Powering battery powered tools from the power grid

Question

I have a number of Ryobi One+ tools that have enough power and quality for me in most cases. They all share the same 18V battery form factor which means that I can invest in 2-3 batteries and use them for any tool, which is nice. However, for longer jobs I would like the option of plugging the tools into an electrical socket and run off of that, to not have to charge or change batteries all the time.

Understandably, companies make lots of money from their battery sales and aren't interested in providing such a solution themselves. I would also break the warranty if I did something like this. However, practically, this this possible?

There are 3D-printed products that act as an adapter from a regular 18V laptop transformer and lets us plug that cable into the empty battery socket of a power tool. These are quite expensive and some reports vary on how well they work compared to the actual batteries. Before buying them I would like advice on if I can build one myself, and what transformer specifications would be necessary.

None of my tools or batteries specify the number of watts they consume at maximum load. When fully charged my multimeter says they output 20V DC. There must be a limit in the batteries so the tool doesn't drain the entire battery instantly and burn up, but is it possible to calculate that somehow without jacking in the multimeter between the battery and the tool and putting it under load? I don't think it would be safe to attempt that. My idea is that if I ran the tool from a 100-120W 20V transformer, or whatever the power I would need, it would simply be a matter of properly connecting the positive and ground wires to the tool battery socket. The easiest way would likely be to sacrifice a battery by taking it apart, removing the battery cells and rewiring it from the transformer. Is this very foolish or not?

Thanks for any help. I have some wired tools that are more powerful, but would really like to use my Ryobi kit both ways when their power is enough.

Answer 1

No reasonably-sized and priced 20V PSU will be able to deliver the power that your 20V tools need. You are trying to design a corded tool that can last through a big job. The correct solution is to buy actual corded tools. Just the ones that you actually need for such big jobs. Plan B would be to buy more, bigger, batteries and a multi-battery charger that you can keep near your work location.

I don't know what current is drawn by battery-operated tools but we can guesstimate. A typical corded 120V saber saw or rotary saw is rated at 10 or 12 amps, therefore consumes say 1400 Watts. At 20V that means a tool with similar power needs to draw 70 amps from the battery. You could improve this guess by rigging up one of your tools to actually measure the current from the battery but let's start with this. You are looking for a 20V 70A PSU.

Forget about laptop PSUs .... even the biggest ones the size of bricks meant for whacko-crazy gaming laptops only provide a small percentage of that power. A typical laptop PSU is 65W. Big ones are over 100W but nowhere near 1400.

20V 70A PSUs do exist, but they are very expensive, well over $500 and they are enormous and typically designed for permanent installation in a workshop. They are specialty items. You won't find one that is practical or economical for your intended use and it certainly would not be more convenient than charging and swapping batteries.

One last thing: If you did manage to rig something up that could deliver the kind of power you want to your tool, you would probably electrocute yourself. Unless you designed it with the same safety standards as an AC-powered tool. Double insulated cables, sockets, and plugs, fuses, etc etc ... all the things designed to protect you with dangerous power delivery. Are you going to design and build that into your 20V solution? No. So you'll eventually get killed. Just go buy one or two corded tools!

Answer 2

Sure you can build a dc supply for your corded tool.

You ask how to calculate the power supply size. Take a known battery with a known amp hour rating. Many batteries state the AH on them some may show a value like 3300 ma hours this makes the number look bigger but in this case it is 3.3 amp hours. Then put that tool under a normal load and see how many minutes it lasts. Calculating the use will be only as accurate as your load. Now for safety double the value you came up with or even 5x and size your components based on that. If trying to go simple transformer there are 18v models out there and full wave bridges rated at 20 amps in small packages you can put in a project box and have your low voltage dc feed the tool.

This could be a fun project and cost less than a new corded tool but is it going to be something that you use? I have thought about it I have Vmax batteries up to 6AH that makes the tool heavy but it lasts long enough for me to get the job done in most cases if I wanted light it may be an option but I have not done it because I already have corded tools in every style of battery tool I have.

Answer 3

Yes you can, with a modification of your plan. I've actually done this with some older 12v Ryobi tools, and it's a nice result; super lightweight and runs all day.

A beefy laptop supply is good, but even that will only put out 10A, max, probably more like 7A. While the general operating current of most tools is only a few amps, the surge current of many is a dozen or two amps.

Instead of a laptop supply I would get a fanless meanwell PSU that spits out 36v and 4 - 10 amps, and a 10-20A "buck regulator" that has adjustable constant current. Set the constant current adjustment on the buck under a heavy load to deliver ~7A, above which it will start dropping voltage from ~20v to keep current at or below 7A.

Now you have the problem of surge current. Nothing can match a battery, so use them. Take a small "power wheels/scooter/farm" 12v and a 6v lead-acid battery (match the AH), wired in series to create an 18v nominal battery pack. Wire that in parallel to the buck converter's 20v output and your tool's input.

The PSU will keep the battery pack mostly charged when not going full blast. When you do hit more than 7A, the batteries will start delivering power instead of charging, and they can spit out dozens of amps. As soon as the load drops the battery resume charging. Don't leave them charging 24/7 for longest shelf life.

Project Cost

PSU: $25
12v Battery: $25
6v Battery: $20
DC-DC 20A Buck: $10

Total: $80-100

You could also use a 20v laptop supply and skip the buck converter by placing a 100W 1ohm resistor between PSU's + and 18v battery pack +, and wire the tool to the battery, so that the batteries trickle charge at 1-2A and your tool mostly pulls from the pack. That seems a little more nerdy/hacky than the DC/DC, but would be cheaper.