Rather like connecting in series, is there any reason to stop at only 2 in series, i.e would it be possible to connect 3 in series or 4...
I haven't built this or any other lithium battery, so just thinking it through.
Some people build a 48V system with four, independent 12V batteries each having its own BMS.
Only some BMS allow that; I think the problem is that it's disconnect switch sees the full 48V.
Within each 4-cell 12V pack, the BMS provides some balancing. But there is no way to balance between 12V batteries. If SoC drifts too far apart, BMS would disconnect to prevent overcharge even though pack voltage seemed OK. The 12V batteries would require occasional independent (or parallel) recharging to top balance.
Unlike that 12/48V configuration, your 48/96V would always have separate recharging of 48V batteries. Each day they would be independently recharged to the same SoC. Also, they can be drawn down at different rates with separate loads (48V inverters), or maybe one would serve as your house battery and the other would never see discharge except by the 96V motor.
Each 48V BMS would need to be able to disconnect loads (96V motor, 48V inverter) for over-discharge of a cell, and charger for over-charge.
That disconnect would have to handle the 96V+ of a full pack (with its internal transistors, or external relay)
With 48V BMS, it could communicate with its associated charger (and inverter if there is a reason to.)
A 96V BMS could also be used, but probably couldn't communicate with separate 48V chargers because the two 48V sets of cells would be of different SoC. Also, only isolated communication could work; if electrically referenced to negative terminal of battery, one 48V charger would be offset.
Like for 12/48V, there would be limit on how many in series because each disconnect switch sees full voltage of the whole battery.
With multiple 48V chargers/loads, each needs its own disconnect. If using a 96V (or higher) BMS, its disconnect signal would be used for relays on each 48V circuit.
This would work with lead-acid as well, but the problem is that entire battery voltage is the only way low SoC is detected, so with one 48V pack lower than the other, it would be over-discharged. A BMS (or monitor) of each 48V pack is required to stop discharge of the 96V battery when either 48V battery is fully discharged.
Main reason was to use off the shelf 48V equipment and liability / insurance issues. Rumours have it that 48V systems are classed as "ok to be build by layman" where as above 48V a professional is required.
I consider this a 48V system. Twice.
By grounding the middle, and using relays/fuses to disconnect at either end, the highest voltage is 48V nominal. or -48V nominal.
The only way you get 96V is by grabbing both wires.
Similar to house wiring - nothing is 240V; all wires are 120V, just of different phases.
If 10kW propulsion motor is the largest load, using 96V cuts the current in half compared to 48V, and the power loss (heating of wire) to 1/4.