Li-ion Battery Protection Circuitry

During the Formula Sun Grand Prix 2001, we realized that the manufacturer supplied protection circuits for our lithium ion battery pack would not be sufficient. So I had 2 months to design, build, and test something that would work. It had many of the same constraints as the sensing circuits, but would serve a different purpose.

Design Constraints

• It should be supplied by the batteries it is protecting, and the static draw should be as low as possible.
• Prevent the cells from charging above 4.30V += .01V
• Must fit within the already built tray and box enclosure system.
• Response time needed to be relatively quick, on the order of several milliseconds from an applied load to action being taken.

In the time allowed for research and prototyping, the only topology that I was comfortable using was a resistive bypass scheme. In this method a resistor is electronically connected across the batteries during periods of overcharge. For simplicity and manufacturability I chose to do the protection at the cell level, so each of our 27 series cells would have one circuit protecting it.

To reduce possible feedback issues between the boards I used a pulse width modulation approach to activating the dump resistor. The duty cycle was proportional to the voltage of the cell once it had reached a pre-programmed threshold. This eliminated the need for a current sensing element, and provided a smooth response to a wide variety of load conditions.

Finally, to reduce the static power draw, a comparator with a slightly lower threshold voltage was used to power on the pulse-width modulation section. Now the static draw was less than 10uA, and the circuit only consumed significant power when it needed to dissipate it anyway.

For this circuit we knew that 27 was beyond our ability to manufacture using the protyping methods we knew, therefore we went the PCB route. I created the artwork for this board using Eagle Layout. Myself and two other team members spent approximately 4 non-stop days assembling the boards and testing them. This was my first experience with medium scale surface mount IC assembly. After some tweaking they all functioned as designed and were installed into the battery pack.

PICS

This is a very blurry close-up of the board. Need a better camera.

This is the battery tray system without protection or sensing circuits.

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