Monday, January 12, 2015

Power Tools Off-Grid

Corded or Cordless Tools - Which are Better for Cruising?

by Tim Allen

Before we went cruising on our catamaran "Unbound," I shopped around and bought a set of cordless power tools to bring along. I thought this would be the greatest thing since sliced bread. Like many people, I had worked on projects away from convenient power. You know the projects - building the kids' gym set in the back yard, attaching a bracket to a wall from a ladder, hanging a mirror. Many of us have done these things.

One morning, after a couple weeks aboard "Unbound" I took on the project of mounting a GPS display near the helm. I needed to drill four holes. I grabbed my trusty cordless drill and ... it was dead. I plugged it into the charger and turned on the inverter. Fifteen minutes later the inverter control panel was yelling at me (with yellow and red lights) that the batteries were too low! Now to crank up an engine to charge the battery bank, so I can drill four holes.

According to the label on the drill's one-hour battery charger, it uses 65 watts at 120 VAC. The charger puts out 2 amps at 16 VDC. That's 32 watts, so the charging efficiency is less than 50%. Our Xantrex inverter/charger is about 90% efficient at supplying 120 VAC, so to charge the drill's battery for one hour uses about 6 amp-hours of house battery current.

Now let's look at drilling those holes with a regular, old-fashioned corded drill. My Milwaukee drill (in storage at the time) is rated at 3.5 amps at 120 VAC, under full load. Now, I was drilling fairly small holes through fiberglass, so lets say it would draw 2 amps (240 watts). Each hole took about 15 seconds to drill. That's one minute at 240 watts, at 90% inverter efficiency - 22.2 amps at 12 VDC. The total battery drain would have been only 0.37 amp-hours! That's only 6% of the power used for the cordless drill! And, I wouldn't have had to wait an hour to do the job.

If your getaway plans include a tool kit with cordless power tools - don't waste your money! Their batteries are almost never charged when you need them, so you have to run your inverter or generator for an hour to charge them up (if you remembered to get the fast charger) before you can get started with your project. A cord-type power tool is ready all the time, and you only need to run your inverter while you are using it. Corded tools also take up less space (no bulky batteries or chargers). Saves time and house battery amps!

True off-grid life is different! Whether on a boat or in an RV, or in a mountain cabin, electricity usage off-grid is totally different. The number three priority for any cruiser (after safety and water) is battery power. Without it we have no communications, navigation, lights - or engines!


Battery technology for power tools is constantly changing. The latest Lithium Ion batteries hold their charge for a very long time. With this in mind, cordless tools get a second chance on board. I love the power and consistent tool speed that the new batteries offer. However, I don't like the surprise when the battery is discharged - the tool just stops without warning!

The charging efficiency of Lithium Ion batteries is similar to the NiCd battery example above. If you are seriously pinching amp-hours, you may still want to use your corded tools.

Happy cruising!

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