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OK, if you want to use a balancing current of maybe 5 A, then you have 18 watts to dissipate and a fet bolted to a heatsink might be a reasonable way to do it. In my car, with 45 cells, that would mean 810 watts to get rid of. Not very efficient is it?
Practically, if you want to use a balancing current significantly greater than 1/2 amp or so, then you really have to move to an active balancing system where power from a "high" cell is shunted to a "low" one. There are some quite good designs out there for such things, but they all cost a great deal more than the simple, lossy resistor.
By far the simplest and most cost effective way to do it is to balance all your cells before installing them into your battery and then keep them in balance with the little 1/2 amp shunt which is all it takes once you have done the initial balance.
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