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Forum Index : Electronics : the ubiquitous 555

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Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 4406

Posted: 08:33pm 22 Dec 2019

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Slight update...

When setting this up initially, its possible to get everything right so that the cell voltage right at the cell, and at the Nano are absolutely identical, well within one bit or one millivolt resolution. But beware of voltage drops in the common ground system.

So be sure to connect the negative side of the flying capacitor system right to the analog ground pin on the Nano through tracks that carry no continuous ground current, when laying out your circuit board.

I have tried quite a few different algorithms to control the turning on and turning off of the individual cell discharge loads. They all worked, but the simplest and best strategy so far, has been to measure each individual cell voltage and compare that to the average cell voltage.

If the particular cell is below the average, leave the cell discharger off. If its above the average cell voltage, turn the cell discharger on. I do not calculate the average in software, although that could easily be done. I use a voltage divider off the total battery voltage, and measure that as just one more analog voltage to measure.

My cell discharge load is 27 ohms or about 120mA which is more than sufficient for the small 50Ah cells I use. It should be sufficient at a guess, for up to 200Ah cells.

By running the cell balancing system continuously, it takes very little to slowly get the cell voltages to converge and stay that way. Its neither top balancing, or bottom balancing, but depends on how long the cells stay either at full charge or discharged.
So its a sort of self adjusting "overall" balance which seems to give excellent results.

Cheers, �Tony.

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