One of the in-efficiencies of Lead-Acid batteries is that a current run through the battery even when fully charged and the Volt never grow. This also accomplishes balancing cells or batteries in series over time. When it comes to Lithium chemistry is the efficiency very high and no current bleed through without the volt increase. Using individual Lithium cells for a battery requires a very precise solution to balance each cell, this can often be solved with devices intended for managing battery packs for remote-controlled airplanes. When looking at a series of whole modules or batteries is devices not that easy to come by and the amount of energy needed to consume can be much higher. This could be the case if using two Tesla Modules in series for a 48 Volt battery or building a 24 Volt battery from two Battleborn 12 Volt batteries in series. Even these modules are very similar can many charges and discharge cycles bring this out of balance so one part charges higher than the other.
We have a 24 Volt Battery Bank in the trailer consisting of 2x2 12 Volt Battleborn batteries. A solution, of course, could be to get 24 Volt batteries to begin with but based on the wish to have batteries that could the "borrowed" to other in need of a 12 Volt battery and the fact the trailer is all aluminum and conductive we wanted to have a fuse in the middle of the battery box that could cut any shorting of a cable.
This setup now has the potential to get out of balance and over more than 2 years of hard use and abuse was that the case. One part of the battery never charged more than 80% State of Charge and when drained ran out of power when the other part still had 20% of the Energy left. The graph below is a day when the State of Charge went to 20% (Yellow) on the whole battery bank, the voltage (Blue) started dropping fast because of one side being almost drained. If this had continued much longer has the Battleborn battery on one side disconnected to protect the internal cells.
The Battleborn Battery Bank is charged to 28.8 Volt or 14.4 Volt for each half when balanced. If the Voltage on one side gets to be over ~15.2 Volt is the internal protection of the battery disconnecting, this happened all the time during the Absorption phase of our charging. The 15.2 Volt caused the other side to be 13.6 Volt and that is not enough to finish the full charge of a Battleborn battery so we ended up with only one had of the battery fully charged and this kept on being worse and worse for each day.
One of the biggest problems balancing the whole battery pack is the amount of energy needed to remove and the higher voltage. With simple voltage protectors and large wat resistors bought from Amazon was a simple system built able to handle several Ampere of balancing current. The devices are Voltage protector and cut the power on the output based on the Input Voltage. The protector has been programmed to connect the resistor if the Volt is over 14.7 Volt and disconnect when it is below 14.5 Volt. This ensures the balancing only happens on one part of the battery bank and always only during the absorption charge cycle. Because the system was so unbalanced, to begin with, was the standard 100 Ohm resistors for one side put in parallel with a 20 Ohm resistor to speed up the balancing. When the Battery Bank is now balanced can the 100 Ohm resistor easily keeps it balanced.
After the balancing was the battery bank again drawn down to 20% State of Charge (Yellow) an the Battery Volt (Blue) stay much more stable.
The balancing is going to be mounted permanently on the Battery Bank with some wires with small fuses connecting to the Terminals.
This approach can also be used when building a 48 Volt Battery Bank from Tesla Modules. Each 6S Tesla Module is balanced with simple balancers and each 24 Volt half balanced with a simple resistor setup. The precision for the cells needs to be 0.01 Volt or less, but the modules can be balanced with a 0.1 Volt precision that this simple system can accomplish.