Hybrid battery temperature and fan speed (Hybrid Assistant)

Hi all,

I have a Lexus UX which is gen4 based and I wondered if anyone had any issues with their hybrid battery overheating on long distance trips and long regens, and the fan not increasing in speed appropriately?

During a recent long trip there were a lot of long regen periods which meant that the battery was increasing in temperature quicker than normal. It normally sits around 38 degC, but the battery eventually hit 45 degC which caused it to start limiting charge and discharge power. The hybrid battery fan speed did not go any higher than 3 on hybrid assistant, and could not be heard.

Using Dr Prius I was able to force the hybrid battery fan to go faster (not supported on Hybrid Assistant annoyingly) and this was able to cool the battery quickly, and was very audible. It seems that the car's default fan control does not appear to work properly in the Lexus UX, and I wondered if any prius gen4 owners had this same issue?

I'm looking to report this to Lexus to get the software updated but I need to know if other people have observed the same thing.

Thanks

Edit: Attached the Hybrid Assistant graphs for the hybrid battery charge/discharge and temperature.

 

I have searched the forums but I didn't find anything relevant.

Dr Prius doesn't override it properly, it speeds up and slows down as the car overrides its commands. I have a fast OBD adapter and Dr Prius set to super fast update speed so this might help, but it's not perfect.

I'm surprised that they didn't prioritise battery health over a little fan noise, as I would prefer this if I'm honest. This is not to say the battery isn't protected, 45 degC is the highest it allows it to go which is plenty safe, but it does restrict performance at that point. I did find this odd behaviour as my CT200h did increase its fan speed when this condition occurs.

I will probably raise this with Lexus when I go in for a service in a few months. It doesn't affect the car often, just on long trips or where there is a lot of regen braking which heats up the battery.

 

It won't be to save energy. The fan uses very little power. I suspect it's mainly noise given that in 99% of driving situations the battery isn't going to get up to that kind of temperature. I've only encountered it a handful of times and it's always been from long journeys with lots of hills. A higher fan speed would allow for more heat to be moved away to stop this from happening at the expense of more noise.

45 degC isn't going to harm the battery and that's its max limit as it becomes very inefficient above this temperature anyway (look at Ni-MH temperature specs). However, it ceases being a proper hybrid when this happens as it's not capable of supplying enough power other than stop-start of the engine. It won't provide nearly enough traction power to move the car.

I'd argue that the design of the battery is fine, but the software control for temperature isn't set up properly. It would only need to run at full fan speed if it reached 45 degC and below that it can ramp down quite significantly. Not a lot of headroom is needed, just a couple of degrees. 38-40 degC is currently where it normally sits as you can see on my graph in the first post, but it doesn't have to be this warm. 25 degC allows max charge/discharge, and so anywhere between 25-35 degC would be a good place to let it sit.

I wish we had a more direct route to technicians who could get this kind of update out there, or allowed us to update our own profiles.

 

It's not fine when the battery overheats when doing its job. When you regen you put a lot of energy into the battery and when doing it repeatedly this generates a lot of heat in the battery which doesn't dissipate very quickly unless the fan is running strongly. I found that after 3 or 4 long regens in a short time (5-10 mins?) from a high ish speed (60mph) the battery was in a state of being too hot. That 6 degC gap between the 'normal' operating temperature and overheating isn't a lot. It would then take up to 20-30 mins for the temperature to fall back to 38 degC when cruising and the hybrid battery isn't used much.

There is no need for the battery to be held at 38 degC when it can be fully functional at a lower temperature which is achieveable with an increase in fan speed. I get the need to go for silence but I would prefer that the battery be able to perform its job and put up with a little fan noise in return. Better still, they could route an a/c duct into the battery fan duct so that it receives better cooling.

In any case, I don't think it's operating correctly. Whilst I can override it with Dr Prius (somewhat badly) I shouldn't need to. I got better performance after doing this which was helpful when I was doing a lot of fast to slow to fast speed transitions on my route.

 

Their hardware design is adequate, their software is not. I'm not confident in their programming of the fan when the battery is allowed to overheat and the fan is not even close to its maximum speed. When I can forcibly set it to a higher speed with a 3rd party app to provide more cooling and then it performs better, their software is clearly wrong.