RCinFLA
Solar Wizard
- Joined
- Jun 21, 2020
- Messages
- 3,562
Many BMS's that have added low temp charge prevention have an issue on how it is implemented.
It is very easy to add the firmware to disable the series MOSFET of the back-to-back MOSFET pairs that are responsible for passing charging current.
Problem with doing this is the turned off MOSFET's body diode is then carrying the discharge current which adds voltage drop and heating during discharge. In this condition the maximum discharge rate can only be relatively low current rate or the BMS will get too hot.
To fix this, the firmware detects when the discharge current through the current shunt resistors is too great to be handled through the MOSFET body diode (greater than about 5 amps) and reactivates the MOSFET responsible for carrying charge current so the discharge current can be normal BMS rated current.
If low discharge current, or a charging current condition reoccurs, it shuts down the MOSFET responsible for charging again, going back to carrying discharge current through the MOSFET body diode.
Most BMS MOSFET gate drive is low current driving strength. This means turn off and turn on time is relatively slow, a few milliseconds. In addition, the current sensor's ADC must read and interpret the current. All this takes a little time which can set up an oscillation problem in the control feedback loop.
Oscillation can occur when a secondary charger, like PV SCC, and/or inverter load variations, are such that thrashing charging and moderate discharging currents is impressed on battery when it is in low temp charge prevention mode. It will show up as sporadic short charging periods at cold temps.
It is very easy to add the firmware to disable the series MOSFET of the back-to-back MOSFET pairs that are responsible for passing charging current.
Problem with doing this is the turned off MOSFET's body diode is then carrying the discharge current which adds voltage drop and heating during discharge. In this condition the maximum discharge rate can only be relatively low current rate or the BMS will get too hot.
To fix this, the firmware detects when the discharge current through the current shunt resistors is too great to be handled through the MOSFET body diode (greater than about 5 amps) and reactivates the MOSFET responsible for carrying charge current so the discharge current can be normal BMS rated current.
If low discharge current, or a charging current condition reoccurs, it shuts down the MOSFET responsible for charging again, going back to carrying discharge current through the MOSFET body diode.
Most BMS MOSFET gate drive is low current driving strength. This means turn off and turn on time is relatively slow, a few milliseconds. In addition, the current sensor's ADC must read and interpret the current. All this takes a little time which can set up an oscillation problem in the control feedback loop.
Oscillation can occur when a secondary charger, like PV SCC, and/or inverter load variations, are such that thrashing charging and moderate discharging currents is impressed on battery when it is in low temp charge prevention mode. It will show up as sporadic short charging periods at cold temps.
