My diy bank -balancing

[Barold]

I returned to my boat after being away for a week or two and the 12v system cut out - when I checked the BMV it read 12.5V but SOC was in the 90% range (AH) the LiFePO4 bank is about 5 years old as is the 220W of solar so something is starting to fail.

Checked the BMS and see that the cells seem quite out of balance.

Is this to be expected of 280AH cells from 5 years ago? Its about 30% shorter than i had anticipated.




ted.

 

[AntronX]

Balance the cells, charge to 100%, discharge to 0%, get your new Ah capacity.

 

[sunshine_eggo]

I returned to my boat after being away for a week or two and the 12v system cut out - when I checked the BMV it read 12.5V but SOC was in the 90% range (AH) the LiFePO4 bank is about 5 years old as is the 220W of solar so something is starting to fail.

Your BMV is likely not configured correctly.

Checked the BMS and see that the cells seem quite out of balance.

These cells are all at low states of charge, and it's relatively normal to see notable disparity in the lower leg if they have been top balanced.

Is this to be expected of 280AH cells from 5 years ago? Its about 30% shorter than i had anticipated.

Without knowing what happened, I don't see any evidence of a cell/battery degradation issue. What I do see is a max battery voltage of 13.7V which may indicate inadequate charging.

 

[Wrayt]

I returned to my boat after being away for a week or two and the 12v system cut out - when I checked the BMV it read 12.5V but SOC was in the 90% range (AH) the LiFePO4 bank is about 5 years old as is the 220W of solar so something is starting to fail.

Checked the BMS and see that the cells seem quite out of balance.

Is this to be expected of 280AH cells from 5 years ago? Its about 30% shorter than i had anticipated.

View attachment 250310


ted.

If those are your BMS readings the -30C temperature looks off. Your BMS should shut off charging before the battery gets to 0. Bad sensor? Lifepo4 should never be charged when at zero or below.

 

[LakeHouse]

12.5V is pretty close to 0% on LFP. Your cells are likely way out of balance. Maybe not damaged, and maybe entirely salvageable, but there's some work to be done.
A few recommendations:

1. Set you BMS to not balance unless cell voltage is above 3.45V. It looks like it's 'balancing' at 3.231V, which will contribute to imbalance, possibly causing much or all of the current cell imbalance.
2. Top balance your cells. Probably the easiest way to do this would be to disassemble the battery. If that's not an option, an active balancer might be helpful. The balance feature on your BMS will take months to do it.
3. Post the settings for your BMV. It's reporting 90%, which is obviously incorrect.
4. Tell us how you're charging this battery. You mention 220W of solar, but your screenshot shows 482W going into the battery. So some sort of shore power? Maybe we can help with getting settings right so you can avoid problems in the future.

 

[sunshine_eggo]

If those are your BMS readings the -30C temperature looks off. Your BMS should shut off charging before the battery gets to 0. Bad sensor? Lifepo4 should never be charged when at zero or below.

This is a very good point, and I find it concerning that balancing is occurring at such a low voltage.

 

[Barold]

This is a very good point, and I find it concerning that balancing is occurring at such a low

 

[Barold]

Thanks for all the replies.

ONe of the two temp sensors is damaged. I have tried to find a replace to no avail (the plugs i buy are wrong and i can't seem to find a source - even on aliexpress for replacements

The solar is the only charging when I am away - i flicked on shorepower charging when everything shut down. (It has a lithium setting that charges just 13.8v and i am leery of leaving it on for long periods but perhaps that would have enable the slow BMS balancing? Is there a better active balancing BMS i should consider if these cells are still ok?

Will take the entire battery home after haulout to top balance again. I bought one of computer fan devices that will track the cell AH. Is that the best way to capacity test these 280AH cells?

Agree the BMV seems really off. The solar panels are 110W sunpower semiflexible and I am wondering if they are aging out as well (all bought at the same time)

 

[LakeHouse]

I bought one of computer fan devices that will track the cell AH. Is that the best way to capacity test these 280AH cells?

Best? No, that would be a ZKE-Tech tester. I have this little guy, but they also make a 40A version that would speed things up considerably. But for 4 cells and (it sounds like) all winter to do it, how much of a rush are you really in?
But the little fan devices will also do a fine job.

 

[sunshine_eggo]

Thanks for all the replies.

ONe of the two temp sensors is damaged. I have tried to find a replace to no avail (the plugs i buy are wrong and i can't seem to find a source - even on aliexpress for replacements

The solar is the only charging when I am away

With what and what charge parameters?

- i flicked on shorepower charging when everything shut down. (It has a lithium setting that charges just 13.8v and i am leery of leaving it on for long periods but perhaps that would have enable the slow BMS balancing? Is there a better active balancing BMS i should consider if these cells are still ok?

Will take the entire battery home after haulout to top balance again. I bought one of computer fan devices that will track the cell AH. Is that the best way to capacity test these 280AH cells?

Agree the BMV seems really off.

If it's a BMV-712, you need to set the "Battery SoC on Reset" to "Keep SoC"

You likely have it set to "set to 100%"

If it's not a BMV-712, set to "Clear" - in this case it will read "--" indicating it has no idea what the SoC is because it lost power.

Charged voltage should be 0.2V below your absorption voltage.

The solar panels are 110W sunpower semiflexible and I am wondering if they are aging out as well (all bought at the same time)

The general consensus is that semi/flexible panels don't last.

 

[Rednecktek]

Make sure the setting for "Balance on Charge" is set to OFF so the balancing continues when you're not there actively charging.

 

[Barold]

Make sure the setting for "Balance on Charge" is set to OFF so the balancing continues when you're not there actively charging.

Is this a setting in the BMS? Does this mean it only balances when it is not charging but discharging?

 

[sunshine_eggo]

Is this a setting in the BMS?

Yes.

Does this mean it only balances when it is not charging but discharging?

Or neither. If you have high cells, you want the BMS to burn off any excess even if it's just sitting there.

The reality is that discharging the battery will rapidly pull the voltage down below your typical 3.40-3.45V threshold, and you don't really get balancing during discharge.

A common balancing technique is to hold the battery at elevated voltage and force the BMS to burn off excess voltage from high cells. This often toggles between charging and protection modes with charging being a much smaller portion of the total time vs. protection mode, so balancing only during charging is not effective in this case and notably slows the process.