Repeated dead 12v battery

You need to have someone properly connect a clamp on or series DC amp meter to the battery circuit and find out what is happening when it's off. Be careful, this can be dangerous. It should be 20 to 50 milliamps (thousands of an amp) See links 1 and 2. You can also check the battery voltage on the MFD after it's off for the night and then again in the morning before it's started. See third link.

Understand that an occasional discharge because of lights left on followed by short commutes may not fully charge the battery causing sulfation of the cells. Try watching the battery voltage on the MFD while driving during the day without the headlights and make sure it is getting to a full charge within 10 or 15 minutes by observing the voltage drop from 14.8-14.7 vdc to around 13.7-13.6 vdc. Finally, (or maybe first) I would consider finding a competent auto electric shop to check things out.

Link 1 & 2
Unsolved Battery Drain (with detailed troubleshooting) | PriusChat
Leaving a Toyota Prius Undriven

Link 3
http://priusdiy.com/tutorials/MFD/12vbatterycheck.htmlhttp://priusdiy.com/tutorials/MFD/12vbatterycheck.html

 

Keep in mind Prius charges the 12v battery very slowly, so if you receive it not fully charged, that could be a problem. Best practice is probably to charge up before you install...I skipped that step the last time, but I won't miss it the next time I replace it.

 

You have an electrical short that is discharging the vehicle or a constant load that is discharging the battery.
You have to investigate and solve the root cause before replacing another 3 more batteries......

 

OR the dealer he has been going to really IS totally incompetent and it isn't being charged properly.
I suggest finding a different dealer or qualified shop AND contacting Toyota to twist somebody's tail too.

 

Based on the information you provided, the first battery died of old age, and the second one was a dud. A bad cell occurring so soon can't be blamed on the car, or the dealer or Toyota.

The battery should be under warranty, but personally, I would have chosen something else.

 

If you want to check for parasitic drain, have a look at posts 26~28 here:

2010 - Sudden 12V Battery Failure | Page 2 | PriusChat

Basically, I disconnected the negative battery cable clamp, connected an alligator clamp lead to it and ran it out through over the hatch sill. Then did the same thing with a second lead, connected to the negative post. Both leads had bare wire at their far ends, separated.

Then I closed the hatch (they are barely pinched, still movable), and just let things sit, without touching any doors, and my key fob away in the house for good measure. Hooked up a multimeter set to amperage. This puts the meter in the circuit, showing the flow of current when the car's at rest.

My meter was auto-ranging, but if you need to set range start with the larger setting, to avoid possible overload.

Readings I got were around 16 milliamps, with intermittent spikes to around 40. I believe anything up to 50 is considered acceptable.

 

This is a very good hint but very technically for most Prius owners.
With this instructions he better go to a mechanic with a good quality Multimeter for findings for a few bucks.

 

I can just imagine shorting the leads across the battery terminals. Ouch!

 

At this conditions, you are just closing the electrical circuit with (assumed) very low parasitic load. The same negative post to chassis ground.
If it sparks, then "Huston we got a problem"................

 

Based on my reading of Harrinj's sad story, he is on his 4th (Fourth) battery in the last 9+ months. And Toyota was involved in evaluating "the problem" with Battery #2 and Battery #4.

 

Either find a different shop to take your car to, convince them to do some better diagnostics, or take it upon yourself. A friend of mine had problems with his PiP discharging the 12V battery while charging the traction battery, and finally took a bunch of measurements himself to show to the dealer. (12V Battery near-dead after charging HV battery :-()

If you elect to do it yourself, uou'll probably want to measure battery current and voltage when off, and at least voltage while on. When off, you want to make sure it's not draining too much. When on, you want to make sure that it's charging correctly. It's also possible that it's only an intermittent problem - i.e. every few weeks, a computer gets goofy and draws tons of power while off, draining (and killing) the battery - but because the battery dies, the computer gets reset in the process. So it might be difficult to track down for sure.