i have a about 12 , li-ion 18650 , 3.7 volt batteries , 6 " sure fire " types and 6 " gtl " brand.
is it possible to connect them all up in series ,in lots of two ( 7.4 volts) , then paralell them all together for more current ? charging doesnt have to be at max , even min charging current will be fine.
i`ve had a decent look around on the net , but all i could find was charging in 2 cell lots.
Subject to the usual caveat regarding safety and suitability of charging system, you can charge them in series strings of whatever length/number you desire. However the difficulty is in achieving matching SOC in series configurations. While protection modules guard against unequal SOC, they don't actually remedy it.
If you configure them in parallel, the SOC *will* equalise, as LiXX charging is voltage driven, unlike NiXX.
I had it the first time but I think you might be better making two paralleled packs of six cells for 3.7v and then put those in series to get your 7.4 volt pack. AFAIK, you probably should do some matching of capacity between the cells but I'm not sure how much having non matched cells will impact on overall performance and longevity.
Perhaps you could include a switch of some kind that enabled you to 'separate' the parallel packs and charge them as two individual packs? That might solve your charger problem too if you can find a charger that can provide the power needed.
Charging in series is to be avoided unless you have voltage limiting circutry across the terminals of each battery. Further, minimum charging currents are also not recommended. If a trickle charge is fed to these batteries you risk irrepairable damage to the batteries that could result in fire or worse.
18650 "Protected" batteries can be purchased that do have overvoltage safeguard however, they are still not safe to trickle charge.