Best way to balance a large AGM yacht house bank

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keith7wa asked Apr 5, '24

Best way to balance a large AGM yacht house bank

I'm refreshing the batteries on a large yacht battery bank used for house loads. I'm using 12 Victron 12v 200Ah deep-cycle AGM batteries (BAT412181164) wired in series/parallel to create a 24v 1,200Ah bank.

As background, I am aware of Victron's official (but sometimes disputed) recommendation to limit parallel strings to only 3-4. However, it's common practice in the boating world to build house banks that exceed this and most, including myself have experienced little trouble and long life from such banks. Also, I am fairly well-versed on Lithium and have opted to stick with AGM in this application for various reasons.

My question:
I know that keeping the bank balanced when charging is key to performance and long life. In my application is there a way to achieve this using fewer than 6 Victron battery balancers? (which would be 1 for each series pair)

AGM Battery Battery Balancer yacht

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Answer 1 · 2024-04-06T01:14:06Z

Paul B answered · Apr 06 at 01:14 AM

No you cant do it with less. any way good luck with your project

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Answer 2 · 2024-04-06T03:40:26Z

kevgermany answered · Apr 06 at 03:40 AM

You have 6 strings of 2 batteries.

You can join mid point of pairs of strings and use one balancer per pair of strings. Making 3 pairs of strings would mean 3 balancers. There's an example in the manual, from memory it shows 6 batteries set up this way.

Question is can you reduce to 2 balancers with 2 sets of 3 strings. I think so, but balancing would take longer as the balancers only pass a low current and only work once string voltage is over 27V.

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keith7wa commented · Apr 06 at 07:16 PM

Thank you @kevgermany - I did have another look at the manual and that makes sense. 3 balancers is perfectly do-able, 6 would be cumbersome.

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Answer 3 · 2024-04-06T07:04:55Z

JohnC answered · Apr 06 at 07:04 AM

@Keith7WA

Victron balancers are designed for use within string(s), not between strings. Your issue is controlling current between strings.. Eg. you may well have designed the charging of your 6x 200Ah strings for a lazy 0.1C charge rate for longevity. And maybe 0.2C as peak. Trouble is, you don't know (nor can control) what each string accepts. So it's conceivable that one string might be doing 0.5C, and the others picking up the remainder - and you don't know.

Granted that this is a (too) common practice, and many get away with it. But then the aggressive type sees a max 14.6V Absorb on the spec sheet and applies that to the whole bank. Then goes off whining about the brand and demanding warranty for his failed batts.

Li batts are even more susceptible to this, that's why they have a bms. Pb's don't.

You've chosen to build a bank with an inherent weakness. Best be gentle with it, Charge A and Absorb V at the low end of the spec sheet (or even lower), and any sign of one batt's Temperature rising higher than the others is a bad sign.

And your wiring will need to be 'perfect'. It's all about string resistance, and the resistance of the batts is down there at the level of the wires themselves. Eye opener - http://www.smartgauge.co.uk/batt_con.html

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keith7wa commented · Apr 06 at 07:27 PM

Thanks so much @JohnC. That smartguage doc is very helpful. Our plan is to connect using Method #3, using bus bars and equal length cabling to control for resistance differences, at least as far as the cabling is concerned. Wired in that manner my hope is that all the batteries should be subjected to as even a discharge/charge experience as possible.

If one needs a large Ah battery bank built with AGM's is there a better way to do it?

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JohnC ♦ keith7wa commented · Apr 07 at 11:38 AM

@Keith7WA

2V batteries is one way. Might cost more overall, but they're usually designed for long life.

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Answer 4 · 2024-04-06T15:33:45Z

frankmsc68 answered · Apr 06 at 03:33 PM

@Keith7WA..hi there, i have got for about 6 years a bank of lead carbon batteries.Twice a year i do a balancing in this way: I leave the batteries connected in parallel only(taking away the series connections) and bring them to a complete charging cycle and leave them for some hours to balance with each other...this i keep on doing with the rest of the batteries.I don't pretend(i have got 4S3P Setup) to have them all exactly balanced,but as close as possible.Like JohnC said it is very important to handle those rather large banks with care to avoid overcharging or overstressing the weekest links.....Best regard Frank

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keith7wa commented · Apr 06 at 07:29 PM

Thank you Frank. Sounds like the best we can hope for is "well balanced" and not "perfectly balanced"!

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