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deeperid avatar image
deeperid asked

24v cells in series out of sync in 48v system, can 2 MPPT controllers balance?

I have 2 24v batteries connected in series for a 48v system but one of the 24v is getting more charged than the other. Currently I have 2 150/35 MPPT controllers in parallel charging the 48v battery and it's monitored by a BMV-712 which needs to be the only thing connected directly to the negative 48v terminal to track the current in & out. The negative terminals for the solar panels & the MPPT controllers (both for solar in and the other for battery out) are connected to the chassis (to reduce loss from heat/save wiring, this is in a motorhome - side question: is this helping or hurting?).


  1. If I connect the +/- terminals of one 24v battery directly to the +/- of one MPPT controller (and do this to the other 24v battery too), will this work or short? If so, how is the + terminal in the middle (of the 48v set) connected to the - of the other battery protected against a short when that - terminal is connected to the - of the MPPT controller?
  2. Should I use a battery balancer for this issue instead?
  3. Whether balancing with a balancer or using the MPPT controllers on the individual 24v batteries, how will the BMV accurately track current unless it's the only thing between the battery and ground?
MPPT ControllersBMV Battery Monitor48v battery
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2 Answers
Alexandra avatar image
Alexandra answered ·

@DeeperID Taking them out the system and charging them separately is a good idea. It will help you see if one one them is not doing so well. Then put them back together and let them equalise themselves.

You dont say what batteries you have but a battery balancers are designed for 12v batteries.

The bmv will only track the one battery if you separate the two electrically to charge them.

I have never tried your idea with connecting the two solar chargers while they are still part of a system; maybe somone else can answer this definitely. But I dont see why it would not work to charge the batteries as long as the voltage etc in the solar charger is programmed correctly. The BMV would not be able to track both charges.

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JohnC avatar image
JohnC answered ·

Hi @DeeperID

From a previous post I suspect the batts are Li's, and the bms should deal with this. But some aren't compatible in series strings (including some Victrons). So like Alexandra suggests, maybe charge them separately to balance them periodically.

What you're thinking has the issue that you'd be creating an 'earth' of 24V on one mppt, and that would even get passed back to the panel -ve. I don't know what would happen, nor do I want to try it to find out..

What you *might* be able to do is charge at 24V over the first batt on the -ve end, and 48V with the other mppt over both, concurrently. If you had an inherently weak batt, make that the 24V one.

But I can envisage issues doing this too, so take care if you attempt it.


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Related Resources

MPPT Product Page

MPPT Error codes

MPPT 150/60 up to 250/70 Manual

Victron BMV battery monitors product page

Additional resources still need to be added for this topic