LFP Battery Rest Period After Discharging before Recharging?

robby

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May 1, 2021
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I tried looking this up but found only one paper on the subject and it was not applicable to LFP.
My current system uses 16KWh of LFP batteries and these are turned on Automatically at 11pm and run until 5:30am. The Sun is up enough by 7:30am to fully power the house and start charging the Batteries. By 11:30am they are typically charged to 100% SOC and are cycling on and off a bit until 5pm to compensate for any cloud coverage on the PV.
After 5pm they are offline unless their is a power outage and typically get back some charge to go from on average from 93% to 95%+ SOC by 5:30pm.

Since I will be adding more batteries my question revolves around the Two hour time period in the morning when they are at 20% SOC and then start charging.
If I add one more battery I could remove that Gap and they would last until 7:30am and start charging back up again right away. I got a feeling that having Zero down time on a battery before recharging is not a good idea. Should I instead start using the batteries earlier from say 9pm and let them go to 5:30am and keep the 2 hour resting period in place? Anyone have any info on this subject?
 

fafrd

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Aug 11, 2020
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I tried looking this up but found only one paper on the subject and it was not applicable to LFP.
My current system uses 16KWh of LFP batteries and these are turned on Automatically at 11pm and run until 5:30am. The Sun is up enough by 7:30am to fully power the house and start charging the Batteries. By 11:30am they are typically charged to 100% SOC and are cycling on and off a bit until 5pm to compensate for any cloud coverage on the PV.
After 5pm they are offline unless their is a power outage and typically get back some charge to go from on average from 93% to 95%+ SOC by 5:30pm.

Since I will be adding more batteries my question revolves around the Two hour time period in the morning when they are at 20% SOC and then start charging.
If I add one more battery I could remove that Gap and they would last until 7:30am and start charging back up again right away. [b{I got a feeling that having Zero down time on a battery before recharging is not a good idea.[/b] Should I instead start using the batteries earlier from say 9pm and let them go to 5:30am and keep the 2 hour resting period in place? Anyone have any info on this subject?
Did you get that feeling out of thin air or do you read or hear something that caused it?
 

Bob B

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With lead acid batteries, they need to be recharged as soon as possible to prevent sulfation ...... there is no effect like that with LFP chemistry.
They could sit at 20% for a long period of time with no ill effects.
 

time2roll

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I equate this to letting a generator or air conditioner have "rest" periods during extended run times. Battery is the same. It is a machine, does not need rest, does not get tired, does not need a lunch break. Let it run until there is a maintenance issue to address.
 

acdoctor

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Central Arkansas
Unless my calculations are wrong that is an average discharge of 0.1C. That C rate should not raise the pack temperature any more than changing would so I see no reason to “rest” before recharging. Besides unless you have east facing panels, battery will get a break from discharge while panels are just carrying load because that’s all of the watts they have for a period of time.
 

robby

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Did you get that feeling out of thin air or do you read or hear something that caused it?
Mostly based on a feeling. Batteries do get warm so I started to wonder if jumping right back into a recharge was a bad idea. Just wanted to make sure there was no known LFP rule about this that would shorten the life.
 

acdoctor

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Just last night my mobile system ran at 0.09C my 24 volt 280AH pack didn’t raise 1 deg F. It did raised 2 deg F. Recharging today though.
 

Bob B

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Mostly based on a feeling. Batteries do get warm so I started to wonder if jumping right back into a recharge was a bad idea. Just wanted to make sure there was no known LFP rule about this that would shorten the life.
Some have reported that the pack warms more the lower the SOC .... it seems logical also since the IR increases at a low SOC.
If you add more batteries, that should keep you operating more in the upper range of the SOC.
 

robby

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798
Some have reported that the pack warms more the lower the SOC .... it seems logical also since the IR increases at a low SOC.
If you add more batteries, that should keep you operating more in the upper range of the SOC.
Makes sense but I am going to always push them to 20% SOC unless my electric bill drops down to zero usage.
I know some people go down to 10% but the trade off in decreased life expectancy is too high.
 
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