Some times, users of a high voltage battery pack (> 500 V) would rather not buy a high voltage charger, or chargers that can be put in series to achieve a high voltage.
So they may come up with this crazy idea of splitting the pack in two, connecting the two halves in parallel during charging, and in series during discharging. For example, a 700 V pack can be split into two 350 V half packs, charged with a 350 V charger, and then used with a 700 V motor driver.
That’s not too hard to do, except for the BMS, which needs to be able to handle both modes of operation. And that can be really challenging.
Here is a clever solution that uses a single contactor, 2 rectifier diodes, and a Lithiumate Pro BMS.
When the AC is available, the charger is on, and a contactor opens the series connection between the half packs.
Rectifier diodes are forward biased, effectively connecting the two half packs in parallel, and the charger charges both half-packs.
The BMS needs to be told that the half-packs are in parallel, so that it can report the correct pack voltage.
When the AC is gone, the contactor closes, and the packs are connected in series.
The rectifier diodes are reverse biased, disconnecting the charger from the half-packs.
The BMS needs to be told that the half-packs are in series, so that it can report the correct pack voltage.
The Lithiumate BMS is the only BMS that can handle strings in parallel, and specifically the only BMS that can be switched on the fly between 1 series string and 2 strings in parallel.