Nov 21 2010

Cordless Tool Charger Safety: Put your Chargers on a Timer Circuit.

Published by at 5:06 pm under Shop Electrical

Rechargeable tools ultimately need recharging. Most chargers will recharge batteries in less than two hours. Ordinarily we don’t give this process much thought, trusting that the charger will do what it is designed to do, provided we are using only the proper charger for the battery that we are charging. It seems to have been designed to require little oversight from the end user. Simply plug the battery in, and after a while, it is recharged and ready for more work.


What happens after the battery is charged depends on the brand, style of charger, type of battery, and other things that are too hard for most people to keep track of in a mix and match world. Some chargers shut off, some change modes from fast charge to maintenance mode. Usually the battery is recharged safely, but there have been issues and product recalls on these devices.

Some people have reported on the Internet in various places, that battery chargers can sometimes electronically fail and become the cause of fires that burn shops or houses down. Other people have reported that their battery chargers work just fine and they don’t see a problem. I have personally noticed that I have tool battery chargers that continue to maintain batteries after charging them. When I return to remove a battery from the charger, sometimes a day or more after I put it in there, the battery is warm. This is normal based on the type of charger this is.

For my own peace of mind, I did phone all the manufacturers of my chargers, with the models and serial numbers of the units in hand, and affirmed I was using the correct batteries with them, and that none had been recalled, and learned I was in good shape. It is probably good to affirm we are not using recalled chargers and not know we are doing this. It is also good to affirm we are not mixing and matching batteries, which with some charger designs I have heard can happen. I could be wrong, but why not check to be sure. Anything that can prevent a hazardous condition is better that having one and not knowing it.

Rather than debate or research battery charging safety endlessly, I decided that all manufacturers of these devices are doing their utmost to assure product safety. I also decided that removing power from any brand or style of tool battery charger with a battery in it, after I initiated the cycle and the proper charging interval has been achieved, seemed to be the safest way to charge tool batteries to me. However, I may not be able to remain present for the duration of a charge cycle, as they vary, and may even forget to manually remove the batteries once they are charged. Creating a timer circuit is a good way of limiting charge time, and adding an extra layer of safety to the battery charging process.

Tackling an electrical project isn’t for everyone. It does take some understanding of electricity, designing circuits and knowing what is acceptable practice to accomplish a timer circuit. It isn’t difficult overall, but the fine details of electricity theory and how to wire for safety and to code are really beyond the scope of what I am sharing. Good electrical wiring practices are universal, but all codes are local, I leave all that to those who would like to have a timer for themselves to consider.

If you know how electrical circuits are developed and made, then the following is something you’ll already understand. If you do not, then my recommendation is to take this idea to someone who already understands electrical circuits. It really is the safest way to go. Many DIY oriented people have some electrical skills, and if nothing else, an electrician would likely be more than happy to wire up a timer circuit for a nominal fee.

I went to the orange box store to pick up my supplies.


My intent was to purchase the parts to make my circuit with a design rating of 15 amps. I picked up an Intermatic brand 12 hour timer that has Single Pole Single Throw switching and is rated 20 Amps at 125 Volt AC Capacity, and a 20 amp Plug receptacle. I chose a 20-amp receptacle, because that is what was available as the highest quality part when I purchased the parts. Had they offered a 15-amp receptacle in comparable quality, I’d have bought it instead. It was a color match thing.


The wiring consists of a 15 amp six foot Air Conditioner cord, and I bought a one-foot length of 14 gauge stranded conduit wire for additional wiring I needed inside the box. Overall, based on the amperage rating of the power cord I used, this circuit will be only good for 15 amps, which is more than plenty for running a charger or two at a time. No matter how much you overbuild this, it should never exceed the amperage of its lowest rating, or the rating of the circuit supplying power to it.

The other pieces were the box. I got the deepest box I could find, so as to accommodate wiring behind the thick timer. The cover plate is punched for a receptacle and rectangular rocker switch. The wire clamp to hold strain relief for the wiring inside the box comes in a package of five.

I only needed one, but that is the way they sell them. I have extra for future projects, and that is fine, I have made a number of foot switches for my electrical tooling as well.

Some of the little details will involve breaking off some of the tabbed mounting points on the receptacle. You will find that the timer does not have break off tabs, but some of the extra mounting holes will be in the way for this box. I ground them off with a bench grinder and dressed the rough edges out with a bastard file.

Look at your box and consider where you want the cord to enter. I decided to come in through the lower right for me, but these boxes are filled with options, go with what makes sense for your set up. Once you decide, these precut holes have covers that pop out easily with a hammer and punch.


Mount the wire clamp to the box where you punched the hole, cutting the molded female receptacle off the cord, and threading it through the wire clamp into the box was next. Then strip back the outer grey wire jacketing taking care not to nip through any of the jacketing on the inner wires. You’ll need to consider how much of the jacketing will need removed to properly negotiate all your connections.


Next, you’ll want to mount the timer and the receptacle to the cover. The cover is made so it can be mounted with either the receptacle on the left or right, so now is the time to choose the orientation. You may find that it will be easier to make some of the electrical connections before you mount the timer and receptacle to the cover. I’ll leave you to sort that out. It’s handy to pay attention to the wire stripping guides on the Timer and Receptacle prior to stripping the jacket back to expose the wires.


Remember that one foot piece of wire you bought extra? This is needed to make a jumper from the timer to the receptacle. One more wiring tip. Throughout the circuit, remember to observe polarity.


Now that most of the assembly is ready, there is one more decision before mounting the cover to the box. I made my timer circuit so I could easily screw it to a wall wherever I needed, without having to open the box to accomplish this. You alternatively may have a permanent location, or may even be choosing to build this circuit right into the wall.

timer_strainrelief.jpg I decided to mount my box to a backing board. You can use plywood or hardwood or whatever you like. I used a scrap of Poplar. Go ahead and lay out the mounting points for both the box and the board, then do the necessary drilling and countersinking if you are using a backer board, or mount the box to the wall before you attach the cover. Now you should have a complete assembly.

Before using the timer circuit, I recommend using a circuit tester to affirm you have observed polarity.


Some electrical appliances are not double insulated and prefer that the wiring be correct. Many circuit testers will tell you what is the problem and how to correct it.

Once you have tested and all is well, you now have a timer circuit you can set to shut off your battery chargers up to twelve hours after you set them. While most chargers will require less time, it is hard to find a timer that offers a time which is long enough for most charging intervals. To me the twelve-hour timer seemed a good fit. Hopefully having a timer circuit on your battery chargers will give you the peace of mind mine is giving me.

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