xed888
In my opinion series is safer then parrallel, with parrallel the voltages of the batteries have to match or the batteries are trying to discharge and charger them selfs as soon as they are connected this can be a high quick burst of amps thats not good for the cells and will increase there internal resistance, witch will lower the amount of power the cell can give and shorten its life and it will affect the resting voltage of the cell over time in my opinion.
With series balance charging the cells can be diffrence voltages, diffrence mAh and even diffrent sizes, just more of a diffrence in either will make the charge time a bit longer, but charging multible cells in series will always be faster then one by one or 2 at a time with cheap dedicated charger.
If your intrested here is a thread on balance charging li-ion batteries, the tittle says 18650,s and 26650,s but it can be used for all the common torch batteries.
https://budgetlightforum.com/t/-/3118
hobby chargers: ive got some pictuers in there of some handy battery holders