What charges the battery?

Its a fair point. There are two different things that may be of interest.

One is SOC as michael mentions, ie how well is the battery being kept topped up. On a good lead acid one can pretty readily infer the SOC from its "resting" voltage, ie with no load and at least a few hours after its last charge. There has been historical evidence that the Prius doesn't always do a great job of this, which can contribute to reduced lifespan. I think the understanding is that the dc:dc converter puts out a lower voltage than a traditional alternator, so the battery charges more slowly. So particularly if you make a lot of short trips it may not have time to recover fully on each trip. I'm not sure if this is still believed true of the latest generation though.

The second is battery aging, whereby the capacity and maximum resting voltage degrades over time. On a traditional vehicle rozi is probably right, the best indicator of battery health is likely how well the battery responds to a load. A weaker battery will get pulled to a much lower voltage under the same load, even though its resting voltage may seem ok. The difference is traditional car batteries have a capacity ~10x that of the battery in the Prius, and routinely deliver current of 100s or even 1000 Amps during cranking compared to the 10 or less Amps the Prius battery has to put out to startup the computers. Resting voltage can still give you a decent idea of health, but can make you think the battery is in better shape than it is.

The challenge with measuring the Prius battery under load is the ACC load can easily be greater than the starting load, and can drain the battery fairly quickly giving you a moving target and stressing the battery unnecessarily. It would probably be fair to test it under the same load every time, perhaps headlights on, radio, interior and ventilation system off? But I would do as short a test as possible. A minute or so should have no impact, but I wouldn't put it in ACC mode with headlights and everything else on, then pop the hood, open up the test points, and then go puttering around in the garage for 20 minutes trying to find your volt meter

That's kind of what I liked about the cig lighter socket solution. Easy to run a quick test frequently, under controlled conditions. You could do this with a certain load on, and it would probably be consistent, but you will likely introduce some error due to voltage sag in the wiring by measuring under load compared to measuring it at the battery directly. With little to no load, there is little to no current flow, which means little to no voltage sag, which makes the cigarette lighter voltage more or less the same as the battery.

More important than the exact procedure you follow is probably just making sure you do the same thing every time (same load condition, same time span, same measurement point), and make it easy enough that you actually bother to do it Or you can do what I've done in my '05, wait until the original battery died and then put in an Optima and didn't worry about it too much

Rob

 

Rob and others thanks for this good info. I just bought a 5 pack ($20.58 w/shpg) of a different brand than linked to above so I can have one in other vehicles that do a lot of sitting around. Good piece of mind.

What Optima model did you get? Was that for a GenIII (ie would be the one for our PiP?)?
Google inquiry from some 'one' said " Optima's do not respond very well to being discharged and need to be kept full charged." Do our Gen III PiP's re charge sloooowly (or relatively quickly from Traction batt. transfer). If slowly, would many short trips not let an Optima/AGM battery get a full charge?

I'd assume with hour commutes each way, rate of charge is a total non issue. But curious, none the less.

 

Optima makes several versions of their batteries. The above quote is accurate for the starting version. It's also accurate for the battery that comes in the Prius. You'd want the 'true deep cycle' version, which would do much better being deeply discharged.

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Pretty much all lead batteries fare best when maintained at a full charge. The solar and EV folks have definitely found this to be true of deep cycles too.

The one that I used (and I believe most folks use) is from the Optima Yellow top series. It is a deep cycle battery, compared to the red top which is a starter battery. I'm not sure how much difference there is in the maintenance voltage vs. lifespan, but its just a much better battery all around and should last longer under pretty much any circumstances. Being a deep cycle it is also much more tolerant of being run down by leaving lights on etc, which is fairly easy to do in the Prius due to the smaller capacity of the battery. Doing that just once or twice on the OEM battery can really kill its lifespan, which is probably a big part of why so many folks have issues with the OEMs.

I bought the kit from the site linked below (as I believe many others have) which came with the necessary cables / clamps to convert the Prius from a Japanese terminal to a more standard terminal type to work with the Optima. It looks like they have a Gen 3 kit now as well.

Installation Kit for Optima battery in Prius

Rob

 

Looks like they also offer one now that is a direct replacement without having to change the terminals:
Optima DS46B24R Optima battery direct fit replacement for Prius 2004 + , Prius V & Plug-in Prius with Instructions

 

When I wrote " You'd want the 'true deep cycle' version, which would do much better being deeply discharged." I meant "much better than a non-deep-cycle battery" not "much better than if it were fully charged."

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