Lipo battery charger and monitoring module

[akis]

I have made an arduino based module that monitors my 6S lipo battery, it looks at individual cell voltages and current draw and pulls the plug if anything went wrong.

I am thinking of adding a small lipo battery charger as well, all into that module, nothing extravagant, say 1A or 2A max. But the battery in question is 6S and am not sure what the proper method is to balance lazy cells. Do we try to charge them to a certain level individually (with all the problems that entails) or do we bleed the other cells to match them down (I seem to have read this somewhere) or do we bleed them while they are being charged so somehow they do not really discharge but they just do not charge as fast?

Has anyone got any experience on the matter? Many thanks

[Zbig]

Both are valid approaches. The basic charger that came with my RC helicopter used the method you've mentioned first: charged each cell separately via the balancing plug. The main power lead was left dangling. It was meant to be used with relatively low capacity packs so neither the balance harness gauge nor the balance plug rating wasn't posing much of a problem. My slightly more advanced Turnigy Accucell-6, as well as other "big boys'" RC hobby chargers, do the other thing you've described: pump the charging current through the main power lead (so, effectively, the cells are being charged in series) and "bleed" through the balancing connector the individual cells that approach their terminal voltage sooner than others.

[Siwastaja]

How are you monitoring individual cells? This is where things usually fail catastrophically; the designs sometimes (often) draw different current from the taps, resulting in accumulating inbalance, and finally, over-discharge destruction of the cells with highest current draw during a storage period.

[sleemanj]

I think i would go with bleeding down cells as the simplest solution,  a mosfet and series resistor across each cell, sized to draw  maybe 100ma.  Balance at top of charge, or if any hit say 4.3v before then.