Why does my multiplus keep trying to charge while its at or above float?

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motorboatin asked Mar 5, '24

Why does my multiplus keep trying to charge while its at or above float?

So, I have a used multiplus 12V 3000VA 120A inverter charger. I just got my lithium batteries in and I have configured the multiplus according to the battery specifications. I usually turn off the multiplus if nothing is going on or if I'm working on it. But when I turn it back on it immediately tries to bulk charge and then goes into absorption mode for the specified two hour time period even though the battery voltage is above the float voltage. Is there anywhere in the settings to stop this from happening? I just don't want the batteries to become abused by the charger unnecessarily.

LiFePo4 battery

Thanks in advance!

battery charging

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Al commented · Mar 06 at 12:37 AM

"I just don't want the batteries to become abused by the charger unnecessarily."

Are your charge settings recommended by the manufacturer?

2 hour absorption without a tail current will overcharge most LFP (unless for periodic balancing)

A float voltage of 13.8V (3.45V per cell) is what most use for the absorption voltage, LFP float at around 3.375V (13.5V) again you will overcharge LFP if it's left charging at 3.45V per cell.

I think your charger settings will abuse your batteries, not the charger, I'd certainly change them!

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Justin Cook answered · Mar 05 at 11:30 PM

When you turn it off and back on, you've reset its charge logic so it has to go through its cycle again. Nothing to be concerned about, it can't push current that the batteries won't accept and as long as you've set the voltages per the battery manufacturer's recommendations then you're fine!

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Answer 1 · 2024-03-05T23:35:01Z

motorboatin answered · Mar 05 at 11:35 PM

Hm. Figured it'd have a button cell or something for memory... Okay, thanks! I appreciate the quick response!

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Justin Cook ♦♦ commented · Mar 06 at 12:30 AM

If I recall correctly, the "Absorption Time" can be set as low as 1 hour if you'd like; depending on the battery bank, though, that may be inadvisable depending on their charge recommendations - ie Battle Born recommends .5hr absorption per battery or per 100Ah of capacity (though I can tell you're not on Battle Borns) so for example, a 400Ah Battle Born bank would call for 2 hour absorption period... your batteries may have a similar spec somewhere.

Personally I wouldn't worry about it, 2 hours at 14.2v even when unnecessary isn't going to adversely affect any reasonably-built LFP battery bank, but at least you know there's some configurability there if you want it!

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Al Justin Cook ♦♦ commented · Mar 06 at 10:56 AM

Yeah, 2 hours absorption is probably not too bad, but still not ideal at those voltages.

LFP chemistry is much better understood today, even though the goop inside (electrolyte) may vary a bit between manufacturer, they are all affected similarly by extended high voltage charging regimes. Which is where Victron's tail current settings can be really useful to minimise overcharge.

I don't have Battle Born, I have a 1020Ah DIY bank, so according to them I'd need a 5 hour absorption! Wow, I'd be really worried about lithium plating with their charge recommendations.

Unless, they have closed loop communication with the charger and just sit at absorption voltage for balancing with no charge current? But that should be an unnecessary length of time if the cells are reasonably balanced and the balancer works properly.

When LFP first came out 3.8V per cell was used for absorption, then 3.65V and now 3.45V - 3.5V, there's little benefit running so hard into the voltage knee. Victron recommends 3.46V for Pylontech with 3.4V for float. (52V and 51V for 15s cells)

LFP 'floats' around 3.375V, that's to say it can be 100% charged with zero amps charge current accepted at that voltage. Allowing it to sit floating anywhere above 3.375V with continuous charge for extended time when at 100% can cause cell bloating and lithium plating, and is generally considered a bad idea for longevity.

Each to their own, but here's a good bit of info on charging:

https://nordkyndesign.com/charging-marine-lithium-battery-banks/

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