I work in a collision shop. Our headlamp aimer is cordless, but the battery is always going bad, it is an internal battery, and we charge the aimer with a brick style charger that plugs into the base of the aimer, it has one of the round dc connectors with a wire attached to the brick, and the brick puts out 15 DC voltage. What I would like to do is use a power tool battery(we use Milwaukee tools here) to just power the aimer all the time, so I would want to remove the internal battery, wire up an external battery instead that we can easily swap out and recharge when it goes dead. The problem is, my choice of battery is either an M18 or M12 battery. I have not searched yet for a tool that has 15 volt batteries, I would like to use best case the M18 battery, but not sure how to drop the voltage down to 15 so it is safe for the aimer. My knowledge of electricity is limited but I am not a newb, just wanted to run this by some experts to get suggestions on best way to go about this. So this ends any confusion and so the question does not get closed, I do not need knowledge on using the tool or the battery. I need help designing a circuit that will use the M18 battery in the 15 volt tool.

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    \$\begingroup\$ @jsotola Some cordless tool brands use batteries that are standardized across their tool line, so the same battery that fits a cordless drill also fits a cordless saw, or work light, etc.. This way you can have spare batteries on charge all the time that will work on any of your tools. \$\endgroup\$
    – GodJihyo
    Nov 14, 2023 at 22:25
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    \$\begingroup\$ Not sure why this was closed, seems a reasonable question on how adapt a battery of one voltage to run something designed for a different voltage. Seems like it might require some design. \$\endgroup\$
    – GodJihyo
    Nov 14, 2023 at 22:28
  • \$\begingroup\$ To clarify, I have a ton of M18 batteries, this is why I want to design a circuit to use the M18 batteries in this particular tool that does not currently use them. It appears to use a very small rechargable 15 volt battery internally, that appears to be one you might find in an electric R/C car. \$\endgroup\$ Nov 14, 2023 at 23:12
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    \$\begingroup\$ Since you're going to open it to remove the battery, why not open it to check the battery voltage first? Just because it's powered by 15V doesn't mean the battery is 15V, it could be 12V for example. \$\endgroup\$
    – bobflux
    Nov 15, 2023 at 2:53
  • \$\begingroup\$ What are M18 and M12 batteries? \$\endgroup\$
    – Hearth
    Nov 15, 2023 at 5:04

1 Answer 1


Battery powered devices usually have some tolerance to voltage input, so the voltage doesn't need to be exact. But, you should drop some voltage to get close to the target range.

Buy a battery adapter to make the mechanical connection, something like this: https://www.amazon.com/Milwaukee-Conversion-Terminals-Connector-Robotics/dp/B0BVG1HTM2?th=1 This has an inline fuse, this is very important, these batteries are powerful and bad things can happen if they are shorted. Realize that 18V and 15V are nominal voltages, a fully charged battery may put out more. Put 4 or 5 1N4001 diodes in series to drop the voltage 3 volts or so. Hopefully, the device has some tolerance to overvoltage, an 18V battery may put out 20V fully charged.

The adapter I bought came with a 20A fuse. You probably want a much smaller one, maybe 5A. They are common automotive blade fuses.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Thanks Mattman944, this is exactly what I was looking for. \$\endgroup\$ Nov 15, 2023 at 4:58

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