Mobile Power From Cordless Tool Batteries

For years, [Michael Davis] has been using a large lead-acid battery to power the electronic components of his custom Dobsonian telescope; but that doesn’t mean he particularly enjoyed it. The battery was heavy, and you always had to be mindful of the wires connecting it to the scope. Looking to improve on the situation somewhat, he decided to build an adapter for Ryobi cordless tool batteries.

[Michael] had already seen similar 3D printed adapters, but decided to make his the traditional way. Well, sort of. He used a CNC router to cut out the distinctive shape required to accept the 18 V lithium-ion battery pack, but the rest was assembled from hardware store parts.

Bent mending plates with nuts and bolts were used to create adjustable contacts, and a spring added to the top ensures that there’s always a bit of tension in the system so it makes a good electrical contact. This setup makes for a very robust connector, and as [Michael] points out, the bolts make a convenient place to attach your wires.

With the logistics of physically connecting to the Ryobi batteries sorted out, the next step was turning that into useful power for the telescope. A stable 12 V is produced by way of a compact DC-DC converter, and a toggle switch and fuse connect it to a pair of automotive-style power sockets. Everything is held inside of a wooden box that’s far smaller and lighter than the lead-acid monster it replaced, meaning it can get mounted directly to the telescope rather than laying on the ground.

If you want to build a similar adapter, the 3D printing route will potentially save you some time and effort. But we have to admit that the heavy-duty connection [Michael] has rigged up here looks quite stout. If you’ve got an application where the battery could be knocked around or vibrated lose, this may be the way to go.

24 thoughts on “Mobile Power From Cordless Tool Batteries

      1. Damn, now I’ve gone and seen you can get a reciprocating saw free with the 2 battery + charger pack and am contemplating breaking my cordless are for idiots rule.

          1. Depends somewhat on the tool/idiot.
            Cordless tools cost more, don’t run forever (assuming you paid your electric bill), batteries will need replacements and spares etc.. So it could be ‘cordless are for idiots’ in that you would have to be an idiot to want them if your needs don’t align well with them – with extension cord or generator there is no reason you can’t use corded tools anywhere you like and they are cheap and potent.

            On the other hand even cheap garbage cordless is great for quick simple jobs a great distance from the nearest power socket. And good ones are no longer massively inferior to a corded counterpart, and being wireless can be more convenient by far.

            Personally almost all my tools are corded or air powered as I rarely work anywhere out of range of my airhose (and air tools let me work in the rain safely and are pretty damn cheap so wearing them out isn’t so bad)

          2. Unless you are regularly working at the top of tall towers or elsewhere that it is really hard to get an extension cord then if you can only have one tool I would definitely recommend a corded one.

            But if you have the room and money to get two of everything then the convenience of having a good set of cordless tools that all use the same batteries is awesome. But yes, you will still be happier with a corded tool when you have a tougher piece of material to cut through or a quick job to do but all your batteries are discharged. And no, having to use an extension cord is not a big deal for an indoor project.

            The availability of 3d printable adapters for mixing brands between tool and battery definitely make cordless tools a more practical choice than before.

            Also, the 1980s are over now. Friends don’t let friends buy NiCd battery packs.

          3. For a while, cordless tools really sucked. I’m guessing RW’s experience dates back from those days – NiCD with poor-quality trickle chargers was the standard (NiMH was basically skipped due to higher internal resistance).

            But the same advances in battery technology that allowed us to have all-electric cars that are viable also has possible cordless power tools that don’t suck. Modern 18v lithium systems are quite good. Charging is much faster since lithium forces you to have a real BMS so you can’t be lazy and just trickle-overcharge to equalize (which is what they did back in the NiCD tool days).

          1. Agreed, for the hobby use version of cordless, only drill and driver are worth.

            Once you tried the construction grade (Makita, Stihl, etc…), with brushless DC motor (BLDC) then it will blow your mind. Sure, you still need a bunch of battery, multi-slot fast charger, change the battery regularly. Overall, the price is x10 for cordless with similar performance of entry level corded. but it will save time and hassle, and for contractor, that’s a no brainer
            For home/hobby use, not really worth it yet (excluding the drill & driver)

            ps: I dont like how cordless mnfg tie you to the battery and charger, as they change model every 5 years, and some tools are nicer in one brand than another.

        1. I’ve never used a battery powered reciprocating saw that I was happy with, even a very new 20V model lacked the power I expected. I bought a Milwaukee “Sawzall” (the original) 15yrs ago and it’s never missed a beat…the first decade it was an every workday tool on construction sites and got it’s fair share of abuse; I still use it weekly.

          1. Ah dang, I thought the power had caught up by now. However, the whole reciprocating saw thing has been weird for years, like good ones are gold dust. It’s kinda how I’ve haven’t got a decent one yet, there’s so many crap ones. The used ones that come up are either flogged to death or being offered at 90% of new price. Well I do have a “hail mary” one, that was pocket change and obviously clogged to hell with drywall dust, someones store brand I think. Cleaned that one up and it sorta runs but screams it’s head off. Funny though, it does seem to have decent power at only 7A until it heats up and goes double screamy. Weirdly, you get some people saying the 7A ones are quite powerful and other people saying 12A ones are weak as crap. I’ve also borrowed a couple and didn’t like them, one seemed super weak, the other seemed okay, but had a short stroke so it would keep grabbing and just rattling you up and down rather than doing anything. So anyway I thought they’d be able to do a decent equivalent of the 7A corded in cordless by now.

            Last couple of years the good value corded stuff has been slipping away though, so apart from used, the midrange is all in cordless now I guess, and they should have got them right by now.

        2. In my experience as a tradesperson cordless tools have supplanted their corded brethren in nearly every category other than pneumatic nail guns and corded chop saws. Running extension cords everywhere you need to use a tool is a pain that I do not miss in the least. Even in a workshop environment I would be hard pressed to convert back to corded tools. Most of the tools I use now have taken the battery weight into account with the ergonomics of the tool and I find them less cumbersome to use than the corded equivalent.

          1. Don’t get me wrong, modern cordless tools are great…the “top shelf” models of drills and impact guns have been there for years.

            About the same time I got the Sawzall, I picked up an 18V Bosch “Brute Tough” drill with a 1/2″ chuck…that thing would run a 1″ auger bit through double toe plates (two 2x4s stacked to make a 4×4) and a 1″ plywood deck, while cutting nails. It has also thrown me off more ladders than I care to admit.

            I dont mind running extension cords when needed, just like I don’t mind running air lines for a nail gun.

        3. Cordless tools have gotten a lot better than they used to be. I pretty much never looked back after going cordless. The only corded hand tool I still own is a large circular saw. I still use my cordless one for most things. Every once in a while I just need a bigger blade and more oomph than the cordless skill saw can handle. The one thing I don’t like about cordless tools is getting locked into one ecosystem by incompatible battery designs.

        4. Most cordless tools are intended for light duty, around the house occasionally. You get what you pay for… There are a lot of construction grade tools, but they come with an ugly price tag. I use my cordless reciprocating saw, more than any other saw around the house. Only need to make a few cuts, here and there. Don’t miss wrestling an extension cord, or worrying about water outdoors. I’ve got a 12 inch blade that makes quick work of a few branches. Really nice during hurricane season. Haven’t needed the chainsaw in years. Probably not ideal for large projects, but it’ll get through most simple things, metal, plastic, wood, as needed.

    1. I actually considered doing that. Problem was that we were under lockdown for the Covid 19 virus at the time. There was no going out to go shopping. I might have been able to buy something online, but Amazon was prioritizing food and medical supplies at the time. Besides, I enjoy a challenge. So I figured out a way to do it with what I had on hand.

    2. My solution was buying a ryobi flashlight, then replacing the incandescent bulb with a LED (I suppose an astronomer would use a red led and filter to make it dim) and install connectors and power supply in the vast empty interior of the device. When you’re working at night you can never have too many flashlights…

      Amazon and/or ebay are good sources of 18 volt LEDs. Modern ryobi tool flashlights might already be LED of course.

      My application was a portable ham radio power source. A ryobi one large lithium battery will power a small ham radio for a VERY long time, and is durable and easy to recharge.

      I’m holding a ryobi battery that claims 72 watthours in my hands while typing this. Even assuming those are “marketing watthours” and power supplies to convert to 13.8 volts are not 100% efficient, that would run a 5 watt ham radio for at least ten hours continuous transmit, and given the handwaving estimate of 10% transmit time to get real world usable lifetime, this battery should run my radio for a cool 100 hours per charge.

      The nicer lithium batteries have a little bargraph LED gauge of stored charge, and I’ve never drained an entire bar in one use. I’ve considered locating a vastly uprated power supply capable of 30 amps or so while still having good RFI qualities, and running a 100 watt-output class ham radio off a drill battery. Certainly, a fractional horsepower rated drill battery could handle a mere 200 or so watts for a few seconds of dits and dahs. They sell an air compressor that uses ryobi batteries that must draw a horsepower for minutes at a time, so a mere 100 watt radio shouldn’t be an issue?

      1. For a transceiver power supply, put a small linear supply in parallel with a large switching supply, such that the switcher is only active at high current draw. Very low noise when receiving, very economical way to get a lot of current when transmitting.

  1. Tweepy says:
    June 13, 2020 at 1:06 am

    “ps: I dont like how cordless mnfg tie you to the battery and charger, as they change model every 5 years, and some tools are nicer in one brand than another.”

    Ryobi has kept their ONE+ 18 volt battery and tool line compatible for over 20 years.

  2. Seems like this would quickly damage the battery contacts, since it’s just a screw head scraping them. Maybe a polished finish carriage bolt that’s completely smooth on top would work better.

    1. That is something I worried about too. So far no problem, but it is early. Being locked down at home when I built it, I had to use what was on hand. Now that I am free to move around again (with appropriate PPE of course) I may get some brass screws at the hardware store to replace the steel ones.

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