Bypassing cells in certain conditions

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reikiman
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Bypassing cells in certain conditions

Any thoughts on the value of bypassing cells in a pack?

I've been at the JavaOne conference all this week, it's a trade show & technical conference about the Java programming language. This year the LincVolt car was at the show because the car's control systems are entirely implemented in Java by a Robotics expert (Paul Perrone of Perronerobotics.com). I spent quite a bit of time talking with him about the implementation and they've got some cool stuff under the hood.

The battery pack is a Lithium-ION (white large format blocks) pack with 100+ cells (hence the car is over 320 volts) driving a Unique Mobility 150 kilowatt drive motor.

He mentioned an idea they don't have implemented but may do. It is to bypass individual cells under certain conditions. They would do it using big honkin' FET's rather than a mechanical switch or relay. I think right now they may be doing this manually but would be doing it in software.

I'm wondering -- why would one want to do this?

An obvious idea is if some cells are weaker than the rest, and a few cells are close to the low voltage cutoff. If so it might make sense to switch the weak cells out of the pack (?? and maybe charge them somehow ??).

Just had a thought - maybe under different performance conditions. Such as cruising on level ground at low speed might require 160 volts and climbing a mountain might require 320 volts. Maybe you could dynamically reconfigure the pack on the fly while driving?

Mik
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Re: Bypassing cells in certain conditions

During charging it would be much easier to do this, because with present day battery technology the charging currents are quite sedate compared to the peak discharge currents.

It would be more efficient to not put more energy towards full cells than to shuffle it away again afterward, if you know what I mean.

A typical BMS seems to be redistributing current.

If individual cells could be turned off, then the current is only distributed. More power can go to the remaining cells until they are also full. This will be especially important once batteries can accept more current than the charger can provide.

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