Converting 6c (3s2p, 4.5 V) DC motor to 1s4p 18650 charging/protection

Question

I am having a hard time figuring this one out. I am trying to convert a 4.5 V, 25 W DC fan from an air mattress pump to run off a rechargeable battery pack made from 18650s.

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 3.7 V just fine and there is more than adequate power for my purposes. 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, but I plan to charge them overnight so time isn't super important.

Lastly (this is not necessarily related to charging/protection): when I run the motor off a single 18650 for testing purposes it seems to be drawing more current than with the 6c batteries at 4.2 V vs 4.5 V with the alkaline batteries. The DC motor no-load speed is noticeably higher. 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?

Answer 1

A 25-W motor running at 4.5V is drawing 5.5 A. The effective motor impedance is therefore 4.5/5.5= 0.82 Ohms. This is a serious load for small consumer-grade batteries.

For example, specifications for Energizer D-cell (E95) say they have 200 to 400 mOhms ESR. Even in best case of 200 || 200 mOhms = 100 mOhm for a parallel pair, times 3 for 3 cells, gives 300 mOhms of battery ESR best case.

Therefore, the 820 mOhm motor will take about 4.5 V / (0.82 + 0.3) = 4 A of current, and the running voltage will be about 4*0.82 = 3.3 V. And 1.2 V will be dissipated on internal ESRs inside the batteries.

The Li-ion 18650 likely has under 100 mOhms ESR, and the drop will be only ~0.5 V, so with a freshly charged cell the motor must be running at 3.7 V, and thus the it takes more current and likely runs faster. So the Li-ion is much better choice for this application.

Now, since the motor is rated for 4.5 V, you shouldn't use any "trimmers", this is a very bad idea, waste. You can run your pump directly from 18650 cells.

Regarding "protection" and stuff, just buy cells with built-in protection, it is usually listed even by junk merchants.

One little problem is total capacity. With the 25-W motor your 4P battery will run likely no more than for 1.5- 2 hours (4 cells with typical 10 Wh capacity give you only a 40 Whr battery). If your current design (from 6 D cells) runs the motor at 3 V and this is sufficient for overall functionality, you might want to consider to reduce the driving voltage from Li-ion cells to that value using a buck switching regulator.

But if you want a really nice enclosed device with charging capability, you would need to design in both charger and BMS. And for 8 hrs run time you will need a battery with ~20 18650 cells, as Tony rightfully noted. Maybe you should be considering bigger Li-Ion cells, automotive or bike style, or sealed lead-acid batteries.

Alternatively, you might save youself a lot of time and money by looking for a professional design as Goal Zero Yeti portable generators or similar, Suaoki, Webetop, Paxcess, etc.