Eneloop charging series vs parallel

is it possible to charge AA eneloop cells in series, or only parallel is possible?

also can i use my arduino to charge them?

It is possible to charge almost ANY battery cells in series and some times preferable to do so, however (and its a BIG however) the charger needs to be rated to the series voltage. A parallel AA charger delivers a nominal 1.5 volts at a set current, however if you connect two AA's in series you get approx 3 volts so the charger needs to output a higher voltage.

I don't see how you could use an Arduino on its own as a charger, maybe a PWM output could work but I will leave that to the experts to answer.