The electrical resistance of a battery pack and even an individual cell can be complex. However, in it’s simplest form it is Ohm’s law:
Voltage = Current x Resistance
V = I R
Hence, the larger the resistance, the larger the voltage drop for a given current demand.
When we look at the components in a battery pack then we need to consider the following when looking to estimate the total resistance:
- Cells
- Cell connections
- Module busbars
- Connections between busbars and components
- Busbars themselves
- Fuses
- Contactors
- Current sensors
- Connectors
DCIR of a Cell
The DCIR of a cell is the Direct Current Internal Resistance. This is the resistance in charge and discharge to a direct current demand applied across the terminals.
BMS Resistance Algorithm
At the simplest level, the pack has an overall voltage sense and current shunt and each cell group has a voltage sense. So simplest form of BMS resistance is just a real-time calculation of (Pack voltage drop – cell group voltage drops) / shunt current to give the total interconnection resistance. You can run additional voltage sense lines within the pack to split up the sections for more granularity.
See also
Pack Internal Resistance, Internal Resistance: DCIR and ACIR, Four-Wire (Kelvin) Resistance Measurement