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mondeoman avatar image

Disable "Repeated Absorbtion" for a Multiplus

How do I disable the "Repeated Absorbtion" for a MultiPlus Compact?

It's a Stand-by UPS application with a drop-in 12.8V Lithium battery with built-in BMS.

Floating at 13.4V keeps batteries fully charged. I'm thinking I should take them even lower than that, to prolong battery life.

No need to repeat Absorbtion, it's just pointless stressing the batteries, even at 45 days.

MultiPlus Quattro Inverter ChargerLithium Batteryabsorption
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2 Answers
JohnC avatar image
JohnC answered ·

Hi M. If you don't need Abs *at all*, just reduce it's setting to the same as Float, so in effect you won't see any change, just a flat V.

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Yes, set the float the same as absorb so float becomes the indefinite CV at your lower cell voltage. The only way you can 'Float' a Lithium.

Going on the demo for the Multiplus you can't set absorb and float exactly the same. It sets a 0.10v if you try. You can however get to a 0.01v offset below. I seem to remember that being the case with the MPPTs as i wanted to test out how it would work out. Now you can even set the float above the absorption witch is interesting.

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mondeoman answered ·

Thank you all for your replies.

My Float setting is already lower than what is considered normal for Lithium. And I was thinking of taking it even lower than that. I don't mind if SOC hovers at 80% if that helps to prolong the battery life.

But if I set Absorb almost the same as Float, when power is restored after a blackout, the initial re-charge will take ages...

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If you set the float below the absorb voltage what that is doing is shutting off charge.
it is not a float at all. current cannot compensate for any small self discharge. It will only become active if the battery voltage falls to equal that of float voltage where the cells will take on some current. re-bulk would kick in before that though.

With lead acid you have bulk, absorb, float all of those voltages exist above the lead acid resting voltage when charge is not applied.

With Lithium you have CC constant current ( bulk) up to the equal voltage as charge voltage then you have CV, now this could be an indefinite absorption or float equal to absorption voltage. They are both 'constant voltage'. Provided the voltage is below the resting voltage of the cells when there is no charge applied then it is not a forced charge. The cells just sit equal to the charge voltage. they only take on the current that they lose through self discharge.

I am not an expert but form what i have read a big factor in going for a lower voltage is cell temp. When cells have been temp controlled researchers have keep them at a higher voltages mitigating degradation.

lithium ion phosphate is already more stable than NMC it has a greater high cell voltage tolerance. There is more head room so to speak of the voltage that the cells can be taken to and where the capacity lies. I would suggest just focus on keeping the cells away form heat and at stable temp as the more important factor here.

The other thing that has come up as a consideration when using lower voltages for UPS applications is cell balancing. BMS systems tend to only balance cells at higher voltages / state of charge. You will have to get info on that form the battery manufacture. I am afraid i don't have the best answer regarding that. When cycling my cells i tend to run them up to a full state of charge max cell voltage at random intervals just so that the cell balancing of the BMS kicks in.

You kind of have to be realistic about function and longevity. They are preforming a function so you can't place longevity too much over function to the point that they no longer serve your need. The cycle life is not like lead acid they do remain usable longer after they have met their rated cycle life.