12V battery goes down to 9.5 after car switched off

Doesn't hold the voltage because its become sulphated. I bet you have jump started it many times. First time a G2 needs a jump start I would get a new battery.

The car has been throwing a bad battery alert your just not paying attention. When you boot up the car watch the dash you should see a red triangle flash momentarily. That means the 12 volt battery is bad. Its pretty noticeable as that RTOD is the only icon that does not illuminate on the boot up bulb check. That's all you get before it strands you.

 

Welcome. Your 12V battery is dying. If it's original, it's 10-11 years old. That's way longer than the average car battery lasts. Better replace it before it strands you.

 

Or you try to jump start the car and damage something. Owners manual says battery is dead at 11.5-11.9 volts. You are way below that.

 

The car charges the 12 volt battery at a very slow rate. How long are your typical trips?

What the owner's manual says about "11.5 to 11.9 V" (page 466) is that, if the temperature is 68 °F, the battery is discharged at that point. It doesn't say dead. The first thing to try with a discharged battery is to fully charge it. But to fully charge the battery from that condition would require 15 hours or so at the recommended rate of charge (or 10 to 12 hours of leaving the car continuously in READY, if you have a safe place for doing that).

If a good charge of ample duration can't bring the battery back up to 12.6 to 12.8 (68 °F, 20 minutes after car and lights turned off), that will be a powerful argument for replacing the battery.

If the battery is many years old, you may prefer to just replace it without bothering to experiment with charging it.

If it has been sitting at this sub-discharged level of charge for long periods already, it is probably a good candidate for replacement; sulfate layers build up on the plates under those conditions, and are notoriously hard to reverse.

If you get a trusted, fully charged, battery in there and you notice its charge slowly declining from day to day, it might be that your daily use of the car isn't long enough to replace the battery's charge, at the fairly slow charge rate that is used. If that's the case, you might want to look for a maintainer charger that you can plug in (or a solar one to let the sun shine on) during some of the time you're not using the car.

 

Well, if you got 10+ years from the OEM, that's hard or impossible to beat with other makes. I always recommend the OEM battery for the Prius. You might find another one that might last that long, but will it have an 84 month warranty?

 

Nothing has beat the life of the original battery. I got a solid 7 years out of mine. I recommend spend the extra money and get the factory battery.

Optima is ok they cost alot more now than i spent 4 years ago. My first Optima lasted 2 years 11 months a 3 weeks. I was lucky I went back to the store thinking maybe they would prorate me into another one but the manager was cool and gave me a brand new one for free.
I'm always in there. But was not pleased about it. I thought should have bought the denso battery. But stuff happens.

 

Am I getting a hint the OP is not in the USA?

If so, that makes some of this a bit tougher. A new reliable brand of 12v battery in US dollars will cost around $200 at the dealership (I think the MSRP is now $239, but coupons, etc. can often bring that price down and some dealerships will discount parts.)

A Duracell brand replacement isn't a horrible choice (I would go with the Toyota brand though), but the price is about the same. Batteries Bulbs+ stores sell them and you can "save" 10 percent if you order online and pickup in store. Some (not many) of these locations will also install for free.

Then you get to the other batteries, such as Edthefox5 mentioned.....Personally, again, I would stick with the Toyota brand.

And, I would not bother trying to charge, test or otherwise mess with your current battery. Replace it. Odds are good all will be well. If you find it is quickly losing charge, then you need to track down that problem, but irregardless, you need another 12v

Just be certain, if you do it yourself, that you get the CORRECT battery. There are two different sizes, depending on whether you have the Smart Key System or not.....

 

Is it the same manufacturer that makes the battery these days?

 

Checked sometime last year and it was still YUSA -- of course that may have changed.

Opps, that looked wrong to me -- I meant YUASA...

 

Click his Location link, the ensuing map shows him near London.

My sympathies.

There is plenty of stolen CAT activity from the London area written up elsewhere on this forum. Far far too much of that going on.

 

Hey, I am a genuine Washingtonian (born and raised in Hollywood)....did not know about that "click thing." Thanks...it must be one of those newfangled internet features....

So, is BritPrius still around? And, no, I do not know how to do that blue color "calling BritPrius thing" But he seems the best on British batteries.

 

Yes, he is still around. Hasn't posted since February, but was logged in to read here over the weekend.

His most recent battery-related post is here (for contact reasons), there have been plenty more: Re-hydrating the battery modules. | Page 16 | PriusChat