What happens to the controller when the BMS Faults?

[Frank Smith]

I installed lithium battery because my lead acid batteries were failing.

I have utility power available but don’t sell back to the utility. My system consist of 3400 watts of panels, 60 amp Renogy MPPT charge controller, a 4KW Schneider 4048 inverter and the 48 volt 280 AH lithium battery. The charge controller and the inverter are connected to the battery.

I can easily switch between inverter power and utility power through a relay. I programmed an arduino to control the system. This works by the battery voltage. When the battery voltage drops to 48, the system switches to utility power. If the voltage rises above 50, it switches back to the inverter.

This worked well with the lead acid batteries, but will it work with lithium?

What happens if the BMS detects that the batteries are overcharged or at the limit? Does it open the circuit to the batteries until the sun goes down and the voltage from the charge controller drops?

If it opens the circuit to the batteries, will the charge controller feed the inverter with the batteries temporarily disconnected? Or do I need to arrange to shut down the charge controller if the voltage gets too high?

 

[chrisski]

What happens if the BMS detects that the batteries are overcharged or at the limit? Does it open the circuit to the batteries until the sun goes down and the voltage from the charge controller drops?

It will shut off oncoming current for an over voltage. Discharge should work.

These BMS limits should be set above what you charge to prevent the BMS from tripping.

I have not experienced an overvoltage, so I don’t know how it awakens. May requires the cell to settle down to a voltage before allowing discharge.

I have experienced overcurrent causing a BMS trip and that required me to disconnect and reconnect the charge source.

I’ve also experienced an over discharge and that caused me to bypass the BMS with a power supply momentarily which rest the BMS.

 

[chrisski]

These are my BMS settings on one build. This has trigger and release settings. I’ve never tripped this one.

The BMS I described that tripped was on a different build and has no adjustable settings.

 

[Frank Smith]

Thanks! I have been looking in my BMS software. It uses different words and I'm trying to match them to the ones you sent. For example there is an entry Monomer high voltage alarm 3.5 Volts and monomer overvoltage protection 3.65 V. These may correspond to your Cell over voltage release and trigger voltages

 

[chrisski]

I have 2 x 280 ah 8s 24 volt batteries in the BMS pic above.

These are my charge settings on the MPPT for that battery:

I charge to 3.475 volts per cell, about 95%, even though max for a cell is 3.65. This came to an MPPT setting of 27.8 charging.

Since I don’t charge above 3,475, I set the cell shutoff to 3.54, with a release of 3.5.

For me, I set the float the same as absorption so that my batteries would charge when there was sun from solar.

Sun came out and the battery would charge. With a lierr float, I found when I had it set lower, I’d be in a situation where I was using 500 watts, but panels were charging less, perhaps 300 watts, which slowly depleted the pack, the sun would set and I’d go into the night not fully charged, and lower than I’d feel comfortable.

I raised the float voltage to where the battery would stay at 100% SOC when the sun would provide.

Some say this higher float voltage will cause the battery to not last as long, but going into a cold night with a low SOC and having the battery die and the heat go out has its downside.