Weird battery behavior (and solar)

[corn18]

I went out to the camper to fix up some wiring. I have 2x12V SOK 206Ah batteries in parallel. I checked the voltage with my shunt before I turned anything on and it sho0wed 12.8V. WTF? I left them @ 13.2 last week. So I unhooked everything from the batteries and they both still showed 12.8V. So I took them home and put them on the bench. The Li charger wasn't putting much out so I checked the voltage and both are showing 13.4V with nothing hooked up. I hooked up my bench DC power supply and could push 10A into them.

It was 20 deg F last night and when I got to the trailer it was 34F. The garage is 54F.

Any idea why they were reading so low and then just fine? I need to get my cell voltage monitor installed.

Also, I have 2 sets of 200W panels wired as 2 series. Each pair has its own SCC. I turned them on and was showing 20W from each pair. Odd. It was noon and no clouds. I was seeing 70-90W from each pair last week.

Here's my setup:

 

[Gizmo740]

Would guess temperature and bms settings for both issues. Spec sheets on 206ah SOK show nominal V at 12.8 reading that at 20-25F is actually good. You warmed battery in garage ~30 degrees so voltage would naturally increase. Even though ambient temp was in 30's would not be surprised if camper internal temp or battery compartment temp was less...ie in mid-high 20's. BMS doing its job and not allowing battery to charge below 32 would explain low PV output as power had nowhere to go above self consumption/losses of system.

 

[corn18]

Would guess temperature and bms settings for both issues. Spec sheets on 206ah SOK show nominal V at 12.8 reading that at 20-25F is actually good. You warmed battery in garage ~30 degrees so voltage would naturally increase. Even though ambient temp was in 30's would not be surprised if camper internal temp or battery compartment temp was less...ie in mid-high 20's. BMS doing its job and not allowing battery to charge below 32 would explain low PV output as power had nowhere to go above self consumption/losses of system.

Thanks @Gizmo740. I attached monitors to each battery to monitor individual cell voltage. I charged them in my garage @ 60 deg F. After settling, they rested right @ 13.46V. It is now 40 deg in the garage and they are @ 13.3V. I can imagine being 0 deg F would drive that voltage down a lot.

I searched the interweb and couldn't find a 100% SOC vs. temp. Do you have access to such data? Would be a good ref to have. Also, I am irritated that the Victron BMS which has a temp sensor hooked up does not know to adjust SOC for temp.

 

[sunshine_eggo]

Would guess temperature and bms settings for both issues. Spec sheets on 206ah SOK show nominal V at 12.8 reading that at 20-25F is actually good. You warmed battery in garage ~30 degrees so voltage would naturally increase. Even though ambient temp was in 30's would not be surprised if camper internal temp or battery compartment temp was less...ie in mid-high 20's. BMS doing its job and not allowing battery to charge below 32 would explain low PV output as power had nowhere to go above self consumption/losses of system.

This.

Thanks @Gizmo740. I attached monitors to each battery to monitor individual cell voltage. I charged them in my garage @ 60 deg F. After settling, they rested right @ 13.46V. It is now 40 deg in the garage and they are @ 13.3V. I can imagine being 0 deg F would drive that voltage down a lot.

Nope. Voltage is more a function of how long they have sat since last charge. Temperature effects on voltage are minimal as LFP does not need temperature compensation to fully charge through a wide range of safe charging temperatures.

I searched the interweb and couldn't find a 100% SOC vs. temp. Do you have access to such data? Would be a good ref to have. Also, I am irritated that the Victron BMS which has a temp sensor hooked up does not know to adjust SOC for temp.

It does, but it can't guess what the battery manufacturer specifies as the temp coefficient. That's your job:



LFP doesn't lose that much capacity in cold temps compared to lead-acid. Lead acid loses about 30% at freezing.

This garbage study (unscientific and biased) shows how battleborns retain capacity at lower temps, and it's not linear:

Lead is Dead White Paper: Cold Charging Study

Click here to read the breakdown of our White Paper Study and find out why lead is dead and LiFePO4 batteries are the best in the industry.

battlebornbatteries.com

Scroll down to "The Results".

Would expect the SOK to behave very similarly.

 

[corn18]

This.



Nope. Voltage is more a function of how long they have sat since last charge. Temperature effects on voltage are minimal as LFP does not need temperature compensation to fully charge through a wide range of safe charging temperatures.



It does, but it can't guess what the battery manufacturer specifies as the temp coefficient. That's your job:

View attachment 79506

LFP doesn't lose that much capacity in cold temps compared to lead-acid. Lead acid loses about 30% at freezing.

This garbage study (unscientific and biased) shows how battleborns retain capacity at lower temps, and it's not linear:

Lead is Dead White Paper: Cold Charging Study

Click here to read the breakdown of our White Paper Study and find out why lead is dead and LiFePO4 batteries are the best in the industry.

battlebornbatteries.com

Scroll down to "The Results".

Would expect the SOK to behave very similarly.

Coolio. I will check into the temp coefficient for the shunt.

I don't use my camper when it's this cold. I do like to work on it though. I may keep the SOKs in the garage and just bring an LFA with me when I want to work on the camper.

 

[mikefitz]

Any idea why they were reading so low and then just fine?

All due to the common port BMS. The low temperature protect activated and opened the charge path FETs. You can still measure volts but there will be a diode volt drop , around 0.6 volts.
Taking the batteries inside raised the battery temperature and the BNS closed the charge path allowing a charge and the voltage measured increased.

I turned them on and was showing 20W

Again if the temperature was low the charge path will have been opened by the BMS preventing charge into the batteries. You still have a power requirement for rest of the system.

Mike