12v Battery voltage sag under half load in the cold?

[invento123]

I have a Renogy 12v 100ah LiFEPO4 battery and an Ampere Time 12v 100ah LiFEPO4 battery setup for use in my shed. The Renogy battery is installed in a Goal Zero Yeti 1250 with the Ampere Time battery connected in parallel to it. I have battery State Of Charge meters on both of the batteries to measure current draw and SOC.
Tonight it was a bit chilly in the shed, about 58 degrees Fahrenheit and I was running a 600 watt heater to keep warm along with a soldering iron and some lights. All together pulling around 850 watts or 70 amps at 12 volts from the 2 batteries; 35 amps each

When the batteries both reached around 50% charge the Yeti 1250 shut off due to low voltage, around 10.6 volts. Is this normal voltage sag for LiFEPO4 batteries?? I know they perform worse in cold temperatures like all other batteries but this seemed pretty extreme. After letting them sit for a minute they both rose back up to around 12.2-12.3 volts. The Renogy battery also has a built in SOC monitor and in the app it showed around 50% charge as well so I don't think my battery SOC meters are off. Any ideas???

 

[sunshine_eggo]

I have a Renogy 12v 100ah LiFEPO4 battery and an Ampere Time 12v 100ah LiFEPO4 battery setup for use in my shed. The Renogy battery is installed in a Goal Zero Yeti 1250 with the Ampere Time battery connected in parallel to it. I have battery State Of Charge meters on both of the batteries to measure current draw and SOC.

Please link this product.

Tonight it was a bit chilly in the shed, about 58 degrees Fahrenheit and I was running a 600 watt heater to keep warm along with a soldering iron and some lights. All together pulling around 850 watts or 70 amps at 12 volts from the 2 batteries; 35 amps each

Okay, so you have a MAXIMUM of 2560Wh of battery capacity.

That means you can run a 850W load for:

2560/850 = 3 hours

Assuming your batteries start at TRUE 100% SoC, i.e., they were recently charged to 14.4V and held there for about 15-30 minutes.

This also ignores inefficiencies in the inverter and the inverter's own power consumption.

When the batteries both reached around 50% charge the Yeti 1250 shut off due to low voltage, around 10.6 volts.

10.6V is a totally reasonable level to expect the inverter to shut down as the cells are completely dead at 10.0V.

Is this normal voltage sag for LiFEPO4 batteries??

You're not seeing sag

I know they perform worse in cold temperatures like all other batteries but this seemed pretty extreme. After letting them sit for a minute they both rose back up to around 12.2-12.3 volts.

Based on voltage, they were about 12% charged.

The Renogy battery also has a built in SOC monitor and in the app it showed around 50% charge as well so I don't think my battery SOC meters are off. Any ideas???

They are WAY off as 12.2-12.3V is about 12%. Again, please provide a link to these "SoC meters". If they are voltage based, then they would be correct at 50%, but that's for lead acid that's sat for 24 hours without charge or discharge. In other words, voltage-based SoC is relatively worthless in the vast majority of cases.

Bottom line, is you used a relatively large load of 0.3C... which means you can power that load for about 3 hours from a full charge.

Battery monitors and BMS SoC measurements require regular full charges to the vicinity of 14.4V to periodically synchronize to 100% SoC.

 

[invento123]

Please link this product.

This is the meter. I use one on both batteries, connected and configured as the manual says.

Okay, so you have a MAXIMUM of 2560Wh of battery capacity.

That means you can run a 850W load for:

2560/850 = 3 hours

Assuming your batteries start at TRUE 100% SoC, i.e., they were recently charged to 14.4V and held there for about 15-30 minutes.

This also ignores inefficiencies in the inverter and the inverter's own power consumption.



10.6V is a totally reasonable level to expect the inverter to shut down as the cells are completely dead at 10.0V.

I know this and was not surprised it shut down.

You're not seeing sag

Based on voltage, they were about 12% charged.

They are WAY off as 12.2-12.3V is about 12%. Again, please provide a link to these "SoC meters". If they are voltage based, then they would be correct at 50%, but that's for lead acid that's sat for 24 hours without charge or discharge. In other words, voltage-based SoC is relatively worthless in the vast majority of cases.

Bottom line, is you used a relatively large load of 0.3C... which means you can power that load for about 3 hours from a full charge.

Battery monitors and BMS SoC measurements require regular full charges to the vicinity of 14.4V to periodically synchronize to 100% SoC.

It could be the SOC meter's are off however I did calibrate them 4 months ago and I find it odd the integrated SOC meter on the Renogy battery would also be off.

They do sit outside in a non-heated shed that has been reaching temperatures down to 30 degrees fahrenheit. Could this cause something like parasitic drain on the batteries that would result in the 50% capacity lost?

 

[sunshine_eggo]

This is the meter. I use one on both batteries, connected and configured as the manual says.

Those are fine.

I know this and was not surprised it shut down.

It could be the SOC meter's are off however I did calibrate them 4 months ago and I find it odd the integrated SOC meter on the Renogy battery would also be off.

There is no question that all devices reporting SoC are off. They are definitely off. Again, if batteries aren't regularly fully charged to allow the monitor/BMS to re-sync at 100%, then their accuracy will drift.

They do sit outside in a non-heated shed that has been reaching temperatures down to 30 degrees fahrenheit. Could this cause something like parasitic drain on the batteries that would result in the 50% capacity lost?

No, but if both batteries have low temp charge protection, they aren't being charged. If they don't have low temp charging protection and are being charged below freezing, they are being damaged.