Low temp disconnect and voltage

[jdavis]

Hello,

Last night it got down into the 20's here. This morning I noticed my 95% full LiFePO4 battery was showing 12.7vdc and was in low temp disconnect and would not take a charge. I warmed my shop up and it came out of low temp disconnect and the voltage jumped up to 13.4vdc. Is that the normal behavior I should expect from a battery in low temp disconnect, to have a very low voltage reading, even if its full.

Thanks!

 

[Wallowa]

My understanding is that the BMS [BB info] stops any incoming charge at or below close to freezing to stop any damage to battery structure...you still have full use of the amps in the battery at that temp and can continue to draw current from the battery which should still have a voltage in keeping with the SOC....when you heated up the battery again, I assume the battery started receiving a charge, hence voltage reflected that...only my WAG....

This is why I have a heated BB; so that it can be charged regardless of how low the temp [35-45 control]....when away from Hawk and sun shining even with temps in camper below zero....

 

[jdavis]

Once it warmed up I did not charge it. I actually only assumed the battery came out of LTD. One second it was reading 12.7v and the next it was 13.4 and remained 13.4 the rest of the day. I was actually pointing a hair dryer at it to heat it up quicker. Tonight it will be even colder, 9f, so I suspect I'll see the same thing in the morning.

I guess 12.7 would run everything I need, but it was a shock to see that knowing it was fully charged 12 hours before.

 

[Jon R]

I’m curious what the BMS is actually doing if the voltage is suddenly lower like that. The cells themselves certainly don’t step change the voltage.

 

[jdavis]

Here is a thread from the victron forums where a guy had the same experience. No update though :\

https://community.victronenergy.com/questions/118305/voltage-drop-with-bms-low-temp-cutoff.html

 

[jdavis]

I think this post explains it exactly:

https://diysolarforum.com/threads/bms-mosfets-explained.29318/

tl;dr when either of the diodes in the BMS are blocking current there will be about a 0.6v voltage drop.

 

[Wallowa]

I think this post explains it exactly:

https://diysolarforum.com/threads/bms-mosfets-explained.29318/

tl;dr when either of the diodes in the BMS are blocking current there will be about a 0.6v voltage drop.

Good find...with non-linear V vs SOC on Li battery small voltage changes can equate to large SOC changes...

Yes, though minor, I tested my BB at 40* and then at 65* with no input and voltage went up about .3V @ 65*...temp does affect voltage...

 

[Jon R]

Good find...with non-linear V vs SOC on Li battery small voltage changes can equate to large SOC changes...

Yes, though minor, I tested my BB at 40* and then at 65* with no input and voltage went up about .3V @ 65*...temp does affect voltage...

I think he’s saying that below a certain temperature a diode is effectively inserted in the circuit, and that alone causes the sudden voltage drop. No change in state of charge.

 

[Wallowa]

I think he’s saying that below a certain temperature a diode is effectively inserted in the circuit, and that alone causes the sudden voltage drop. No change in state of charge.

Way beyond my MOS but I suspect that different battery manufacturers use differing circuits...BMSs not being equal...