BMS: Proper way to measure the voltage across Pack+ and Pack-

[Blackgar]

Hello,

What is the proper way to measure (with isolation) the voltage across PACK+ and PACK- (voltage across the load) of a BMS? It seems complex since PACK- contactor may open at anytime (due to failure etc.).

I was thinking to add a resistor across the terminals and measure the current flow using a current transducer or calculate the Load voltage by summing up 2 different measurements with reference to BAT-.

I am not sure which is the best way or what other choices I might have.
 

[max_torque]

If this is for any pack above 60Vdc, then be careful, because there are a LOT of rules for packs considered to be "high voltage"  (above 60Vdc and below 1KV)  These rules include things like leakage currents and capacitiev energy storage etc

Most systems use the pack -ve as the reference point, which is fine for internal (ie pack side) voltage sensing of the various Pack+ voltages, but care must be taken for the pack out +ve measurement point, because your resistive divider forms a leakage path around the positive contactor.

You don't need galvanic isolation as long as you condsider this leakage path, and most systems simply use a large value resistive divider (typically 4MOhm per leg for example)

Take a look at the Isabellenhütte IVT-S series of current and voltage measurement devices for example