Aftermarket Cell Balanceing for NiMH

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antennahead
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Joined: 09/27/2009
Points: 17

Thanks all for the input on my Denver thread- please keep it coming.

I went for a bike (bicycle) ride today and put some thought into the cell balancing problem.

I work occasionally with a battery pack manufacturer in Denver (Nexergy, Inc.) and have a little background on cell balancing.

The tall stack of cells in the Vectrix is indeed tricky. However, I think a reasonably useful active cell balancing module can be created on a per-cell basis, i.e. 102 little circuit boards, each one across an individual NiMH cell.

Components would include, 1.2V->3V boost converter for powering the electronics, an MSP430 ultra-low power MCU, voltage reference, MOSFET, and power resistor. The quiescent current for such a unit should be well under 20-30 uA, and in the noise compared to the self-discharge rate of these cells. This would be a tiny board and I think it would fit right on top of the cells, based upon the pictures I've seen.

The idea is that when the cell voltage exceeds ~1.40V (or whatever the forums deems most appropriate) the MOSFET shunts about 1.5A-2.0A of charging current around the cell through the resistor. This energy will still be dissipated as heat, but at least it won't be going into the cell and boiling the electrolyte. The MCU can PWM modulate the MOSFET to achieve lower average shunt currents if needed.

This technique won't balance a stack at once- instead during charging the full cells will have their current shunted, preventing heat/electrolyte damage, while the weak cells continue to charge. Over time this should bring the stack into balance, and help maintain balance. A severely unbalance stack probably still couldn't be brought back with this, but would probably still be helpful.

The main benefit to this setup is backwards compatibility with the existing charger and cells, assuming the board is small enough to nest in between the terminals, etc...

Has this been discussed before? I've been pouring over this site for a day now and haven't seen anyone try to address this.

Best regards,

Steve

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AndY1
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Joined: 06/17/2008
Points: 1071
Re: Aftermarket Cell Balanceing for NiMH

When I didn't know, whether I'd get my NiMH pack replaced or not, I was thinking of making my own LiPO pack. I'm a RC helicoper flyer and we fly with LiPos. I also know a guy, who converted 3 cars, so far, to electric drive.
I'd use 38 Kokam Lipo Cells, with 40Ah each. I'd use a series of Schultze balancers to balance the pack.

When charging, the max. charge cut-off I've seen at Vectrix is 152V. With having 38 Lipo Cells, each having 4.2V as max. Voltage, that would make 159.6V cut-off Voltage for them, so Vectrix' charger would be far from reaching their highest voltage. Also, high power 40Ah Kokam cells are just exactly wide enough to fit into the Vectrix battery compartment.

The only concern I have is, what are those extra 2 leads, that go to the battery pack, for?

Quote:

A) There is no active monitoring, let alone transfer of charge between individual cells. All cells get charged with the same current, all the time. No balancing other than over-charging is possible. Only three sub-sets of cells are being voltage monitored: 27 cells, 48 cells, 27 cells.

http://visforvoltage.org/book/ev-collaborative-hand-books/6719

38 LiPos should give aprox. the same voltage ratio in a string of 10-10-18 as above NiMH pack. But how precise Vectrix is? Would a 1V declination make Vectrix undrivable as the onboard computer wouldn't let you drive the bike?

marylandbob
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Joined: 06/22/2009
Points: 520
Re: Aftermarket Cell Balanceing for NiMH

Sounds OK, but more important is a method of monitoring DISCHARGE, so that cell reversal can be avoided! Perhaps each cell can be monitored, to notify when a pre-programmed discharge voltage is reached, for instance? A warning at 1.1 volt, and an ALARM/current interruption at 0.9 volt cell voltage would be nice! If safe "Full Performance" usage of the vectrix battery down to as low as 115 volts were enabled, range would be significantly improved. (This might require better cooling fans, for hot summer weather)-Bob

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Mik
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Joined: 12/11/2007
Points: 3727
Re: Aftermarket Cell Balanceing for NiMH

If you have a handle at making such very small boards with a few components on it, look into "Switched Capacitor Balancing" instead of the resistor current bleeding approach. That would be nice, because it would increase range in capacity imbalanced batteries!

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