question

colin avatar image
colin asked

Will a Victron MPPT start up a Lithium Superpack from a low voltage cut off state?

Hello, I hope you're all having good holidays!


I am planning a portable power supply along the lines of the picture below.

My concern is what will happen when the superpack discharges until the internal bms cuts off due to low voltage?

I suspect the MPPT will not sense a battery voltage and hence will never commence charging the battery?

I'm guessing that the best thing to do would be to use a Battery Protect to isolate the loads before the internal bms cuts-off - thereby ensuring that the MPPT is always able to charge.

However, if it *would* work ok as per the drawing below, without a Battery Protect, I would prefer that - due to simplicity, space and cost considerations.


SuperPack
equipment-connection-cb00.png (29.2 KiB)
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1 Answer
Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

Hi @Colin

When the sun is shining, the MPPT will start pushing voltage to the DC bus, even if it is 0V.

So you do not need to worry. Once there is voltage on the DC bus again (provided by the MPPT once the sun is shining) then the super pack's internal protection will reconnect the battery.

So while it is not necessary, there is still some merit in having a Battery Protect in front of the loads, it will mean the BMV maintains power, which is very useful.

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colin avatar image colin commented ·

Thanks Guy for this.

That's great about the mppt!

My next thought now is - what happens if the loads (various possible combinations of LED lights, dc fridge and/or inverter) remain connected? When solar is low the mppt might not have enough power to maintain ~12V under the loads so would cut in/out? Or the voltage would fluctuate? Neither of which would be good for the loads.

Indeed, it would be good not to lose power to the BMV...

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ colin commented ·

That's another reason to use a Battery Protect to disconnect the loads before the BMS cuts off.
It is not so good for the battery ti get discharged so deep everytime that the BMS have to cut off the loads.
Connect the MPPT(and the BMV) directly to the battery and all the loads behind a BP and setup the BP to cut off the loads before the BMS has to do it.

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colin avatar image colin Matthias Lange - DE ♦ commented ·

Yes, that looks like the sensible thing to do!

Thanks Guy and M.Lange for clarifying

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