Victron Multiplus 12/3000/120 and SOK Batteries in Parallel:
It has taken me a while to wrap my head around what seemed to be a serious problem with the MultiPlus and three 206 AH Lithium Iron Phosphate batteries. Almost everyone who speaks about setting up charge and inverter settings for these batteries only approach the settings for a single battery. With a single battery the settings published by SOK and the video presented by Current Connected works perfectly. The single battery can be fully charged and discharged without issue.
The charge problem only becomes apparent when more than one battery is involved. My battery bank consists of three SOK units arranged with the positive lead connected to the first battery and the negative lead connected to the last. I have observed that each battery charges and discharges at a slightly different rate. Three separate batteries being controlled by their own independent battery management system is when the charge cycle becomes much more complicated. Under a 120 amp charge (40 amps per battery as recommended by SOK) the charge cycle works great until the batteries begin to reach their maximum state of charge. It is then that one of the batteries will reach full charge first and enter an overcharge state on one cell that will shut that battery off and not allow any further charge. This causes the other two batteries to experience an elevated charge rate which puts another one in an over charged state causing the voltage to fluctuate enough to temporarily shut the MultiPlus down and occasionally go into a fault condition. This generally occurs when going from bulk to absorption.
The only solution I have found to counteract this situation is to lower the charge voltage to a level that will allow the MultiPlus charging to naturally terminate without pushing any of the cells into an over charge situation. Currently my absorption voltage is set to 14.30 which seems to allow the charging to complete and move into absorption and float without pushing any of the battery cells too far. I have also lowered the charge amperage to 70 (23.33 amps per battery) to prevent pushing a single BMS and battery beyond BMS specifications for charge rate in the event two batteries are shut down by their BMS.
Since changing these voltage and amperage settings I have not had any issues charging and none of the batteries have been turned off by their respective BMS and it takes about eight hours for a full charge. I plan to periodically charge each battery independently with a bench power supply to enable the BMS to do it’s top balance task at the specified 14.4 volts.
In my opinion, there's no economical way to bring all three batteries up to full charge in a parallel system since they arrive at full state of charge at different times. The voltage has to be reduced, through trial and error, to meet the cell with the lowest resistance. There also does not appear to be a way to charge the bank at full recommended amperage since that will put the last active battery in an overcharge situation possibly damaging the BMS and battery.
I am new to Lithium Iron Phosphate batteries and would appreciate input if I am missing something.