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This post is not really a request for help; rather it's to document some strange electrical behavior (strange to me) related to cold weather.
I've got a Lithium (LiFePO4) battery bank, solar panels with charge controller, and 110 volt charging system. I had read that Lithium batteries shouldn't be charged below 32 F, but I never really thought about the implications -- mostly because we don't camp much when it's cold. I recently (during December in Minnesota) had to take my rig in for a recall, and simultaneously had some leveling-jack problems. These unplanned utilization's resulted in my battery bank being discharged to around 30% SOC.
30% seemed a little low for lasting until Spring and still having enough juice to come out of storage without issues (think slide-outs, leveling jacks, quick-y use of the inverter, etc.). No problem, I thought. I'll just wait for a sunny day and enable my solar setup. No dice. The solar monitor showed plenty of voltage being produced at the panels and the output of the charge controller, but it also showed 0 amps flowing to the batteries. 0. Outside temp was 33 F, so I was confused. Turned out, the recommended charge controller settings for my battery included a low-temp cutoff of 34 F. Hence the 0 amps. Right?
Well I changed that low-temp setting to 32 F, but still 0 amps. Huh? Should have been 10-12 amps at the voltage I was producing at the panels.
Turns out all decent Lithium batteries have a built-in BMS with their own low-temp cutoff -- because charging below freezing will wreak a Lithium battery. I'm pretty sure that the 33 F I that was sensing on the case of the battery was actually below freezing inside the battery. So the BMS was restricting the incoming current flow – even though my charge controller believed it was good to go.
It was a sunny day and warming up to mid 40's that day, so I brought the rig home from storage so I could use my 110 volt charging system. My battery monitor temp-sensor was showing 41 F, so I enabled the inverter/charger for charge. The charger was delivering 14+ V to the battery, but still the BMV-712 battery monitor showed 0 amps current flow. The solar system was engaged, and it's monitor also reported 0 amps and it indicated the charge controller was in Float mode. ???
I'm really confused by this 0 amp current flow and the charge controller being in Float, not Absorb or Bulk. I conduct a test: disable inverter/charger and reset the charge controller. Charge controller switches to Absorb and delivers a fraction of an amp to the battery.
So I think there were two things going on with the solar system. With the inverter/charger delivering 14+ volts to the battery, the charge controller thought the battery was charged so it switched from Absorb to Float. The fractional amp current was the BMS still restricting current flow, because the internal battery temp was still lower than the BMS cutoff temp.
So I decide to use my heat gun and (gently) warm up the battery bank. Re-enabled the inverter/charger and began the warm-up process. After about 10 minutes, I decide to see what the BMV-712 monitor was showing. Still 14+ volts and 0 amps and, and – 100% SOC! Crazy.
The Lion Safari UT 1300 batteries have a built-in SOC monitor that you can activate by depressing a momentary switch on the battery itself. I checked, and the batteries were at 20% SOC. Pretty sure what happened is the BMV-712 SOC algorithm shows 100% when voltage exceeds a threshold for a given length of time.
Anyway, on a whim, I try one more thing. I had been running the heat gun with the inverter/charger in charge mode. I decide to reverse it and run the heat gun off the inverter – pulling current from the battery bank. Just for a minute or two. Then I switched back to charge mode – and low and behold, I have started charging and delivering around 55 amps to the battery. The solar system is participating, too. I've now got current flow from everywhere.
Couple of hours later, and I've got my battery bank charged back to where I wanted it.
My take-away from this experience is that Lithium's are very sensitive to cold weather and the internal BMS does some funky things that can block your intentions and confuse you and your monitors.
-Steve
I've got a Lithium (LiFePO4) battery bank, solar panels with charge controller, and 110 volt charging system. I had read that Lithium batteries shouldn't be charged below 32 F, but I never really thought about the implications -- mostly because we don't camp much when it's cold. I recently (during December in Minnesota) had to take my rig in for a recall, and simultaneously had some leveling-jack problems. These unplanned utilization's resulted in my battery bank being discharged to around 30% SOC.
30% seemed a little low for lasting until Spring and still having enough juice to come out of storage without issues (think slide-outs, leveling jacks, quick-y use of the inverter, etc.). No problem, I thought. I'll just wait for a sunny day and enable my solar setup. No dice. The solar monitor showed plenty of voltage being produced at the panels and the output of the charge controller, but it also showed 0 amps flowing to the batteries. 0. Outside temp was 33 F, so I was confused. Turned out, the recommended charge controller settings for my battery included a low-temp cutoff of 34 F. Hence the 0 amps. Right?
Well I changed that low-temp setting to 32 F, but still 0 amps. Huh? Should have been 10-12 amps at the voltage I was producing at the panels.
Turns out all decent Lithium batteries have a built-in BMS with their own low-temp cutoff -- because charging below freezing will wreak a Lithium battery. I'm pretty sure that the 33 F I that was sensing on the case of the battery was actually below freezing inside the battery. So the BMS was restricting the incoming current flow – even though my charge controller believed it was good to go.
It was a sunny day and warming up to mid 40's that day, so I brought the rig home from storage so I could use my 110 volt charging system. My battery monitor temp-sensor was showing 41 F, so I enabled the inverter/charger for charge. The charger was delivering 14+ V to the battery, but still the BMV-712 battery monitor showed 0 amps current flow. The solar system was engaged, and it's monitor also reported 0 amps and it indicated the charge controller was in Float mode. ???
I'm really confused by this 0 amp current flow and the charge controller being in Float, not Absorb or Bulk. I conduct a test: disable inverter/charger and reset the charge controller. Charge controller switches to Absorb and delivers a fraction of an amp to the battery.
So I think there were two things going on with the solar system. With the inverter/charger delivering 14+ volts to the battery, the charge controller thought the battery was charged so it switched from Absorb to Float. The fractional amp current was the BMS still restricting current flow, because the internal battery temp was still lower than the BMS cutoff temp.
So I decide to use my heat gun and (gently) warm up the battery bank. Re-enabled the inverter/charger and began the warm-up process. After about 10 minutes, I decide to see what the BMV-712 monitor was showing. Still 14+ volts and 0 amps and, and – 100% SOC! Crazy.
The Lion Safari UT 1300 batteries have a built-in SOC monitor that you can activate by depressing a momentary switch on the battery itself. I checked, and the batteries were at 20% SOC. Pretty sure what happened is the BMV-712 SOC algorithm shows 100% when voltage exceeds a threshold for a given length of time.
Anyway, on a whim, I try one more thing. I had been running the heat gun with the inverter/charger in charge mode. I decide to reverse it and run the heat gun off the inverter – pulling current from the battery bank. Just for a minute or two. Then I switched back to charge mode – and low and behold, I have started charging and delivering around 55 amps to the battery. The solar system is participating, too. I've now got current flow from everywhere.
Couple of hours later, and I've got my battery bank charged back to where I wanted it.
My take-away from this experience is that Lithium's are very sensitive to cold weather and the internal BMS does some funky things that can block your intentions and confuse you and your monitors.
-Steve
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