At what voltage should the OEM 12 volt battery be replaced?

I have one of the oldest 2010's (serial number 1379) and have had no problems yet. I had to replace the 2008 Gen II's battery in the spring. That would have made it a about 5 1/2 years old.

 

I'll submit that voltage tells you almost nothing. A voltage check *might* be useful in confirming a bad battery, but as a predictor of failure it has no real value. A battery may well have 14v showing one day and be dead the next.

A better question to ask would be: what's it worth to you to avoid the dead battery scenario? There's no question that the OEM battery is the cheapest of the cheap and shouldn't be expected to last as long as a decent retail battery, but it mostly depends on how its used. Lack of use kills them as fast as heavy use, but they can just go belly up for no reason and without warning.
Best test posted around here so far is to put your car in "on" mode, (not "ready"), and run all 4 windows down and up again. If the motion is snappy and seems normal you have a decent battery - maybe.

 

The 12v battery has been lasting about 6 to 7 yrs in the gen2 FWIW.

SCH-I535

 

there's a thread here somewhere that shows how to use the mfd to test the battery.

 

No bisco, that is only showing you the voltage at one point in time. Plus it is not real accurate compared to a calibrated VOM at the battery itself.

Lead acid batteries can die without notice.

 

First, I would not bother to even test battery voltage unless I had symptoms of a low battery, then the test would be a confirmation that it was low. At that point if it was under 12 volts on any of the three tests, I would replace it.

Weird stuff happening? MPGs dropping? Test The Battery | PriusChat

 

I had the battery die in the first 12 months, but that was because I somehow "tricked the head lights into staying on for about 12 hrs and completely flattened the battery. My second battery, also a Toyota battery is travelling fine so far.

 

Two quick points.

1. Several posters have stated that lead-acid batteries can fail without warning. I'll go with that.
2. It's also possible that a lead acid battery will fail with warning.

One failure mode I've seen in a relatively long lifetime is for a cell to short out. In a 12V battery, there's six cells, all containing roughly the same amount of charge per cell. If the battery is allowed to discharge all the way down (i.e., the lights were left on, dummy), one or more of the cells can get reversed in potential, after which they may not take a charge any more. In that case, sticking a voltmeter across the battery, especially if the battery has at least some small load on it, will reveal 10V or less across the terminals, and it's time for a new battery.

Second bit: Over time, some of the sulfate in the electrolyte (H2SO4 in water = sulfuric acid) can result in a coating of lead sulfate crystals on the plates; this reduces the ability of the battery both to take a charge and dump out current; call it the natural aging process. So, even with a good voltage, there may not be much capacity on the battery.

Most service stations keep an automotive battery tester around ($300 for the gold plated version, $12 for one you really wouldn't want to use) which can put up to a hundred amp load on the battery whilst checking the voltage. This is a non-destructive test; in the U.S., Advanced Auto Parts and probably quite a few other places will do a free battery test for you. Since this test consists, pretty much, of clamping the test load to the battery terminals and hitting a button (i.e., dead stupid to do) and a failed battery means they get to try and sell you a new one, it's appears to be a cost effective way to drum up business.

KBeck

 

Just keep in mind this little battery never sees more than ~30A load at boot up.
Those little terminals and the battery itself is not designed for one of those old school "Load Testers" that can pull
+200 Amps.
It may be abusive to load these little batteries with that much current.

In aircraft they test the battery's capacity.
A set current draw is applied to the battery for a set time. Sometimes it will be done at the one hour rate.
Example: 32 Ahr battery should put out 32 A for one hour.
If the battery produces 85% of its rated capacity it is good to go.

 

ted, I don't understand your test.
As soon as you turn on the car on to Ready, the DC to DC converter comes online and the LV bus is now at ~14.2 vdc.
If you had a clamp-on Ammeter you could tell what the draw is from just the headlights and measure the capacity that way. (You stop at 10 VDC doing this type of capacity testing.)

But the dang headlight turn themselves off, correct?

 

Get a battery tester and stop guessing

 

Which exact type of battery tester do you recommend?
There are:
Load
Capacity
The high tech internal resistance and other sulfation voo-doo ones.

 

This one works.
Amazon.com: OTC 3183 Digital Battery Tester: Automotive are:

 

ted, OK, that's not Ready. I'm with you now.

 

From the Toyota Repair Manual: