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diy solar

Float voltages for LFP cells.

rjg13 said:
That was in response to


Do you have a SOC curve? Every curve that I see is showing a steep voltage increase around 3.36V. Even in the resources section on this website is showing this. I don't understand why floating the battery at this voltage would be a problem.
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Floating at 3.35 indefinitely is probably ok. Balancing at 3.35 is very questionable though.

You have to be a lot more careful with your float voltage in a standby application. I float at 3.437 but it's one thing to do that for 6 hours at a time vs. continuously.
 
hwy17 said:
Floating at 3.35 indefinitely is probably ok. Balancing at 3.35 is very questionable though.

You have to be a lot more careful with your float voltage in a standby application. I float at 3.437 but it's one thing to do that for 6 hours at a time vs. continuously.
Click to expand...

I don't understand. If I float at 3.35vpc, I could have some cells at 3.300V, and some at 3.550V, like I do right now. If I balance at 3.35VPC, then I won't be running the risk of holding a single cell at a much higher voltage.
 
rjg13 said:
I don't understand. If I float at 3.35vpc, I could have some cells at 3.300V, and some at 3.550V, like I do right now. If I balance at 3.35VPC, then I won't be running the risk of holding a single cell at a much higher voltage.
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I think we're stuck in circles a bit here.

I agree with you that floating and balancing at 3.35 seems like the best potential option here. Generally it is considered that you "can't balance" at 3.35 because even if you get all cells to 3.35 even, they might have different states of charge.

In a daily cycled pack, that would be true, charging and balancing to 3.35 for just a matter of hours would not reliably balance the pack, even if they are voltage balanced. But, if they are held and balanced at 3.35 for days, weeks, etc., I think a reasonable SOC balance would be acheived.
 
hwy17 said:
I think we're stuck in circles a bit here.

I agree with you that floating and balancing at 3.35 seems like the best potential option here. Generally it is considered that you "can't balance" at 3.35 because even if you get all cells to 3.35 even, they might have different states of charge.

In a daily cycled pack, that would be true, charging and balancing to 3.35 for just a matter of hours would not reliably balance the pack, even if they are voltage balanced. But, if they are held and balanced at 3.35 for days, weeks, etc., I think a reasonable SOC balance would be acheived.
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Okay, so I should probably either swap the BMS, or add an active balancer/equalizer to it. I don't need to utilize the full capacity of this pack, so I'm not worried about squeezing every last drop of SOC out of it.
 
rjg13 said:
Okay, so I should probably either swap the BMS, or add an active balancer/equalizer to it. I don't need to utilize the full capacity of this pack, so I'm not worried about squeezing every last drop of SOC out of it.
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Does your current BMS have settings to balance at 3.35?
 
hwy17 said:
Does your current BMS have settings to balance at 3.35?
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I don't think so. It is a JBD and there does not appear to be a bluetooth connection. I think there is a serial connection that I may be able to interface with, but no idea where to go from there.

I just purchased a NEEY 4th generation 4a active balancer. I think this is probably the easiest way to do this.
 
rjg13 said:
Okay, so I should probably either swap the BMS, or add an active balancer/equalizer to it. I don't need to utilize the full capacity of this pack, so I'm not worried about squeezing every last drop of SOC out of it.
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Did you ever test the battery for actual capacity? It could still deliver 50Ah even with cells not fully balanced.
 
Brucey said:
Did you ever test the battery for actual capacity? It could still deliver 50Ah even with cells not fully balanced.
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Nope, never tested for full capacity. I don't need the full capacity. This was just a better option than spending the same amount of money on lead acid batteries that were going to die every 2-3 years. The reason I care about the cells being balanced is longevity. I don't want to be changing the batteries on the UPS constantly.
 
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