If you charge them individually each battery will get it allotted charge, when charging multiple batteries at once the charger will send a charge to equal the combined voltage of in your case 6v or 7v to the set of batteries.
Problem is one or two batteries could receive a overcharge at the same time 1 or 2 more batteries could be undercharged but the charger will not know it, it will happily charge until the whole pack of batteries reach its 6-7v, possible cooking a battery or two in the process.
For RC battery packs and the like , they use balance chargers which allows all batteries to be individually monitored while being group charged.
In your case charge them individually and the batteries will thank you.
Regards, Doug / ecomodded
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