Current control methods

Choosing the right way to let the BMS turn off the battery current

The method that allows the BMS to control the battery current depends on the battery topology and the devices connected to it.

This application note lists all combinations, and persents a recommended control method for each.

A battery pack may:

• Have 2 separate power ports, one for a charger and one for a load; each port has its own internal power switching
• Have a single power port for both charging and discharging (or multiple ports connected directly in parallel); and:
  • Power switching is inside the battery, or
  • Power switching is outside the battery

The system external to the battery may be:

• A bidirectional device (or multiple bidirectional devices)
• A separate charger and load

These devices may:

• Allow a BMS to limit their current into or out of the battery through a low power signal
• Not have such a provision

A load may be:

• Bidirectional
• Discharge only

Here it is assumed that a charger is only capable of charging (it is not bidirectional).

The BMS needs to control the charging and discharging current separately and independently. But the external device(s) and the battery topology may not be directly compatible with that requirement.

Therefore, some of these applications are straightforward, while others require somewhat complex circuits.

In this table, find the combination that describes your system, and note the recommended circuit number.

		Single-port battery Switching inside battery	Single-port battery Switching outside battery	Dual-port battery Switching inside battery
Single, bidirectional (charger / load) device	Doesn't obey the BMS	Ckt 2		n.a.
	Obeys the BMS	Ckt 1
Multiple bidirectional devices	At least one doesn't obey the BMS	Ckt 2
	All obey the BMS	Ckt 1
Separate load (discharge only) and charger	Both obey the BMS	Ckt 1
	Charger obeys the BMS Load doesn't	Ckt 2	Ckt 4
	Load obeys the BMS Charger doesn't	Ckt 6	Ckt 5
	Neither obeys the BMS	Ckt 2	Ckt 3
Separate load (bidirectional) and charger	Both obey the BMS	Ckt 1
	Charger obeys the BMS Load doesn't	Ckt 2	Ckt 7
	Load obeys the BMS Charger doesn't	Ckt 6	Ckt 5
	Neither obeys the BMS	Ckt 2	Ckt 7

This table shows each circuit (listed in the table above) and gives some details about it.

		Circuit	Waste heat
Ckt 1	Fault contactor		Low
Ckt 2	2 steering diodes and 2 contactors		High
Ckt 3	Load contactor Charger contactor		Low
Ckt 4	Load contactor Fault contactor		Low
Ckt 5	Charger contactor Fault contactor		Low
Ckt 6	1 steering diode and 1 contactor for charger Fault contactor		Medium
Ckt 7	2 steering diodes and 2 contactors for load Fault contactor		High

The rectifier diodes and the contactors' contacts must be rated for the current they handle and for the full battery voltage. In circuits 3, 5 and 7, the contactor for the charger port only needs to be rated for the charger current, which may be less than the load current.

Note that each of the rectifier diodes will waste significant energy in heat whenever the contactor across it is open. That only occurs if only charging or only discharging is allowed; if both are allowed, then they generate no heat.