question

kimsails avatar image

Controller damage if the battery is disconnected but PV is not?

Hello,

I have two SmartSolar MPPT 100/20s that I am planning to install. The Battery is 12V 4S LiFePO4. I would like to use a BMS to disconnect charge sources if the BMS detects an over-voltage on one of the four battery cells. If the battery-side of the controller circuit is interuppted by the BMS, but the Solar PV circuit is intact, will the controller suffer damage?

I could add two relays, one to each controller, to also interupt the PV circuits at the same time. However, I am wondering if this is really necessary?

Thanks,

Kim

MPPT - Solar Charge ControllerBMSovervoltage
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2 Answers
Daniël Boekel (Victron Energy Staff) avatar image
Daniël Boekel (Victron Energy Staff) answered ·

Hi @kimsails

This shouldn't hurt the controller, but it can cause a short voltage peak if there was still current flowing into the batteries. The unit is not designed to have the battery disconnected under load/charge.

The recommended way is to use the https://www.victronenergy.com/cables/ve-direct-non-inverting-remote-on-off-cable for the 100/20, or the remote on/off terminal on the larger models. this is only possible if your BMS has (charge enable) control outputs.

Please note that although a BMS can disconnect a battery, under normal circumstances this shouldn't happen. The battery should be charged to a voltage where the BMS doesn't disconnect, and also discharged not too far so the battery doesn't disconnect on 'low cell alarm'.

The only reason a BMS can disconnect at 'normal charging voltage' is when a battery is out of balance, this should really only happen with new batteries, or batteries that haven't been used for a while.

To 'pre-balance' batteries it's a good idea to keep them charging to the prescribed voltage at low current for a while, this gives the BMS some time to get the cells balanced. The battery manual might have information about this.

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Thank you Daniël and Lange for your help. The documentation for the cable seems to show it specifically being plugged into a Victron BMS. Can I use the cable if my battery and BMS are not Victron? If so, does the sign of the voltage applied to the cable to turn on the controller matter, as long as it is between the 6v-70V limits?

I agree that a cell-level BMS over-charge or over-discharge situation should be rare. This will not be the primary means of maintaining the battery. I have balanced the cells and plan to check them regularly. However, the application is a sailboat and it may be left unattended for a week, maybe two at a time. Unattended, the battery will have very minimal loads. And a charger the will be set to maintain a mid-level 13.2-13.8 voltage on the LiFePO4 battery. No reason to expect that these unattended conditions will trigger a cell-level BMS issue. But the LiFePO4 battery is not cheap and I am looking for insurance to protect the battery in case something goes awry.

Thanks,

Kim

Matthias Lange - DE avatar image
Matthias Lange - DE answered ·

It can damage the charger we had this a few times. Better use the cable @Daniël Boekel mentioned

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