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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?
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
(I accidentally put this as an answer, moving it to a comment on your answer)
Ok so:
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.
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.
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?
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.
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?
