rolandrrr avatar image

When battery disconnects, should solar disconnect to? What is the best way

I have had this problem both small (just one Victron SmartSolar 75/15 in a van) and large (large offgrid installation with Multiplus and Cerbo GX)

The problem is that you remove the battery because of an external reason, or the battery disconnects itself by the BMS, connected devices are still powered directly by solar. Which leads to unpredicting results. Not only from safety point of few, but also for the devices possibly not getting enough power and acting weird.

So in most cases you want the solar power to disconnect as well. I am thinking what the best way is to do this?

On my larger installation is a larger SmartSolar 250/85, which has a remote input (and no load output). This I want to let disconnect by a relais when the BMS kicks in and disconnects the battery. That way the system will go down. Is that the proper way? Perhaps there is an application note or some other document about this.

On the smaller system, the SmartSolar doesn't feature a remote input, so I should probably have a contactor/relay between solar and load output.

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The current way the system works is how I would like it to continue.
If I have a BMS fault but there is enough PV power to run my loads it should continue to do so. The fault might clear and the system return to normal without interruption. The multiplus will fault and turn off if there ins't enough PV power as the DC voltage will drop too low.

As for safety, you should have a PV disconnect that you turn off before you perform any maintenance or work on the system. So I am not sure what you mean by "safety point of view" Also on big systems you use the MPPTs to precharge the system before turning on the battery.

If you do want to turn off your PV if your battery disconnects then you should install a contactor on the output of your MPPT, this contactor is powered from the battery side of the contactor. So once on it will stay on but if the voltage drops too low it will turn off and stay off till you reset it. Keep in mind these contactors/relays will use 5-15W of power 24/7. If you can control this contactor with your BMS that would be best.
Lots of simple BMS have charge and discharge outputs. A CAN BMS or VE.Bus BMS would be better as @Mike Dorsett said.

@shaneyake I think t he contactor needs to be on the PV side of the MPPT, so as it powers the unit down, rather than leaving the output in a state where it can go over or under voltage. Contactor needs to be rated for the PV Voc voltage and Isc current, with 1.25 safety factor on the voltage. MPPT instructions say not to power unit without a battery connected.

I agree that the idea placement would on PV side, however contactors rated for 250V DC are really expensive and would unlatch every time the sun went down. This is for setup where BMS does not have outputs.

If BMS does have outputs I would use Shunt Trip Breakers, which are cheap and rated to 250-1000V DC.

Can you please link to the manual where it says not to power unit without battery?
I could only find the order instructions but nothing that says don't use MPPT without battery. I have a few diffrent systems that are direct-drive, no battery, that only run during the day. Mostly pumps, some communication equipment. Not a recommend setup but something we do, haven't had any problems yet. Pump controller adjusts load based on voltage.

1 Answer
Mike Dorsett avatar image
Mike Dorsett answered ·

The better way is to have the BMS feed back to thew MPPT's and regulate the battery charge current down as they approach full charge. This can be set all the way down to 0 amps. I have also arranged a Solar Disconnect relay on my system, whereby this will open if the charge is not regulated for some reason. This is an essential precaution as MPPT's should not have input with no battery connected. Have a look at


to see if this fits in your system.

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