I have a pair of solar panels, each wired to its own MPPT controller.
I have LifePo batteries and I’ve configured the MPPTs to use 14.25 as the absorption voltage and 13.25 as the float voltage.
I also have a generator powers a MultiPlus II - what I am trying to understand is why do the MPPT controllers switch to ABSORPTION stage when the genset/multiplus has been running? When I run the genes I am charging the batteries from appx. 45% SOC unto about 70% SOC, this ends up driving the batter voltage up to about 13.7, which is clearly above the 13.25 float voltage.
But, if the batteries never reached 14.25, why is the MPPT switching to Absorption stage?
The swap of the MPPT from bulk to absorption is not based on the battery voltage, it is based on the voltage at the output terminals on the MPPT. If you have a voltage drop of 0.55V between the MPPT output and the battery then when the battery is at 13.7V the MPPT output terminals will be at 14.25V. I suggest you check the voltage drop when the MPPTs are running at high charge current with a multimeter on the battery terminals and then the MPPT output terminals. Using a multimeter will eliminate any reading differences between different devices. Then check reasons for high voltage drop, too thin wire, poor contact, loose bolts / screws, loose crimps, resettable breakers not making good contact.
0.55 is about 3.8% of 14.25 - I suppose this is possible, since the MPPT does not have a separate voltage sensing wire to the battery. Wouldn’t that (if it was possible) allow the MPPT to know more accurately the battery voltage and not change to absorption stage prematurely?
I will have to verify this with the multimeter when I have a chance (access is restricted, so I need to move a lot of gear out of the way to get to it).
I will check this next time it happens. Unfortunately, when this occurs then only way (I know of) to get the MPPT to reset is to power it down and re-start it, via a cutoff switch, this also erases history data.