BMS/charge controller issue

[littleharbor2]

I'm basically bench testing a 12 volt 305 ah. LiFePo4 battery build I'm doing for a friend for his RV. I have the battery connected to an inverter for load and a Xantrex C-60 for charging, It appears to start out working fine but then CC goes open circuit soon after connecting. This open circuit voltage causes the BMS to cut off charging due to an over voltage situation. Could there be an issue with the C-60?

 

[sunshine_eggo]

Are you sure it's not the other way around? The BMS is engaging charge protection and then the SCC is spiking because the BMS has cut the battery out of the circuit? This is how it usually happens.

Try setting absorption to silly low values and increment it (13.2, 13.3, 13.4, 13.5, 13.6) and see what happens.

 

[littleharbor2]

Are you sure it's not the other way around? The BMS is engaging charge protection and then the SCC is spiking because the BMS has cut the battery out of the circuit? This is how it usually happens.

Try setting absorption to silly low values and increment it (13.2, 13.3, 13.4, 13.5, 13.6) and see what happens.

Thanks for your reply, You're probably right. With these BMS's it's kind of like the chicken or the egg thing. The voltage setting on the cc wasn't set too high as I had used it for another 12 volt setup before. The battery was 99% full, if that makes any difference. I did try setting absorb lower but not crazy low, yet.

 

[sunshine_eggo]

99% full... If the battery is nearly full, then BMS charge protection is almost certainly the cause. Your cells may be imbalanced.

When you cut a battery out of a circuit, there is a lag between cut-off and the charger terminating output. Since the charger doesn't turn off instantly, it's still trying to sent current to an open circuit. This usually results in a voltage notably above bulk/absorption. If an inverter is in the circuit, it's usually enough to trigger the inverter's over-voltage protection (16V, 32V, 64V).

 

[littleharbor2]

99% full... If the battery is nearly full, then BMS charge protection is almost certainly the cause. Your cells may be imbalanced.

When you cut a battery out of a circuit, there is a lag between cut-off and the charger terminating output. Since the charger doesn't turn off instantly, it's still trying to sent current to an open circuit. This usually results in a voltage notably above bulk/absorption. If an inverter is in the circuit, it's usually enough to trigger the inverter's over-voltage protection (16V, 32V, 64V).

It's not a balance problem. The hi-lo difference is .002 vdc.
Turning on the inverter on seems to put enough load on the system to pull the voltage down out of open circuit voltage.
I'm going to swap inverter and CC in the morning and see what happens.

 

[sunshine_eggo]

It's not a balance problem. The hi-lo difference is .002 vdc.

Are you observing this when it happens, or are you seeing this typical in operation?

 

[littleharbor2]

Are you observing this when it happens, or are you seeing this typical in operation?

Well, I've only had the battery operational for a few minutes. Top balanced cells, sat in parallel for a few days, I assembled the battery and tested it, But to your question, no, I read the differential after this happened, from the BMS Bluetooth.