2006 Prius Traction Battery Acting Strange

It may be a sign that the traction battery is losing capacity. Continue to monitor it. Rapid state-of-charge changes and the HV cooling fan running at high speed (vent is in the rear, passenger side seat bolster) are common signs of a declining traction battery.

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I do everything I can to keep cars away from the service department. I've been managing so far with Youtube how-tos and online parts.

 

You can probably keep driving on it, especially if the cooling fan isn't screaming. Has your fuel economy dropped?

The HV battery life seems to be pretty inconsistent. Some go 300+k. Others quit before 75k.

 

You should be hoping for a battery replacement. You have a 10 year/150k miles warranty in CA. You WANT a new replacement.

 

Avi's Advanced Automotive is the only Prius independent in your area I see referenced.

If you do hear the battery cooling fan running at all, that is not a good sign. Avoid parking in the hot sun, use window reflectors and the rear cargo area cover. Use A/C to keep the battery air cool. Heat is the enemy of battery life, and it is a statistic that batteries from hot climates generally don't last as long.

 

So, are you saying some of the A/C is diverted to cool the traction battery? Or, just that keeping the cabin cooler helps the battery?

 

I don't think that is entirely true. That fan runs at different speeds. Sometimes it's running at low speed silently - and in very hot weather without A/C in use, it can be heard running that bit louder - depending entirely on the surrounding temperature and how much the traction battery is being utilised or charged.

When my UK Gen 2 Prius (T Spirit) is very quiet at standstill, usually after I have arrived home and parked after my 18 mile "spirited" commute home, I can often hear a faint hum coming from behind the dashboard, and an equally unobtrusive whirr coming from the traction battery fan. I have exceptionally good hearing (for my age), and so can pick this up crystal clear at that moment. I don't hear it when in stopped traffic. Considering that the last (less than) quarter mile home is up a steep hill in 27c summer heat, this alone might account for the fan running quietly - but run it does; and indeed, it's supposed to.

You are absolutely correct about using the A/C on hot days to do this. This is something I have always done since I bought my Prius 13 months ago.
I've also noticed that chilled A/C air cooling the HV battery results in the Prius running for longer (and stronger) during those few moments when I'm running on the battery only. So running the A/C is always a good thing for the HV battery, 1 - 2mpg loss be damned.

These days, I tend to pulse and glide a lot, so I no longer run up to a mile on battery power only, like I used to last year. This is also to preserve the life of the battery. It only gets used mostly in pockets of crawling traffic, which thankfully are quite minimal.

It is also worth noting that during summer last year, I achieved my "66.0mpg @ 605 miles" personal record while running the A/C on minimal settings during my commute home in the afternoons. Ditto with my "66.5mpg @ 628 miles" this year. In the mornings, at anything from 12c to 18c, I tend to use just the cabin fan for ventilation. If it gets hot and / or humid, the A/C comes on.

To recap, that HV cooling fan is always running once the Prius gets underway (charging and discharging of the HV battery). I believe it was designed to do that. Within optimal surrounding (read cabin) temperatures, it will run quietly. In very high temperatures (or with a battery overheat / failure in progress), it will likely run at full tilt. Keep the cabin cool and prolong the life of your HV "Traction" battery

p.s. If I got anything wrong here, someone please correct me - I'm here to learn just like everyone else


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The latter: keeping the cabin cooler helps the battery.

 

The HV cooling fan sucks air directly from the cabin space to cool the HV battery. Ergo, the temperature of the air in the cabin area is what goes to cool the battery. This is more critical on some hot days when the temperature in the cabin can easily climb to 40c+ in relevant locations.
Reduce the temperature in the cabin by winding down the windows initially once you get moving, close the windows then crank up that A/C and keep the cabin temp down. Good for the driver, passengers and THAT HV battery


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Hi Chris. Your HV battery is showing some signs of aging, as are many others approaching 10 years old. Take heart though, my 2005 has had very similar symptoms to what you're describing for nearly 3 years now, and is still going fine. I'm currently at about 135k miles, and this issue has been noticeable to some extent for at least 40k miles now. Also, I have discovered that there several things that you can do to mitigate the issue and keep your Prius running well.

Firstly, it's important to understand that it's very much normal (all Prius) to use the HV battery very heavily in the first few minutes. It's a combination of several factors (including high initial DC-DC converter load and also aircon compressor load, both of which that feed off the traction battery) but mostly due to the fact that the car tries to use electric propulsion in preference to engine power until the cats reach operating temperature.

When your battery is new you may not notice this large amount of battery usage in the first few minutes because the SOC display is decidedly non linear, with 6th (and 7th) bars on the display being noticeably broader than the others (see attachment). Once you drop to the 5th bar or lower however, the amount of energy corresponding to each bar is much less, and this definitely gives the perception of much more rapid reduction in SOC (can almost feel like the battery is "collapsing" sometimes).

Now lets look at ways to mitigate the problem.

1. Make sure that your battery cooling is in good health. This is primarily to prevent/limit further battery degradation. Check the rear vents and battery cooling fan for dust clogging and keep these as clean as possible. It's a fairly easy job and you'll find a lot of "how to" info here if you search on it. I include this as a regular part of my maintenance now.

2. Try to prevent your SOC dropping too much at start up. The simple method which helps a lot is just to let the engine warm for about 40 to 60 seconds any time you're doing a cold start. Leave the transmission in "P" (and give the accelerator a tap to kick off the engine if needed) and just let it sit there idling and charging the battery for a minute or so. This has two positive effects, firstly it slightly pre-charges the battery, but most importantly it warms the cats so that the car uses engine power much sooner when you start driving. Trust me that this makes a huge improvement to the problem!

(Note: you may not need to do this if your starting SOC is good and your first minute or two of driving is slow on level terrain. If you have hills or need rapid acceleration however then it's much more important. Just watch for when you are and aren't losing SOC at startup and you'll soon get a feeling for when this is or isn't required).

3. Try to reduce the aircon compressor load in hot weather by delaying a/c usage for a minute or so and using the windows down to flush out the hot air first. Don't forego aircon usage altogether though, as it's vital to keep your cabin cool in order to keep the HV battery cool (as it ducts air from within the cabin space).

4. Try to park your prius with a reasonable SOC level (eg 6 bars) whenever possible. Try not to EV glide too aggressively near the end of a trip, and it may also help to turn off the aircon (and just use fan) for the last minute or two, particularly if that involves slow manoeuvring (like parking) where you're only running on EV.

5. In hot weather try not to park in direct sun whenever possible. When this is not possible, then definitely don't do it when the battery is at full 8 bar SOC. (The battery can easily overheat in this state). Literally just leave the prius ready and aircon on full blast for a minute or so until it drops to 7 bars if you ever find yourself in this situation.

6. Try to avoid getting full 8 bars SOC when possible. It may help to use "B" mode on larger descents if this is an issue. When you can't avoid hitting full 8 bars SOC, then be aware that the SOC may drop unexpectedly rapidly as it comes back towards 5 or 6 bars (for example you might get a very sudden 6 down to 3 bar drop if your running EV). This seems a fairly common issue with older batteries and may be due to the battery management overestimating the amount of charge available under this scenario. I try not to allow it to use EV (full electric) too much after it has dropped from 8 back to about 6 bars.

BTW. By following these tips I'm still getting really good MPG despite this issue.

 

Yes, certainly. My fan motor voltage reads about 7-9 volts when the batteries temp sensors are less than 50 degrees C. My hearing is good, but I don't hear that speed of fan unless I put my ear close to the vent. It's the fan on maximum that signals the overheating condition, which is what most people are going to hear, given the normal ambient sounds.

 

That's good. But just remember that the dealers never check the cooling fan or ducts for clogging, so do make sure you do check those out. It's not a difficult job, and trust me heat is the enemies of these batteries if that rear vent cooling system isn't in good health. Overheating my battery due to a clogged fan was originally what caused my battery get into the same state as yours now appears to be.

Also, loss of battery capacity is somewhat normal with aging and is NOT something that the dealer would pick up on or even test for. As long as the battery doesn't develop other problems, like shorted cells or a high resistance cells, then it can keep going with reduced capacity without triggering any errors. The dealers never test for amp-hours and they honestly just wouldn't have clue about loss of capacity.

 

Not so! Cold will cause the battery ECU to limit the current through the modules, until they warm up a bit, which happens pretty quickly. There will be a limit to acceleration initially as a result. But there is no harm done. Excess heat causes electrolyte loss, which is irreversible. The dealership techs are not the last word, at least when it comes to the Prius.

 

Where on the rear seat is the vent cover? Alan