that's why I said - I'll have to figure out how to connect jumpers to cables so that they DO NOT fall off during all the manipulations. not that simple, actually, as for regular jumpers, there's not enough room inside the battery compartment to hook them up. small jumpers may not have enough grip on the jaws to stay on. may end up removing insulation somewhere on the cables for hook up, and then covering it with electrician tape.

 

I can assure you the car won't start with a dead or mostly dead 12v battery. Voice of experience.

Yesterday evening I went to start the car and the battery was pretty low. The car tried to come on, but after a point the info display in the gauge cluster start going on and off and saying to check the VSC system when it would come on. Some of the other lights flashed too.

I put it on low power charge on the charger for a couple of hours and the car started after that, then I drove the car to let the car's onboard system charge the 12 volt some more.

I'm replacing it with an Optima tomorrow. I'll report back how it goes. I don't anticipate too much difficulty.

 

I forget if the 12 volt battery in the trunk has side or top terminals. I was thinking cleaning the terminals might help the low voltage if they are on top. Another way to read if the battery is low, is to turn on the headlights for 10 to 15 minutes (engine off), then measure the dc voltage across the battery while they are still on.

The new Optima will sure cure your battery problems.

 

FYI, from dealnews.com:

For in-store pickup only and with limited availability, Advance Auto Parts offers the Optima YellowTop 12-volt AGM Car Battery for Toyota Prius, model no. DS46B24R, for $124.99 via the directions below. That's $1 under our mention from six weeks ago (which included free shipping) and the lowest total price we could find by $40. Sales tax is added where applicable. Rebate expires May 31. To get this deal:

Add the Optima YellowTop AGM Car Battery to your cart for $199.99
apply coupon code "VISA" to cut it to $149.99
redeem this $25 rebate for a net price of $124.99

Brand: Optima
Model Number: DS46B24R

 

The 2012 Owners Manual says the following about the 12V battery in the specifications:

12.6 -12.8 V Fully charged
12.2 -12.4 V Half charged
11.8 -12.0 V Discharged
(*: Voltage checked 20 minutes after the engine and all the lights are turned off, at 68 deg. F)

It would seem to me if you were consistently getting Half Charged range when you test with a multimeter, I would start to get concerned. If you then check voltage when the car is running and hopefully charging, I would expect it to be up around 14-14.5 volts. If it does not come up to that, then you can suspect the charging system.

Another way to evaluate the state of a lead acid battery if it has removable cell caps is to measure the specific gravity with a battery hydrometer. They are relatively cheap. If you have a suspect battery, you can confirm it by testing each cell to see how uniform they are. Often you will find one cell is lower than the rest. The only trick of this test is making sure you don't get any acid on your car, hands, or clothes!

On buying a replacement, cranking amps or CCA is irrelevant as this battery is not used for starting. Buying a deep cycle seems to make a bit of sense as there does seem to be a drain on these batteries when the car is not running. Generally you can tell the capacity of a battery for deep cycle by the amp-hour rating, or the reserve minutes. More is better.

Consumer Reports I recall used to test Optima and recommend them, but in the latest tests they do not seem to be included. DieHard Gold and Platinum seems to test near the top in most sizes.

Does the original battery have a vent connection? Venting of the hydrogen and oxygen would seem to be a bit of a concern as it is located in the trunk rather than under the hood where there is lots of ventilation.

 

After a little more research, I believe the OEM Toyota 12 volt battery is a gel type. If so, there will be no way to test the electrolyte specific gravity. I found this graph that shows a bit more accurately what battery condition is based on voltage. It says it is based on no load and a 3 hour rest after charging. Without disconnecting the battery, you can't assure no load, but perhaps the idle drain is low enough that the values still apply.

Expected life of a gel type should be good, unless it has been overcharged. They are more sensitive to charging voltage. That is probably why Toyota recommends no more than 5 amps for charging.

 

The 12 volt battery is the power source for everything in the car that uses 12 volts. Most important for starting would be that it powers the ECU. The ECU in turn connects the traction battery to start the engine, sometimes rolling the car to do so, only if you put it in drive or reverse quick enough.

Batteries are usually not usable if they are 10% below their rated voltage. That would be 10.8 volts for a 12 volt battery. If you can't get your car to start due to low illuminated instrument panel. Make sure everything is off, including closing the doors so the dome and map lights don't come on. Let the car sit for 3 to 5 minutes. Sometimes this makes enough difference to get the car started.

I think it was haroldo that long time back said, be sure the clock is black or not illuminated when you get out of the car. To turn if off, press the power button till the clock goes dark. I do this with my '12 TCH but also sometimes see the mid reminding me to turn the power off.

I understand the deep cycle 12 volt battery in the trunk is vented down and outside the trunk.

 

Sorry to be waking this thread up, but I have been thinking I could connect the backup power cables to the points under the hood before changing out the battery. Does any one have any concerns with this approach, aside from having to be careful with the positive cable end in the trunk not touching any conducting material in between the battery change?

 

Should be fine. Just check that when the car is turned off that there is still 12 volts at the points you choose under the hood. In any case I think all the "problems" caused by disconnecting the battery is just internet mythology.

 

My backup power source did accidentally disconnect for a few seconds, and that left my computer "dumb." It was a number of complete start-up cycles with cool down before the engine light reset itself, and more than a tank of gas before fuel economy returned from normal (it never got real bad, though). All radio settings had to be reset, and a few other settings in the car reverted to factory default. Not a myth in my experience!

-- Alan