligent enough to log into PC, you're not currently residing there NOW.....

3.) Park the car outside over night.

4.) Attempt to start the car.

As indicated before, you can place the battery under a load leaving the high beams on while the car is not booted, and then test it but this will only test the battery one time and in whatever environmental conditions exist AT THE TIME.
Load testing IS a good way to determine battery health, which is why auto parts places have load testers but the Prius battery is sort of a PITA to get to and I'm thinking that if I'm going to the trouble of yanking a 5 year old battery out of the car, I'm not going to CARE about whether or not it passes a penitential checkup!

Remember that the 12v battery does NOT actually start the car.
People always forget about this when they're remembering the good old days when on a cold morning when you hear the solenoid click and the starter motor struggling to turn over a 7500cc motor that's encased in ice.
Yeah...back then you could could warm the battery a bit, or thwack the battery terminals and maybe get that ONE good cylinder to ignite and break the car out of its long nap..... but this is 2018!

Priuses do not care how many cold cranking ampres that a battery is capable of producing, because all the ECU just needs enough current at the right voltage level (12v? 12.1v? there IS a threshold) to close a few relays and let the traction battery start the car.
THAT is why you can start a Prius with a DeWalt power drill battery - or probably 9 "D-cell" flashlight batteries.

What all of this means is that you're going to have no advance warning for your 12v battery's trip to Valhalla.
You're going to go out one day with your little keyfob thingy and the door isn't going to open.

So....
Here's what I would do:
1. Buy a portable car jump starter. They're about $40...and learn how to use it.
2. If you're not able to remove the battery from the car or you just don't want to, replace it proactively.
5 years is enough.

Good Luck!

 

NOT talking about surface charge here.

AM talking about things like sulphation and simply worn out plates which causes the total capacity to go WAY down and the "self discharge" rate to go way UP.

The simple fact of the matter is: An old worn out battery can, and often does, pass whatever test you throw at it immediately after being charged. The reserve capacity dwindles down fast, however, and it will fail rapidly.

This is what an owner with a charger often experiences.
Car starts fine after charging but the next morning it doesn't.

 

It’s in Section 9, Specifications. Page 429 in the 2006 owners manual.

 

Ok, thanks. I wasn't even looking in the right manual, lol: forgot this was second gen.

Still: a check of voltage does not really check the "health" of the battery, the question posed in the tread title. For this (and I'm steadfastedly flogging a very dead horse here), the latest and greatest is the prosumer level digital load testers, such as the Solar BA5.

 

But remember a Prius battery does not have the work load of a normal car battery. It’s not used to spin the engine, just run the computers.

 

Yes, but assessment of the battery's CCA is still an effective way to determine it's "health".

 

True.

 

Good video:

See the proper terminology for these new electronic load testers is "conductance tester".