wolters avatar image
wolters asked

Advice in solar charging 36 battery bank (3x12V)

I have been asked to come up with a solution to solar charge an 36V battery bank.

Current setup:

3 x Victron (110Ah) BAT412101084 Connected in series to provide 36V

the charger is a Pro Sport 20 Plus (mains powered charging 3 batteries individually 12v)

What I like to do is install 3 solar panels 12V, each connected with a SmartSolar charge controller MPPT 75/10 (12V). So each unit will charge each battery individually (like the current installed charger)

My questions:

Would there be an advance/disadvantage in connecting the panels in series and connect a MPPT 150/35 in a 36V setup and charge all batteries in one go?

Will the 12V setup be powerful enough to charge the 12v Victron 110ah batteries

battery chargingSolar Panel
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2 Answers
JohnC avatar image
JohnC answered ·

Hi W. It's not impossible to do what you're thinking, but I doubt you'll find anyone recommending it. Imbalance is an issue with most batt types, and you'd be leaving yourself open to early failure in less than perfectly matched conditions.

You must've done your homework, cos the 150/35 is indeed Victron's smallest that can do 36V. It's bigger than you'd need, but that's what I'd do.

Your '12V' panels are actually meaningless with mppt units, and with the 150/ rating that mppt won't care with a single string of maybe 3x '24V' panels. But come back if you go that way.

The batt's recommended max charge is 0.2C. So @ 110Ah thats 22A, or about 800W of panel nominally @ 36V.

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wolters avatar image
wolters answered ·

John thank you for your reply.

So your advice would be 3x 24v panels with the 150/35 (MPPT) and charge the whole 36v bank in one go. And I can see this is a good solution (would 4x12v panel be a good option too?)

I did try to find the advantage/ disadvantages of individually charging the 12v batteries in a 36v bank vs charging the whole bank on its own. Personally I thought having a individual charger for each battery would be better as this monitors the battery on its own keeping each of them in their most optimum charge, but I take any advice I can get on this.

And what you say I should supply the 75/10 with a 24v panel, to make better use of the MPPT function if I still want to charge them individually.

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JohnC avatar image JohnC ♦ commented ·

Yeh, the 150/35 is the way I'd roll. Batt imbalance is a serious issue, and is the reason why exotic Li's come (or should) with a BMS, and why Victron manufacture balancers for Pb's. I've served my time with imbalance, and the thought of it sends my spine a'shiver.

I won't delve to the theory, it's quite complex. But if you're discharging a bank at 36V, best to charge it at 36V, *especially* if both are happening at the same time.

Nice too if you can string your panels with a comfortable V margin between Vbat and Vmp. Poor light can reduce Vmp to well below panel specs, and it's best to harvest what's available rather than wait for better light.

So yes, if you went with 3x 75/10's, a '24V' would be better on each, and if the 150/35, 4x '12V' panels would be better than 3x in poor weather especially.

Gotta drag yourself away from the ole PWM thinking, where '12V' panels were used for 12V batts. Mppt prefers more V, to extract it's better performance fully.

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wolters avatar image wolters JohnC ♦ commented ·

Thanks John, I appreciate your help

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