Checking the 12v battery status on a 2010 model III

My Scangauge II shows 13.8V most of the time, confirmed with several different voltmeters.
Deep discharge batteries should be floated at about 13.8V, but it should be temperature compensated (it isn't in the Prius).
AGM batteries are slightly different, and slight differences in battery chemistry (we're talking of lead acid batteries here, but I'm also throwing in lead calcium types and "pure lead" types) can affect the float voltage a bit. Note that most cars don't operate at the float voltage because it's not high enough to ensure a good charge level. The reason is most car trips don't last long enough. Float levels are for "non-use" or non-discharge for at least 12 hours.

As the Prius uses 13.8V the Prius 12V battery is often operated at lower than "full charge". Some contend this causes it to sulfate. I doubt this is a problem but it's possible. In my experience I haven't seen a sulfation problem until the battery gets well discharged and left with no charging for at least several days. I'm talking battery voltages of under 12V here.
Starting batteries can be floated at 14.0 or so but they will slowly loose water at that voltage. If you can add water it's not a problem, but a sealed battery will have a shorter life in this service.

I -suspect- this is why the Prius battery dies an early death. It's sealed meaning you can't add water and yet can loose water (in vapour and as H2 and O2) through the vent tube. My experience is after 3-4 years -any- battery that doesn't have recombination devices inside will require water. The Prius battery doesn't have recombination devices.

Now, the anti-Prius (2001 Nissan Pathfinder LE AWD) had a sealed battery that lost water (it held the system voltage at 14.0V). After three years it began to exhibit symptoms it was about to quit. I pried open the caps and added distilled water. It fixed the problem (the plates were about to be exposed to air). Did the same again two years later. When I traded it for Pearl the original battery was still in very good shape after 7 years of use!
This -may- be one way of extending the life of the Prius battery. I'll have a look this summer to see if it's possible as Pearl is now three.

 

Post deleted, bad information.

 

I put my car in storage on 12/4/09 and got it out of storage on 3/24/10. I did start it twice over this span of time. I did not have any problems starting it those two times and I did not have any problems when I took it out of storage a couple of days ago. It seems strange that some can go for months and others just for days. I guess I am lucky to get one that can go more than days.

 

It has been one of the biggest mystry of the Prius 3 and no one seem to have an answer for it as to whats causing some battery to die so early and some can last a long time!

I would like to find out myself since I know soon I will take my car and leave it at the airport for some time.

 

So, can we have a few others report in with the voltage they are seeing from their SGII on a 2010 Prius while in Ready mode? I'm wondering if mine being at 14.6 - 14.7 is a sign something isn't right...

 

I noticed the same when I plugged in my SGII; 14.6 to 14.8 volts when driving. I thought this was a bit high so talked with techs at two dealerships who told me this is normal.

My Prius II was built Jan/10. Could there have been voltage spec changes at the factory, and could those getting the lower voltages be the same ones who later experience dead batteries?

 

Not sure of the build date but I took delivery on 7/8/09 so I'm sure mine was built well before Author's.

 

You are correct, my previous post (now deleted) was wrong . I just looked at mine again and it is 14.5 to 14.7. My build date is July 09.

Edit: Just went out and checked with my meter to be sure our ScanGages were not wrong, meter reads 14.7V also.