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question-tm.jpgRob asks: “I have a Craftsman 18.0V cordless drill.  I REALLY like it.  Yesterday, Sears had a pack of 19.2V batteries on sale.  Can I use these in my drill without hurting it?  Yes, they fit, but I don’t want to damage my drill.”

This is a great question.  My initial thought is that a 1.2V difference won’t do any short term damage and shouldn’t do any long term damage.  There’s some precedence for this in the industry.  For example, Milwaukee recommends its new li-ion V18 batteries as “backwards compatible” with their 18V tools, though the V18s are actually 20V batteries.  Competitive robot builders often over-volt motors, though they’re willing to trade reliability for performance — something I’m guessing Milwaukee wouldn’t be so hot on.

Charging could be another issue, though if you’re referring to the 19.2V batteries that drive the Craftsman drill/driver we’ve got in the shop here, they’re old Ni-Cds, just like the 18V.

Anyway, I hope you don’t mind, Rob, but I thought I’d kick this out to the readers to see what input they might have.  If there’s still a lot of question come next week, we’ll jump on the phone and see what we can find out.


12 Responses to Reader Question: To Over-Volt, or Not to Over-Volt?

  1. Myself says:

    I agree with you that the tool won’t care one bit, but the charging situation is a good question. Without knowing how they measure charge termination, all we can do is measure.

    There are some “any voltage” chargers that look for -dV/t characteristics, and some that have a safety voltage limit, and most probably do both. If I had a datalogging voltmeter I’d camp on the terminals of my charger and try to determine what it’s doing in there.

    I’m not sure calling the manufacturers will give you straight answers, either. I’d like to see commentary from the likes of Cadex and Alexander, plus possibly someone who rebuilds power tool packs, before calling the issue settled.

  2. The motor itself would not care, most motors can use a peak power of 50% greater than the specified (continuous duty) HP/Voltage/Amperage. Working on robotic and CNC motors we will sometimes use up to 3X rated voltage, though this is very bad for the motors and they will overheat very quickly, sometimes you can only go for a few seconds, or you have to add heat sinks.

    The problem is that many ‘simple’ tools like drills now contain electronics, and thus if they aren’t designed to hold back that much current you could overload it and fry the electronics even though the motor was fine.

    Most importantly: Batteries do not provide the same voltage at every moment. A car battery which is rated at 12 volts for example will often be around 13.5 volts when it’s fully charged and as low as 10 volts as it dies. Things like the Dyson Vacuum that just came out had to use a 20 volt battery and only use 16 volts so that it wouldn’t lose suction (power) and thus had complex electronics meant to handle it.

    IN a summary: If the battery goes directly to your drill motor you should be fine, just be a bit sensitive to amount of duty your using it on. If there is a complex sensor board then you could be hurting it, though 1.2 volts is much less than the amount I would expect the standard battery to vary by.

  3. Lee Gibson says:

    I’ve got a question on powering tools, but going the other direction. I’d like to build a “dummy battery” for my Ryobi set, so I can run the tools off of wall power if I run down all my batteries. I imagine that I could take a dead battery pack, remove the cells, install an AC-DC inverter, and plug it in. If I make sure to keep the inverter cool enough, I should be able to run the tool indefinitely.

    Any thoughts? I know enough about electricity to be dangerous, so I’m looking for somebody to point out some pitfalls before I wreck a battery. : )

  4. Myself says:

    Lee, the problem is that most wall-warts can’t put out more than an amp of current, and your drill might draw ten times that much when the motor’s stalled or starting. If you have an old battery pack that doesn’t hold much charge but will still turn the motor, adding an input to it would be a better idea, so the pack can supply momentary demands and then be replenished by the plug.

    The practical problem will be accommodating the connector in the already-cramped battery case. You’ll probably end up mounting something on the outside, and trying to protect the wire leads as they sneak into the case. I’d look at Anderson Powerpole connectors, since they stay together pretty well up until the pull-apart tension, then they separate without damage. The contacts are also self-cleaning when you mate them, so a little shop dust won’t be a problem.

  5. Andy says:

    Lee, I’ve been considering the same thing, but I agree with Myself (wow, that _really_ sounds self-involved) in that I think that putting a prefab inverter into the battery case probably wouldn’t cut it. Consider throwing together an assembly of a transformer, heatsunk rectifier and capacitor as high-rated and big as you can cram in and that just might do it. Or it might catch on fire, but I seriously doubt it’ll damage the drill unit. Wear a hot mitt during testing! Email me if you want in on what little I’ve learned about power supply design in contemplating this mod.

  6. Jason says:

    “Consider throwing together an assembly of a transformer, heatsunk rectifier and capacitor as high-rated and big as you can cram in and that just might do it.”

    The linear topology you describe probably won’t fit into the form factor of a battery pack — not at the amperage required (assuming “Myself” is correct in that a portable tool can draw ten amps).

    You could build an unregulated supply to feed your tools that was external to the tool, however.

  7. Rob says:

    I have another one for you Lee. If you happen to have a 12 volt tool, you can wire it up to a cigarette lighter adapter and use it in your car, no changes necessary. Depending on how often you use your drill around your car, this could be useful or useless 😉

  8. Myself says:

    Rule of thumb for electric motors is that start/stall current can be up to ten times their rated constant-run current. And Jason’s right, a batteryless supply would need to be designed for the peak current, which is why I suggested a battery-assisted scheme if at all possible.

    I thought about mentioning capacitors, especially in light of the incredible energy density of modern carbon types. But their voltage curve means they sag quickly under load, you’d either need a very large cap, or complicated power electronics, for this. The amount you could cram into a power-tool battery pack housing might be enough to help a weak transformer get a motor started, but I wouldn’t bet on it.

    The 12-volt tool sounds awfully convenient, Rob. 🙂

  9. Lee Gibson says:

    Thanks for the tips, everybody. I figured there was a reason nobody had done this. I certainly imagine I’d need something beefier than a vanilla wall-wart. My thought was to use a connector like the D-shell power connectors you probably have on the back of your computer, so you could remove the cord for tidy storage.

    Perhaps you can further educate me. Obviously, you can run a drill off of 120v AC. Is the motor in the drill wound differently to run on DC current in cordless tools?

    My practical understanding of electric motors is pretty limited.

  10. Myself says:

    According to http://www.instructables.com/id/EBLN6A48CVERIE1R92/ they’re actually universal. I’m not big on motors either, or anything with an inductive component for that matter, so take it with a grain of salt. You can certainly find a lot of material about various motor designs online.

    When you say D-shell connectors, did you mean http://en.wikipedia.org/wiki/D-subminiature or the http://www.stayonline.com/reference-iec320.aspx style?

  11. EJG says:

    Wouldn’t it be easier to get a proper AC/DV transformer and plug the drill into that?

    Then you make the drill a lot lighter and have no worries about overheating.

  12. Mike K says:

    As far as making a constant power supply for tools, two things come to mind. 1) Doesn’t that defeat the entire concept of “cordless” tools? 2) There are already tools out there with constant power sources that are already proven tried and true, they are called “corded” tools.

    The exception is the idea of converting a tool to run off a car battery. That has potential. I could see breakdown mechanics (i.e. AAA, towing companies, etc.) using such a tool, but in my experience they usually just tow you to a service station. A do-it-yourselfer may make use of one for camping, RVing, road trips, etc, but I wouldn’t be surprised if there is a nitch market already producing such items.

    I am however interested in more discussion about using higher voltage batteries in lower voltage rated tools.

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