Design challenge n.7: the fuse voltage drop

A project log for Green BMS

Open source Bluetooth Smart Battery Management System

Sergio GhirardelliSergio Ghirardelli 10/16/2021 at 17:250 Comments

The cell module circuit includes a 3A fuse connected immediately to the module's positive power supply.

The fuse has a very low internal resistance (about 50 mOhm)

However, this resistance becomes a problem during balancing, because the balancing current of 1A causes a voltage drop across the fuse of about 50mV.

This drop compromises the cell voltage measurement during balancing, compromising the regulation logics: because the microcontroller detects a lower voltage than the real one of the cell!

How to solve this challenge?

Could I remove the fuse? could be dangerous in case of short circuit!

So I immediately tried to find a way to calculate this drop and compensate it with a mathematical formula in the software.

But this choice proved to be very complex due to the many variables involved to be managed:

different fuse resistance for each board, user variable balancing current, balancing resistors with different tolerances, different balancing voltage between Lifepo4 and Lion ... etc.

Too inaccurate: another solution had to be found.

After a few days of reasoning I had a simple thought:

I have a microcontroller that manages the balancing circuit and I have a mosfet that can switch quickly!

So why not periodically interrupt the balancing, to acquire the real voltage of the cell?

This graphic shows the logic I have implemented in the software:

Every 5000 ms the balance is stopped for 50 ms.

In the middle of this time a voltage reading is taken, which without the fuse voltage drop corresponds exactly to the real cell voltage!

I am very satisfied with this simple solution that I have found.