12v Battery Change Interval

My 12V battery is just over 7 years old, and I'm starting to consider replacing it. I bought a battery tender recently because the Prius is probably only going to get used about once a week, maybe less in the summer - it has been largely replaced by a Bolt EV (which is awesome). But now that it's not being driven on a daily basis, the 12V battery sometimes gets low by the time I need to use it again. Twice, it was only barely able to start the car - the vacuum fluorescent display for the MFD didn't turn on, and other things were flickering for a bit until the HV battery relay closed and the 12V system got better power.

Anyway, I decided to fill the battery with distilled water, charge it up, and see how it does. It'll probably still need to be replaced, but I'm going to drag it out as long as I can! No MPG loss, and I don't mind too much if the car doesn't start when it's parked at home, so it's not causing me any serious trouble yet.

Of particular note; I was able to start it once when the battery voltage was 9.98V (with no load; when I turned it on, it was valiantly trying to run the brake booster pump, so then the voltage would have been even lower when I pressed the power button). So it can go pretty far down and still survive. YMMV, though; a friend with a Plug-In Prius said his wouldn't start that low.

 

Filling an AGM battery with distilled water?

 

I'm driving a 2010 gen3 prius and still have the original battery. 120,000 miles, and I've never needed to jump start it...

 

My buddy just had to replace the 12v in his late 2nd gen Prius. He was having all kinds of weird stuff happen: random dash lights, occasional non-starts, etc. Sometimes it would seem just fine then it would go all wonky again. At my urging he had the 12v checked and sure enough, it needed replacement. Car has been 100% ever since.

 

I'l probably regret saying this, but my 2010 (bought July 2009) with 145,000 miles is still running the factory battery. Never run down, never jumped.

 

I know what you mean, tempting fate and all that.

I'm really tempting it though, I have a Prime on order.

 

no new cars in my future with a high school junior foaming at the mouth for college. Maybe he can take the Prius and I'll take the XC90.