# Operating electromagnet with 18650 cells

I want to make portable 12 V electromagnets with 18650 cells. I'm not sure where I should start looking for a safe and reliable circuit.

My original idea was to use four cells in series to a PCB and then a buck/boost to drop to 12 V to run the electromagnets. Is this a viable option, if not, what should be done differently?

• You can supply the coils with their nominal voltage. Or with the current for the field necessary, which may be less (e.g., same force but an initial gap closed), for prolonged operation. Jan 21 at 3:13

## 3 Answers

When designing with batteries you also need to take C rate and that is the rate a battery can be discharged or the current a battery can withstand over time.

Example if you have a battery of 10A/h

Then will the C rate be

C = 1, a 10A discharge under 1 hour
C = 2, a 20A discharge current under 30 minutes
C = 5, a 50A discharge current under 12 minutes
C = 10, a 100A discharge current under 6 minutes
C = 20, a 200A discharge current under 3 minutes


So choose a battery that can withstand the current you want to load the battery with.

my original idea was to use four cells in series to a pcb and then a buckboost to drop to 12v to run the electromagnets. is this a viable option, if not what should be done differently?

This is a viable option for sure.

The buck-boost regulator will allow the battery voltage to drop to a minimum of circa 2.7 before the batteries need recharging as well as dropping higher battery voltages to 12 volts. You might also consider that a DC electromagnet running from 12 volts wastes a lot of power if it has padding resistors to to set the current. In other words there may be more efficient winding regimes that out-perform what you may be considering and allow a running voltage as low as 6 volts or less.

You also need to keep an eye on the voltage of each 18650 cell to ensure that one hasn't reached it lower discharge limit before the other three. This can be done with differential amplifiers that measure each battery's voltage.

If one (at least) reaches the lower limit, the discharge process must be stopped to prevent possible cell damage.

I don't know what output current your load will take but here's one that works from 9 volts to 15 volts and can supply up to 3 amps. It also has an undervoltage lockout that switches off the output should the input voltage drop below 8.5 volts (set by the two resistors that attach to the RUN pin): -

The force in an electromagnet is proportional to the current and the number of turns of wire. What you really want is to control the current, not the voltage. Otherwise the current is just limited by the resistance of the wires in the magnet. With constant current, you get the same magnetic pull even as the batteries are discharged.

If you google "high power constant current switching regulator high efficiency", you'll find many possibilities for chips that you can use to build a supply. You can also find many low cost off the shelf adjustable constant current switching supplies by clicking the shopping button on the above google search. I would recommend going the off the shelf route. Just make sure that you get one that can run over the voltage range you want to run the 18650 cells.