# NiMH Battery Management System - Balancing Circuit Design

## Main Question or Discussion Point

I am working on a senior design project that is to design a battery management system to monitor NiMH batteries powering an electric truck. The professors who proposed the project set the main requirements to be that it measures the voltage of every cell, the temperature of every 5-10 cells and the system must be able to bypass 1-amp of current around a cell when it reaches nominal voltage.

NiMH Battery Specs: 10-Cell Packs@13.2 Volts Nominal (16 Volts Max Charging) so 1.32 volts per Cell (The specs we were given are attached.)

There are a very limited number of monitoring chips made to work with NiMH batteries because it is not usually necessary to know the voltage of each cell, but my team has chosen to use the Maxim 11068 which claims it will work with NiMH batteries.

Maxim 11068 Datasheet:
http://datasheets.maxim-ic.com/en/ds/MAX11068.pdf

The current problem we are having now is designing the bypass/balancing circuit. The internal switches of the chip can only bypass ~250mA. The datasheet on pages 25-26 provides sample external bypass circuits. The problem with using those circuits is trying to find an external MOSFET with a very low gate threshold voltage that will bypass 1A.

The approximate voltage from my calculations that can be supplied to the MOSFET gate is approximately 0.6 volts.

I cannot find a MOSFET with a gate threshold voltage this low that has an id~=1A.

I've also looked at BJTs, but run into a similar issue.

Does anyone know if there are any transistors that will work for this scenario or have any suggestion of how to design the circuit so that it is possible to balance the 1.32V cells using the IC chip to trigger the balancing?

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I have simplified my design so that I will bypass current around three cells. This will provide a much higher gate threshold voltage, but I still have a few questions about the right MOSFET to choose.

The total battery voltage will be 3.96 volts (3 NiMH cells). This will provide 1.98 volts to the gate of the MOSFET.

The following MOSFET has very low on-resistance and decent power dissipation. My only question is that Id at 1.8 volts is much higher than 1A. As far as I understand this shouldn't be a problem as long as I limit the current across the MOSFET with a resistor above the drain. Do I have this correct? Should I be concerned with any other aspect of the MOSFET then what I have mentioned?

First Option:
http://www.fairchildsemi.com/ds/FD/FDS6574A.pdf

This is one other option I found. I am afraid it may be to limited by max temperature and power dissipation.
A second option:
http://www.fairchildsemi.com/ds/FD/FDG1024NZ.pdf

Any suggestions would be appreciated. Thanks.