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Lithiumate™ Manual

index reference app_notes control_methods

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 schematic diagram Low
Ckt 2 2 steering diodes
and 2 contactors
schematic diagram High
Ckt 3 Load contactor
Charger contactor
schematic diagram Low
Ckt 4 Load contactor
Fault contactor
schematic diagram Low
Ckt 5 Charger contactor
Fault contactor
schematic diagram Low
Ckt 6 1 steering diode and
1 contactor for charger
Fault contactor
schematic diagram Medium
Ckt 7 2 steering diodes and
2 contactors for load
Fault contactor
schematic diagram 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.

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