Battery failure due to years in use or mileage?

For the Fob and 12 volt batteries it is age, not miles.

I am not sure when we will have enough statistics to judge the HV Battery.

(Heat and heavy load seem to be contributors to low life)

 

That sums it up well.

 

Agreed, not enough data at this time.

 

Although we have a lot anecdotal data about Prius battery failure, the difficulty is that it's self reported and we don't really know the underlying numbers from which those failures are drawn.

In general though, we do know the main factors that influence Nickle based battery lifetimes are,
1. Temperature.
2. Cycle number and depth.

As you can see, these factors relate only indirectly to both age and mileage, with climate terrain and driving style being just as important.

One reason why Prius with relatively high mileage for their age tend to have low battery failures (at least according to our anecdotal data) could simply be they do mostly highway miles and (or) have fewer stop/start cycles. So it may really be that age is only an apparent factor here, with usage factors and driving style being the more direct reasons for the trend.

 

By start/stop cycling I'm referring to cold or cool starts. The reason that these effect the battery is that cool or cold starts are very often associated with a relatively deep battery discharge.

For many owners, the most common time that they'll see a low (eg 3 bars or less) state of charge is in the first few minutes of driving. There are well known reasons for why this often occurs. If you're not seeing this then that's a good thing, but that relates to your specific route / terrain / driving style etc.

Warm stop/start cycles have never been a big issue for any aspect of vehicle longevity.

 

Here is the poll we have going on...please chime in:
Hybrid Battery Survey-Gen2 Prius 2004-2009 | PriusChat​

The battery is designed to last life of car. It seems the few failures tend to happen under warranty period, so not too many owners face the need to fix on their own, hence the car retains value well. I do not think a new model 100k Prius would be going for half price on Craig's list without some other problems, or yes great deal.

 

No, those issues with lead acid batteries don't really apply to NiMH cells like that of the Prius HV battery (provided of course you don't store it so long that it gets fully drained). NiMH cells can be stored for quite a long time with a relative low state of charge, and the worst that happens to them is that they take a few cycles afters you put them back into service before they recover their full capacity.

Also, even when the displayed SOC is empty (on the energy monitor display), the actual state of charge is still about 40%. The only thing I'd recommend is letting the engine idle for a minute or so when cold starting with a fairly low state of charge, so that it doesn't try to thump the battery too hard when you first drive off. The main risk of operating with a very low SOC is the possibility of having some poorly balanced modules, in which case you could get an individual cell become fully discharged or even reversed biased under heavy loading.

 

The reason that the battery often gets hammered a bit at start up is that several things often come together to create high demands on the HV battery in the first few minutes after starting.

First and most important of these is that it tries to minimize the loading on the engine as much as possible until the cats are up to temperature. This means that it usually tries to use as much electric power as possible for propulsion in the first minute or so (even with the engine running!).

Secondly, our 12V batteries (especially old ones) often leak quite a bit of charge while they're sitting (with the Prius off). So when first started, the DC-DC converter can also want to draw quite a bit of HV power to keep the 12V system up and replenish the auxiliary battery.

Finally of course there's summer when the car is often very hot when you first get in to start. This causes the aircon (which also runs of the HV battery) to have maximum demand at this time.

 

The Prius tries to protect the battery as much as possible from anything bad happening, but as the pack ages and module capacities decrease and diverge slightly it becomes more difficult for the battery management to do this.

Some of us with older packs are noticing that we sometimes get undershoot and overshoot events. Where for example the ECU is overestimating the SOC and then suddenly realizes that it actually has less charge than it thought, and you get the SOC undershoot (rapid decline in SOC) to undesirably low levels. Not so often noticed (due to the way the SOC display is set up), but I suspect a similar overshoot events also happen where the the ECU underestimates SOC and lets the charge go a bit higher than it should. In other applications, like power tools for example, I've seen where packs can have only a relatively modest amount of degradation, but enough to result in the charger not recognizing when they're fully charged, and the charger will kill them completely in very short time (by overcharging). Fortunately the Prius charging system is smarter and a lot less brutal than that of a typical NiMH/NiCd power tool.

 

I know this is an old thread, but since I have a question related to it, I will post it here. I know there may not be an answer to this, but I was wondering if a 2008 with 50,247 miles might be potentially a bad thing rather than a good thing, because I have read that the hybrid battery likes to be driven rather than sit? I am guessing it isn't nearly that simple after reading this thread. It sounds like trying to judge the shape the battery is in for a car like this on the market might be impossible, depending on why the car has low mileage (sitting? being driven consistently, just not very far? etc), what kind of climate it has been in (temperature, hills?), how it has been driven, etc. I am looking to replace my 2007 Touring model, and this car is a loaded, silver, 2008 Touring edition.

 

I have a 2006 with only 48k miles and it has had two module failures within the pack in the past two years. The first was replaced by the original owner and I bought well knowing that I would be replacing the pack. It has had religious grid charging and conditioning after the second failure and has been running much better so I have put off replacing the pack with a new one for time being.

From my experience, sitting for long periods unused and in a warm climate aren't good. You may fair better up in cooler Illinois (93f here today!).

 

Yes, I think it seems a bit risky, rather than like with other cars when low miles is usually a good thing. It also concerns me a bit about cars that sit at the dealership for a few months. I wonder how bad that is for a Prius? I looked at a 2010 earlier this week that had rust in the rotors from sitting on the lot.

 

Prii work best when they're used on a fairly regular basis. If they sit for over a month without precautions being taken (like 12v battery lead being disconnected), then batteries can suffer. The 12v battery powers accessories that can stay on even when the car is off, such as the smart key system. If the 12v battery starts to drain, it is recharged by the hybrid traction battery. On top of that, the hybrid traction battery will naturally have a tendency to self-discharge. If the car is left long enough that the 12v battery drains the hybrid battery, then that can cause permanent problems with the hybrid battery charge capacity and age it prematurely.

 

That's really interesting. Thanks for the info. Definitely cautionary in buying a used Prius that's been on a lot for a bit.

 

I would say we've had very few 2008/2009 batt fails so far, and I am wondering if Toyota got some better batt life after 2007.

Gen2 has the generic issue if you run out of gas, the computer allows you to totally discharge the HV battery if you keep trying to get as far as you can to a gas station. That's one prob for Gen2's that was fixed in Gen3 by not allowing complete discharge. So if buying a used Gen2 you wouldn't know if the prior user did that.

Toyota is tight lipped on batt fail data, so we got nothing on it. Consumer Reports did one valuable survey and that's all we got.