Battery heater not working?

The weather's been sitting around -25°C for a while here, so I was curious to see what the battery heater was doing. In the last week, I've not seen the engine stop, and it's used only a few km of EV range.

I'd disabled the charging schedule and I've been leaving it plugged in every night, despite the battery always being nearly full, with the understanding that this would be trying to hold the battery above around 0-5°C for up to 3 days after being plugged in, leaving it able to actually use the battery.

I nipped out to plug Techstream in to have a look at the status, and it turns out the battery temperature is reading -23°C.

Surely it shouldn't be at ambient sub-freezing temperature, if it's been plugged in for 18 hours? That would explain why it's been so incredibly reluctant to use any charge.

I used a test function in Techstream to confirm that the heater could be turned on and start heating - heater temperature went up to +20°C rapidly and the battery temperature started to shift, very slowly.

New Car Features says:

And a diagram I can't upload showing the heater being activated in bursts to keep the battery warm, starting from the moment of plugging in. Doesn't seem to be happening though.

Any thoughts?

Could it be that -25°C is just too cold - it's decided it's not going to get any good use out of the EV battery anyway, so there's no point trying to keep it above 0°C? Not found anything about a minimum temperature in the docs though.

 

Well, that's my thought - it's too inefficient to get it into good operating temperature, so they don't attempt it if it's that cold.

The "Greenland/Canada" mode presumably would kick in if enabled at some point to keep it above -30°C.

 

Preconditioning is a necessity for EVs, which rely on the battery, but a PHEV can live without it - it's got a whole backup power source it can use while the battery gets up to temperature.

So the Prius can afford to just "give up" on preheating the battery in a way an EV couldn't.

 

In the Prius's case, the reason stated by NCF is to increase battery usage:

"As the output of the HV battery assembly drops when the temperature of the HV battery assembly is low, the engine may start during EV mode even if the state of charge is sufficient. Heating the HV battery assembly automatically during plug-in charging ensures sufficient output from the HV battery assembly even in extremely cold environments."

(Followed by diagram showing extra battery output being available at higher temperature, hence not needing to start the engine, unlike if the battery was at 0°C).

It doesn't mention anything about aiding charging - indeed it says charging may be slower with the heater enabled.

Now, below -10°C, the "not starting the engine" reason doesn't apply so much, as we're going to want it for cabin heat. So maybe it gives up. Maybe the effective operating range of the heater is something like -15°C to 5°C. (Until you start using it for the Canada/Greenland mode - might try enabling that tomorrow just to see if it kicks in. Might need to be even colder.).

Checked that - it said it was on.

That seems a bit far-fetched. Even if there was such a concern, I doubt few-hundred watt heater is going to be able to overheat a battery with the ambient temperature being -25°C! The same logic would mean that the primary function of "keeping warm for 3 days after charging" would never be invoked at higher temperatures.

 

Been trying to find any evidence of really-cold weather Prius/Toyota plug-in behaviour. Video of OBD displays would be ideal.

I thought john1701a's trip recordings might help - some temperatures down to -21°C (-7°F) there, but his car was never actually parked outside. Looks like the battery is always above 0°C in his videos, but the car was never parked anywhere much below freezing.

Edit - oh, found one:

Parked for 6 hours at -17°C (1°F), battery temperature 13°C (55°F). So working there for him, but still significantly warmer conditions than me. And the driving result is a lot better - actually using a significant amount of EV charge. Which I think is consistent with my experience - I didn't notice it getting really anti-electric until below -20°C, so maybe the "battery heater" cutoff is around there.

(Or could he just have residual battery heat after only 6 hours? 13°C seems warmer than I thought the heater would heat. Edit again: ah - he would have actually been charging up until the start of the video, so it could be charging heat. So not really conclusive about heater behaviour.)

Obviously I need to do some checking of my own - never actually decided to investigate heater behaviour until this drop into the minus twenties.

 

Pretty technical - kind of a companion "rationale" document to the repair manuals. I guess they're written by engineers bigging up their design, and passed by marketing. They wouldn't say anything was done to be cheaper... It would always be more lightweight...

One possible answer is 12V DC load - certainly with the current design it seems that the power distribution wouldn't permit the heater being used while the system is on. The fuse tree is:

• 50A IGCT IG
  • 15A BATT HTR NO.1 > Heater relay, Battery ECU controlled
  • 15A BATT HTR NO.2 > Heater relay
  • 5A PHV BAT > Battery ECU
  • 5A BATT-S > Battery state sensor
  • 10A BATT-S NO. 2 > Battery state sensor
  • 30A IGCT SCENE > IGCT SCENE relay, HV ECU controlled
• 150A BATT MAIN > Other more conventional stuff

So it looks like you could send 30A to the battery heaters, or 30A to the IGCT stuff (basically 12V for all the hybrid electronics), but not both, cos you'd exceed the 50A total. Of course, those fuses could have been rated differently, but there are presumably issues about rating of DC-DC converters too. Main point is that 30A extra on the 12V is not insignificant.

Another possible answer is - if the main battery's that cold, I thought you were supposed to be limiting power draw? Diverting 350W of its available output might not work well. Maybe it's best just to use it and let it warm up naturally while providing useful output. We're not as impeded by a cold battery as an EV is. Lack of battery power only really impedes efficiency - we still have usable performance from the engine. Trying to warm it up actively could impede our HV efficiency

Third answer is - you tend to want to maximise range. Running the heater will quite likely reduce total range. That might not be the case for an EV, but here where we've got fuel to use, may as well use it first, giving the battery time to warm up naturally.

Heating the battery while driving in an EV is a "least worst" option. Spend power to get the battery into the "better temperature for how much power we're using" range asap. In a PHEV we have a better option - avoid using the battery much at all at those low temperatures, and just rely on the engine more while we wait for it to warm up.

That seems to be a function lost in the G5 - the G4 had it. (And it's still described in NCF - they forgot to take it out).

Good question. I assumed ambient temperature - seems simpler. Could be something else. I was assuming it could get to around 0°C even at -25°C if it tried, but maybe not.

Yes.

 

It's going to warm rapidly the next few days. I'll keep an eye on the system.

Currently -18°C, and the battery is at -20°C, parked for 40 hours now.

Tried turning on the "Greenland/Canada" mode. Starting showing the "Traction battery temp is low - System has prioritised charging to preserve battery condition” MID message, but didn't immediately start heating. I guess that's just warning that it won't be obeying the charge schedule, but -20°C isn't quite cold enough to heat immediately. Turned it back off for now - want to see what the normal mode does first.

 

I meant that when I turned it on manually, the two temperature sensors on the heater elements showed the elements themselves ramping from -23°C to +20°C, showing they were working. Didn't leave it on for more than a minute, and the 15 battery temperature readouts looked like they might have budged a little bit in the first decimal place - 22.9°C rather than -23.0°C...

Not found any specific numbers in documentation, but I'd imagine the thermostatic control in the G5 is the same as the G4 - it will run the heater until the battery itself is +5°C.

Thanks - I'll go through that.

 

Car's now indicating a balmy warm -13°C ambient temperature (I think it might be bit optimistic). Battery still at -19°C.

 

I agree they could be being coy. Of course, in Tesla terms the Prius's maximum 3.3kW charge rate is already extremely limited! I think issues with charging over 20x the rate are a different kettle of fish altogether.

Just being coy about damage doesn't make sense though - as people were saying in the thread above - if it was required to prevent damage, they wouldn't let you turn it off. There are plenty of warnings about things you shouldn't do, some extremely trivial, and "turning off battery heater in cold conditions" is not one of them. All we have is "do not charge below -30°C as it is likely that charging will take longer" and "do not leave the vehicle in areas where the outside temperature is lower than -40°C".

On the other hand there IS a protection function that isn't user-accessible, and that's the Greenland/Canada mode, to keep it above -30°C (or something like that).

Anyway, still getting warmer here. Car indicating -9°C, but the battery's still at -16°C to -17°C. No sign of heating yet. Wondering if the session starting so cold may rule out any heating, or maybe it'll kick in when the battery warms a bit more. Let's see where it's at tomorrow morning - should be -8°C all night, which should get the battery back into "normal" freezing temps.

 

Why wouldn't it? Wouldn't make any sense to link them, and from the documentation, it's a totally separate function.

I can see people have got confused - not least because the manual forgets to mention what the battery heater is for - but they're quite distinct - try to keep the battery at a temperature to permit EV operation for 3 days (paying attention to charging schedules) and try to keep the car functional for 30 days (charging schedules be damned).