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They should be charged as a pack I think and not individually.

I disagree - you shouldn't charge packs in parallel as it would be possible to undercharge one pack whilst overcharging the other.

The other advantage of charging them separately is that you can use standard 7.2V RC chargers to do so. Charging 14.4V packs is expensive. But you do have to ensure that packs in series are kept at the same level of charge - mixing a fresh pack with a half used pack can kill the half used one. And avoid discharging the cells below 1V per cell, ie 12V for the whole lot. (This should prevent stronger cells in the pack from destroying weaker cells). You may want to build in a small LCD panel meter to pay attention to this.

I'd advise labelling each set of packs clearly. If you can't find heatsink of that large size in multiple colors then use different color sticky tape. Anything to make it obvious which pack belongs with which. Get into the habit of charging two packs together. (Yes, two chargers - you don't need to get expensive with them)

I'd personally build such that there was a slide switch to select between the two sets, rather than leaving them both wired in parallel. This would mean that you are only discharging one set at any moment. It also means that if you use it for say 30% of total capacity, you only have to recharge 2 packs rather than 4.
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