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AGM storage and Solar question please.
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Hello All, I am hoping someone might have done a bit of thinking on this question, who has greater insight on AGM chemistry than I. My questions surrounds storage of AGMs and solar charging. I have noticed our Victron Multiplus 3000, when plugged in at the dock for extended periods, stores our AGMs at around 26.6v noting “Keeps the battery at reduced constant voltage to limit gassing and corrosion, slow self discharge is prevented by an automatic refresh of the battery with a short absorption charge.” When away from the boat, I generally unplug from the dock, because of lighting issues, and the false sense of security being dependent on the dock breakers, wires, etc as someone is not looking at the boat every day. And we have plenty of solar-which I regard as more reliable than the mains source. So the question is: when away for extended, multi month-long periods is it worth reprogramming the MPPT to mimic the Multiplus at 26-27v as opposed to the normal MPPT cycle of 28.4v absorption and 27.0 float? Very appreciated, Porter McRoberts IBIS A54-152 Newcastle, AUS |
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HiPorter,
Not having any great insight on AGM chemistry, even though I have had AGM batteries for more than a decade, I have ALL the charging devices on Elyse set to exactly the same Absoprtion and Float voltages. So whether Our Victron Skylla 100A charger, or Pronautic 30A chrager, is on shore power, the engine alternator is charging controlled by its SMART regulator, and/or the Victron MPPT 150/35, the charging characteristics are all the same...and for our batteries that is 28.8V Absorption and 26.6 Float ...which, of course, is battery temperature dependent and all our chargers have temperature sensors. Why would you want different voltages depending in the charging source? Seems to me that to look after the bateries as best one can, the voltages should be consistent. Cheers Alan Elyse SM437 |
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Bill Kinney
Porter,
The need for a regular return to absorption is less important for AGM than it is for Flooded batteries. With a flooded battery the acid can stratify over time. The more concentrated acid sinks to the bottom of the cell, and lighter, less concentrated, acid floats on top. Bringing the battery to absorption voltage where it 'gases" for a bit mixes the acid again. Since the acid in an AGM is not a free flowing liquid this isn't an issue and they are fine holding (more or less) indefinitely at the manufacturers recommended float voltage. BUT, if solar is your only charge source, you aren't really "floating" but cycling--a little bit--every day. If you have Victron MPPT controllers, their default setting for "variable absorption" will work well as the only charging source. They look at the system voltage when they first wake up in the AM, and estimate the battery SOC from that, then pick an absorption time to match. I don't know that you can do much better than that unless you have a Venus system and can use the DVCC function. Certainly the risk of damaging overcharging is minimal. If you force the MPPT to stick to the float voltage only, you risk a slow walk-down of the SOC of the batteries with each night, or a string of cloudy days. The amount of actual charging that can occur at the float voltage is small. I think you are better off topping them up every day and keeping the batteries full instead of letting them drift at a lower SOC that you have less control over. All this assumes you have some significant power draw. If everything is off, and the normal power overnight power draw is just a couple of A-hrs, then it matters less. Bill Kinney SM160, Harmonie Le Marin, Martinique |
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Thanks Gents. Appreciate the response.
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The Victron gear is pretty amazing. I’ll just leave it on standard settings. That was my concern with unlimited charging and no discharging I didn’t want to damage the batts. Porter McRoberts S/V IBIS A54-152 Newcastle AUS WhatsApp:+1 754 265 2206Www.fouribis.net On Jun 12, 2022, at 12:33 AM, Bill Kinney <cruisingconsulting@...> wrote:
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