Victron Energy Battery Balancer, auction is for one unit that can balance up to 2x 12v  batteries in series, or of several parallel strings of series connected batteries, or i have 3 units that can balance up to 4 batteries in series, or of several parallel strings of series connected batteries.

The problem: the service life of an expensive battery bank can be substantially shortened due to state of charge unbalance
One battery with a slightly higher internal leakage current in a 24V or 48V bank of several series/parallel connected batteries will cause
undercharge of that battery and parallel connected batteries, and overcharge of the series-connected batteries. Moreover, when new cells
or batteries are connected in series, they should all have the same initial state of charge. Small differences will be ironed out during
absorption or equalize charging, but large differences will result in damage due to excessive gassing (caused by overcharging) of the
batteries with the higher initial state of charge and sulphation (caused by undercharging) of the batteries with the lower initial state of
charge.

The Solution: battery balancing

The Battery Balancer equalizes the state of charge of two series-connected 12V batteries, or of several parallel strings of series-connected
batteries.
When the charge voltage of a 24V battery system increases to more than 27,3V, the Battery Balancer will turn on and compare the voltage
over the two series-connected batteries. The Battery Balancer will draw a current of up to 0,7A from the battery (or parallel connected
batteries) with the highest voltage. The resulting charge current differential will ensure that all batteries will converge to the same state of
charge.
If needed, several balancers can be paralleled.
A 48V battery bank can be balanced with three Battery Balancers.

LED indicators, Alarm relay