Having a hard time figuring this one out, I am trying to convert a 4.5v 25w dc fan from an air mattress pump to run off a rechargeable battery pack made with 18650's. This is going to be charged and discharged often and I desire a long run time which makes c cell batteries considerably cost prohibitive long term.
The motor will run off 3.7v just fine and is more than adequate power for my purposes however That being said I have a few questions.
How would I go about discharge protection? Do I need a protection module for each cell or would one suffice for the entire battery circuit?
How would I go about charging them? The bms boards I have been able to find seem to be designed for series applications. Would a bms designed for a 1s cell work for multiple cells in parallel? I am not opposed to using sleds and charging the batteries individually however it would be nice to keep the batteries internal and charge via usb. I know this would take longer however I plan to charge them overnight so time isnt super important.
Lastly this is not necessarily related to charging/protection but when I run the motor off a single 18650 for testing purpose it seems to be drawing more current than with the 6c batteries at 4.2v vs 4.5v with the alkaline batteries. The dc motor no load speed is noticeably faster is this due to bypassing the existing wiring/switch and running straight off the dc motor +/-? I plan on adding a trimmer pot to the circuit anyway to keep the current down and hopefully extend battery life however I am not really sure what resistance value would be appropriate. Short of pulling the motor from the housing and checking the current draw outside the housing using the alkaline batteries is there a way for me to determine a safe operating current for this motor?