Using a Tesla module from a Model S or X is very dense storage of electric energy. Three main things is to consider: Charging within specs, Keeping temperature stable and Balancing the cell groups. This is going to be about the Balancing of the Cell groups.
The Tesla module consists of 444 individual 18650 Lithium cells arranged in 6 groups of 74 cells. The individual cells are very similar but over time can the groups get out of balance. When imbalance happens is the voltage of each group different.
A Tesla module should never be charged to full voltage 25.2 Volt (6 x 4.2 Volt), any imbalance is going to force the group higher than the 4.2 Volt maximum per cell. Another reason to not charge to 100% State of Charge is degradation of the battery, In my opinion, is staying below 80-90% State of Charge a safe range (we use ~80% as full charge, 24.1 Volt for the module or 4.02 Volt per cell). Staying below 100% State of Charge both reduce degradation and leave room for imbalance.
Already at 1% imbalance is the last cell group entering dangerous territory. This is, of course, an optimal bad case for the calculation, when only one group charges too high. The Tesla modules are very balanced more than 99.9% is my estimate. Simply removing a small amount of energy each day from the last groups can balance the complete module. This happens in the car by the integrated Battery Management System (BMS). Using a single module require a similar function to be applied. Solutions exist that use the Tesla board on the module for balancing too, but there are other options available too. Looking into the options used by Radio Controlled Plane pilots to balance the cell packs is a cheap option.
The first option we deployed was Tenergy 5 in 1 Cell Meter (~$10). This does a great job balancing the groups of cells and is precise to 1 millivolt (0.001 Volt). The downside is that this requires manually to trigger a balance to happen. Very similar to the maintenance task required for flooded Lead-Acid batteries, except no holes in your cloth.
We now use a HobbyKing Battery Medic (~$15). The device has an accuracy of 10 millivolts (0.01 Volt). The device can be set for a threshold voltage and balancing occur automatically above that voltage. We have set ours to 24.0 Volt. So when the batteries are close to fully charged is a small amount of energy removed from the group with 10 millivolts higher than the lowest. This process cycle on and off since the thin wires add a large voltage drop when the resistor is applied, so a few seconds balance, a few seconds stabilizing and measurements repeat all the time.
The device can get hot during operation so it is a good idea to not cover the rear ventilation and keep the device sitting vertically to allow the warm air to rise. We have been using this solution for over a year now with none of our five devices failing. When measuring on the batteries is the balance of the cell groups perfect.
Other option also exists like the Batrium system (~$350) that provide a lot of useful information but has a higher cost and require a lot of wiring since only a single group module exist and a Tesla Module requires 6 LongMon modules and a central control unit.
So balancing a single module is very easy and cheap using the Radio Controlled Plane balancers. What if you want to put 2 modules in series for 48 Volt? This is a case very similar to balancing the 6 groups in a single module, but with a very different level of energy. Using a Battery Medic on each module could seem like a good option, but is not working. Let's say that one module never has imbalances and require no removal of energy, The other module has a slight imbalance and requires removing 0.01% when charged to keep it balanced. Over time is this 0.01% removal of energy from only one module driving the perfect module to a higher voltage and the worse to a lower in each charge cycle. Within months causing the perfect module to exceed 25.2 Volt and entering dangerous territory. A 12S solution must be applied when using modules in series.
A way to reduce the number of balancing modules is also to have all modules on the same balancer. If two modules charged to the same Voltage and perfectly balanced is it possible to parallel the balancing leads and a single balancer used. We decided against this option because a disconnected cable from one module could not be detected with a simple inspection of the balancing devices, but with a more expensive Batrium solution is this both reducing wiring and cost.