I have a collection of 4 LC18650 batteries. I need to draw at least 12V with a current of at least 1 amp, hence the use of lithium batteries. I know I could use lead acid but environmental impact, energy density, and compactness of the system are high priority concerns. My best option is to use 3-4 lithium batteries connected in series.
I also have a battery holder that connects the batteries in series to output the combined voltage. At full charge, the bank should output ~16.8v, and ~12.0v at low charge.
My understanding is that doing this wouldn't be a problem, however recharging the batteries without removing them would be. I personally find the idea of removing the batteries in order to charge them distasteful and unprofessional, but keeping them from exploding is much more important. To that end, I've acquired a BMS from my local electronic component store, which unfortunately did not supply a datasheet. 
As I understand it, I connect the 0v pin to the negative terminal of the first battery. I then connect the 4.2v pin to the positive terminal of that battery and the negative terminal of the second battery. I repeat this for all remaining batteries. Next, I connect the 0V pin to the ground supply of my device and the 16.8v pin to the V_BATT connection. I can then charge all the batteries from said device exactly as I would a single battery. Is my assessment accurate? How exactly does my BMS work? If I'm wrong, how would I design such a system?
Best Answer
Almost certainly [tm]:
The 4S battery string has 0V at most negative and +16.8v at most positive end.
Total of 5 connection points - ground/negative and top of 1st 2nd 3rd 4th cell.
Connect most negative to 0V,
1st top to 4.2V,
2nd to 8.4V etc.
There is a very very very high chance of this being correct.