Reddit mentions of Hongbous Nitecore i2 Intellicharge Charger for 18650 AAA AA Li-Ion/NiMH Battery

WALL OF TEXT ALERT:

The most difficult part of building a proper charger for an 18650 bank will be to ensure that the charge system will be able to balance each battery/cell during charging. You can find a lot of single cell charging solutions at sparkfun. The only thing is that product is only meant to handle a single cell. You could run one of those charging boards for each cell but I could see that being pretty bulky.

You specified that you want the bank to be 12 V with a capacity of 50 Ah. The nominal voltage for an 18650 is 3.7 V but fully charged is 4.2 V. That would mean that you want 4 batteries in series to ensure > 12 V during use while they drain down to 3.7 V each before needing a recharge (This is assuming that your output regulator does not have a boost on it). To achieve the 50 Ah mark you will need a total of 17 batteries (or 16 for a capacity of 48 Ah if the 50 Ah is not a strict requirement). That would need that you need 4 sets in parallel of 4 batteries in series (If that makes sense). I would go for the 4 sets of 4 to keep the parallel sub-groups of the system symmetric and balanced during discharges. You will definitely need some regulation out of the battery bank for the devices you want to charge. Probably a 12 V out and a 5 V out for various devices. This also assumes that the batteries internal behaviors during discharge will let every set of batteries discharge all at the same rates, which will probably not perfectly be the case.

Like I said, the most difficult part of this build will be making the charging side be able to balance the cells during charge. It might be easier to just get a hold of a 4 cell 18650 balance charger, unplug the batteries from your system, and charge the batteries in a pre-made charging system. For the most optimal charge times you would want 4 of those chargers so you can charge each parallel group at the same time. Something like this.

I did find this charging PCB on EBay that could be built into your bank with the input pads all soldered in parallel for each sub-bank of batteries (Make sure the wiring can handle more than 12 A or bundle 4 sets of wires together that can handle 3 A each). The board is rated for a maximum current of 3 A meaning that to charge 4 batteries up to 3000 mAh (3 Ah) would take roughly 4 hours for each sub-group of cells. The big charger would definitely charge each set of batteries much quicker, but would be a separate system from your charger bank.

Keep in mind that everything I've discussed was just a quick breakdown of the difficult parts that I can see right now. I'm not 100% sure what kind of output rails you want to build or the currents you want to be able to run out of the bank. I tried to focus mainly on the charging portion of the project. Since LiPos need a proper charging system to keep the batteries healthy over long periods of use and each cell needs to be balanced for everything to remain in spec over the duration of the life of system any charging system needs to be able to perform proper balanced charging to each cell.

Let me know if you have any other questions or are looking for any particular out-of-box solution for any piece of the project. Something like this could be very useful to have, but you definitely want to make sure that the batteries will be properly handled to ensure longer lifetimes.

https://www.amazon.com/Hongbous-Nitecore-Intellicharge-Charger-Battery/dp/B01F04ZWFI