I don't disagree, and building battery packs for battlebots was a fun experiential way to learn this, but it is important to match chemistries. Chargers for nic

I don't disagree, and building battery packs for battlebots was a fun experiential way to learn this, but it is important to match chemistries. Chargers for nickel-metal-hydride batteries won't correctly charge lithium-ion batteries, or worse nickel-cadmium batteries. Not to mention that some battery packs (the ones in the Thinkpad come to mind) have a "battery monitor IC" actually built into the battery pack so it helps to start with an existing pack and then "re-manufacture" it with cells rather than just wire together the same number of cells. Perhaps to most critical element in these "smart" batteries is the temperature sensor which dials back the current when they start getting hot to avoid thermal runaway. (source of MANY battery fires in laptops from counterfeit battery packs). As for matching the cells, in our battlebot we would take each "unit" (which in our case was 20 NiMH batteries) and condition it by draining it and charging it in cycles while monitoring voltages to level out the batteries and "sync" their charge levels. We still ended up with a couple of fires but that was because we were charging them right at the limit of what is reasonable to do because the time between bouts could be pretty short.