krby avatar image
krby asked

MultiPlus charge settings for third party LiFePO4

I've got a 8S LiFePO4 with an internal BMS (I can't tap into it). For LiFePO4 and other lithium chemistries, I'm used to a CC/CV (constant current then constant voltage) charge cycle. What I would like is to get the MultiPlus to charge at no more than 50A until it hits 28V (3.5V per cell) (then hold that 28V until the charge current drops to 5A. I don't want any float stage or any periodic re-absorb.

Could someone give me advice on how to get as close this as possible?

Multiplus-IIbattery charging
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1 Answer
Paul B avatar image
Paul B answered ·

Give this a go and see if it helps

limit the charge current to max amps of 50 amps and set the float voltage to say 24 volts or the lowesty figure that you want the battery volts to drop to.

this will cause a max amps charge of 50amps until bulk voltage is reached then the current will slowly drop off and when the absorption time period ends float will take over and if thats less than the battery voltage the amps should be zero

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krby avatar image krby commented ·

(I accidentally put this as an answer, moving it to a comment on your answer)

Ok so:

  • charge current will be 50A, bulk stage will be the constant current phase.
  • absorb voltage should be 28V, my target "batt full" voltage. This will be the constant voltage stage.
  • Absorb time should low as possible? It sounds like the Multiplus only stops absorb based on time, not based on charge current (I'd like 0.1C, so 5A in my example). Could there be an assistant to do this?
  • float voltage is 24V or something lower than absorb so it never kick in during regular charging?

One concern I have is that during the float stage as you described it is that the MultiPlus isn't fully disconnected from the battery, even though now current should be flowing. It is my understanding that for Lithium batts, you really want to leave them alone after finishing the CC phase.

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ben avatar image ben ♦ krby commented ·

You don't have to leave them alone, although a low enough float voltage will effectively do so (until/unless their voltage drops below float, at which point charging will resume).

Paul's description is the only way to configure CC/CV charging with MultiPlus today. You cannot use a tail current for stopping the CV, you have to use a time value instead.

For the most part, it will still work fine, with careful adjusting of the voltage setpoints.

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tkrull75 avatar image tkrull75 ben ♦ commented ·

There is still a lot of current flowing in and out when technically the battery should be left alone during float stage - do you experience the same phenomenon?

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ben avatar image ben ♦ tkrull75 commented ·

If you set the float low enough, there will only be current flowing out of the battery for some time.

Once the battery voltage under load reaches the float voltage, yes, there will be current moving both ways.

The (old) advice to "not float lithium" is related more to not holding a high float voltage (as you might do for lead acid), rather than not allowing a charger to keep the battery at a medium-SOC state.

You should set a float voltage that holds an effective SOC of well under 100%, if your goal is to maximize lifespan.

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tkrull75 avatar image tkrull75 commented ·

Hi Paul/krby,

I have a very similar issue and am wondering if what you suggested was confirmed to work? I tried to push the float voltage in the multiplus-II to 48V (I have a 48V system) in order to turn it off - all that did was getting the inverter to pull as much current from the battery back into the grid in order to get down to the 48V - rest assured I quickly reverted it back to the 54V I have it set to. This means my battery still sits on float and I am not able to turn that off - are there any other suggestions?

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