question

stonge avatar image
stonge asked

Tail current-based absorption?

I've got an AGM battery bank on a boat, and the technical specs say that to hit 100% I should charge them at absorption voltage all the way to a 6amp (0.5%) tail current. I've got a smart BMV to monitor the bank, and it can track the system current, but the older Multiplus I have isn't networked, and only seems to charge based on an algorithm, so when I'm running the generator, it always goes from absorption to float well before the system current gets down to +6 amps.


Can newer Multiplusses stay in absorption based on tail current? If not, what setup should I be using with a generator to make sure it stays in absorption until the current gets to the right value?

battery charging
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2 Answers
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pwfarnell answered ·

Tail current to end to absorption has NOT been implemented in the Multiplus in newer firmware. The only way I can think of to work around your issue is to swap the multiplus from "adaptive" charging to "fixed" charging and then set an absorption duration to get to 0.5% tail current by trial and error.

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stonge avatar image stonge commented ·

Ah, got it, that's disappointing. I suppose I could try the "forced absorption" switch if I see it go to float too early, but then I need to carefully monitor the charge. Does Victron make other charging products that can charge based on tail current? Should I be using a separate charger and inverter to get that functionality?

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pwfarnell avatar image pwfarnell stonge commented ·
I know that the MPPTs have this function. However to use it properly you need a shunt on the battery and a GX device, or a Smartshunt may Bluetooth network to a Smartsolar but I am not 100% sure on this later point as mine is networked by a Cerbo.
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stonge avatar image stonge pwfarnell commented ·
I do have the Smart BMV and Smart MPPT combo for solar, so I think I've got the current-based absorption timing there, but I wish I could also have it for my generator- and shore-power-based charging, too. Any idea if the EasySolar devices that combine MPPT and Inverter/Charger somehow get the updated functionality of tail-current monitoring when charging with an AC source?
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JohnC avatar image
JohnC answered ·

Hi @Stonge

A side issue to your question..

Your figures frighten me. 6A at 0.5% suggests 1200Ah of battery. Is that right?

Victron solar mppts have that Tail function, and I use a ~3% Tail to drop to Float. At that they're 99.0% done. Flooded batts, but I would never consider pushing them that far. I have seen them down to 0.5%, but that's at a much lower Float V, and there's a big difference in the Current they can accept at Float. My shunt is set to sync to 100% at 0.6-0.7% at Float V.

And it takes hours at Float V for the current to get that low. A genset running all day at idle to get there isn't practical.

I sometimes shake my head at the figures batt makers publish. Take care..

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stonge avatar image stonge commented ·

Hi @JohnC, thanks for double-checking!


Yes, it's a 1200Ah battery bank and they're AGM batteries. I agree it's not always necessary to get all the way to the manufacturer's spec, but in this case, they really do specify charging at absorption voltage all the way until the charge current is 0.5% of the rated capacity to reach/validate 100% for these batteries, and the Multiplus seems to be going to float at closer to 2% or higher, especially if the batteries are already around 95% when I start a charge.


You're totally right that running a generator at idle for hours isn't practical, and shouldn't be the every day use case, but for my setup there are times where I want to make sure the batteries do get close to 100%, and I'd rather push absorption voltage for as long as I can given the spec instead of wasting energy in float.

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