Won't run in cold weather (below 50f)

[shawnm]

Hi:
I converted a three wheeled Heinkel microcar to EV power using a QS Motor 8kw hub motor and a 72v 24S LiFePo4 pack. It has exceeded all of my expectations except for one: it won’t run in cold weather. I am having problems with my BMS system in cold weather. My EV runs fine when the temperature is above 50F (10C). Below that temperature the BMS does not recognize the cells on board #2, and will not power the Kelly controller. If I charge the EV (also with a DeliGreen onboard DC charger) it will “wake up” the controller, but when you try to apply a load the BMS cuts off again.

I have attached some screen shots that demonstrate the problem. I suspect the problem is with the temperature cutoff settings.
This screen shot shows when I turn on the car in the morning. It was about 4c/ 40F outside. You can see that cells 15-24 have no reading. This corresponds to board 2 on the BMS. Based on the documentation I originally left “board 2 temp num” on the factory setting of zero. I did try to change it to 1 (see screenshot). It did allow me to see the voltage readings on cells 15-24, but the car still would not operate. I have since changed it back to zero.

These are the settings on my temperature control. My understanding of the “dischg low temp protect” is that the BMS will not send current if the temperature is 1C or less (about 34F). I do not know what “diff Temp protect” is, or if I need to change it. I used the settings that were in the documentation provided by DeliGreen. I suspect "diff temp protect" is the culprit.

If the car sits in the sun and the temp exceeds 50 all is well.

Can you tell me what needs to be done to fix this issue?

Thanks,
Shawn

 

[j bjork]

This sounds more like a bad connection than a settings problem to me.
I cant imagine that the cells should show 0v when the temp protection is active.
Bad solder joints on the pcb can act this way. (temp sensitive)

 

[From-A-To-B]

I agree -- it doesn't sound like a settings issue to me.

FWIW, I believe that Temp Diff refers to the difference between the BMS board temp and the BMS probe temp (which most people stick into the battery pack, but lots of times the thermistor is taped to the outside of the pack or left flapping around.) So the BMS will shut off if, presumably, either the BMS or the battery is heating up and the other isn't.

I don't think this is your issue, but you might as well set the Discharge Temp Protect lower, just to get that out of the way for testing purposes. Li-ion batteries can be safely discharged below freezing.

 

[spinningmagnets]

I have read about EV conversions in Canada that added an insulation layer around the battery. And then added a flat heating element against the bottom of the pack.

He tracked the drain, and the heater only used up a very tiny amount of he battery's range.

 

[methods]

I didn't read this thread but listen carefully

* LiFePO4 will be destroyed if you run it below freezing
* Good BMS check for this

Many Battery systems have a self heating feature to warm up so you can get past this problem.

Good luck

-methods