Charge in parallel, discharge in series

For a low voltage charger and a high voltage load

To handle a low voltage charger and a high voltage load, you may split the battery pack in two, charging in parallel and discharging is series.

The Elithion Lithiumate Pro BMS is the only off-the-shelf BMS that allows this operation.

Required components:

• A contactor, AC coil rated for the AC grid voltage, normally closed contacts rated for DC and for the load current
• Two rectifier diodes, rated for the voltage of each half pack, and for the full charger current
• An Elithion Lithiumate Pro BMS

Connect as shown here.

• AC power:
  • Charger AC input to the AC supply
  • Contactor coil to the AC supply
• DC charge:
  • Charger DC+ output to the lower half pack (directly) and to the upper half pack (through a diode)
  • Charger DC- output to the upper half pack (directly) and to the lower half pack (through a diode)
• Load:
  • Contactor contacts between the two half packs, to connect them in series
  • Load to the upper half pack (positive terminal) and to the lower half pack (negative terminal)
• Control (not shown):
  • Charger control to the BMS's HLIM output
  • Load control to the BMS's LLIM output
• Current sensing:
  • Charger current sensor right at the charger's DC+ output, before it splits to the two half packs (doesn't see load current)
  • Load current sensor on the B+ line to the load (doesn't see charger current)
• BMS power (not shown):
  • 12 V power to the VS input of the BMS, present if and only if the AC power is on
  • 12 V power to the VL input of the BMS, present if and only if the load is on

Configure the BMS:

• Number of batteries in parallel: 0
• Source current sensor: the sensor that sees the entire charger current
• Load current sensor: the sensor that sees the entire load current

When the AC is available, the charger is on and the contactor opens the series connection between the half packs. The rectifier diodes are forward biased, effectively connecting the two half packs in parallel. The charger charges both half-packs (first just the lower voltage one, and then both).

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. Current flows through each half pack in series, and into the load.

The BMS monitors how it is powered (through VS when AC powered, or VL when the load is on), to determine which current sensor to monitor, and to determine whether the pack voltage is the sum of all cell voltages (load on, half packs in series) or half as much (AC power on, half packs in parallel).