Ebike Battery Charge Safety: Heavy Duty Cutoff Timer

Add a safety layer to ebike charging practices, with residential/commercial grade components rather than the cheap stuff.

Nip It!

In my previous post I dug into the subject of using a mechanical cutoff timer plugged between your ebike charger and the wall. It gives you an added layer of safety when charging your ebike battery.  Using a timer, coupled with some advance planning and brain power, you physically cut the power to the charger. In doing so you prevent even the possibility of a charger or battery malfunction. I went into the details thoroughly in that post.

To do this job I used a cheap, basic timer bought online that I have been using for years.  Recently, others who also use this timer have taken it apart and looked inside. The verdict: It will do the job, but it is not particularly robust or fault-tolerant.  

How Do We Beef It Up?

Use a better timer. Chances are pretty good if you live in the USA you have already seen and used this exact timer many times. They’re on the wall in almost every hotel or motel bathroom: The timer that turns your heat lamps off after a short while so the bulbs don’t burn out. You also see them on whole-house fans in private homes (you have to live where it gets hot to have these).

The one brand that seems to be everywhere and in use for decades is Intermatic. They seem to last forever, so the hope here is it will also last forever with your ebike charger. Since it is made to conform to residential electrical codes, its internals should be safer and more robust than any teeny tiny timer you bought on the internet.

Following that, what will we use to go along with it? After all they’re called ‘wall timers’ because they are usually built into walls and connected to your power grid. We want this thing to be reasonably portable, and still safe.

Lets Make A Parts List

I have built a few of these. I have bought parts from a local Home Depot, or from Amazon. Neither is the best place for everything. Some parts are cheaper at one source than they are at the other, but the bargains are all different parts. Buying everything from just one or the other, you end up spending the same amount of money. If you want to save a few bucks, shop around locally and buy a few bits in town. You should be able to save about US$15. I am going to stick to using Amazon on this parts list.

Project cost if bought from Amazon is about US$75 if you buy the 10-foot cable. Thats a lot more than the cheapie timer but it is WAY less than your fire insurance deductible on your garage.

Intermatic 12-Hour Spring Wound Timer

Different versions of this timer exist to span ranges from a few minutes to a few hours. This 12-hour version (Model SW12HWK, Single Pole Single Throw) is less common. It is usually an order-in item if bought locally. I chose 12 hours because I like to charge big batteries (35ah!) at low current rates, so a long timer works better for me. Its also almost guaranteed to work for a variety of other jobs, so I am not blowing US$75 on something that is only good for one thing.

The image above is what you will see on order pages. It is a bit deceptive as the timer does NOT come with the decor-style faceplate seen in the product image. You have to buy that separately.


The commercial series by Intermatic has a slightly different look and a faux brushed-aluminum 1-piece faceplate (In days of yore it was made of metal. In modern times its plastic). These are available in a 4-hour version (model FF4H) and a 6-hour version (model FF6H).

Both models, like the 12-hr version, are SPST

This shorter duration may be a better choice, timewise, for more mainstream, manufactured-ebike batteries that tend to be in the smaller 12ah to 20ah range. That is the good news. The bad news is these versions cost more money and can be difficult to find, even on Amazon where you will see some sellers wanting ridiculous prices for them (the links above are the best prices I could find).

This increased cost is slightly offset by there being no need to buy the next item on the list, the decor-style wall plate cover.

White Decor-Style Wall Plate Cover

This is what you’d think the timer would have come with above but doesn’t. I chose white to match the inner timer plate (which DOES come with the timer). The inner plate fits directly into and onto the ‘decor’ style wall plate.

There is no requirement to buy the linked Leviton brand as all decor-style covers are a standard size.

1-Gang Weatherproof Deep Wall Box

This is what we are going to mount the timer into. Heavy duty aluminum boxes like this are ordinarily used to surface mount a switch onto a wall, like bolting one onto a metal wall or similar. The box will come with little screw-in plugs so the big extra hole in the back will be easy to seal up.

I chose to use the ‘deep’ version of a wall box so I have more room to work in. I also used the larger 3/4″ hole version as again bigger holes are easier to work with.

Two Strain-Relief Connector Fittings

One of these comes out the top of the wall box, and one comes out the bottom. They have rubber gaskets inside and when you screw them down, the gasket tightens onto the cord running thru so it is held fast and can’t be pulled apart without deliberate malice aforethought.

The pictured part above has a metal ring that could be used inside some wall boxes, but in this case we’ll discard it and just screw it tight to the box. I have bought this exact pictured part and used it, but in the pictures below you will see I used a slightly different version I bought locally.

Lighted 12-Gauge Outdoor Extension Cord

Read the specs on this cord via the Amazon link above and you can see it is quite a beefy cord. The SJTW rating means its meant for hard-service, rated for 300v and approved for outdoor use. The one I linked is good for 15a and 1875 watts. As importantly, since I am going to be splicing this cable into a residential/commercial electric timer switch, and those switches typically use 12 gauge wiring, I wanted a cord that also used that same thickness of wire, even though it is by necessity stranded and not solid copper wire.

The seller I linked sells the cord in a variety of lengths. You can decide whether you want a short or long one. I went for 10 feet when I put mine together as that is what fit best from my outlet to my charging station.

Lastly, notice the cord has a lighted end. This will be handy for when the timer turns on the power. We know its hot by seeing the light at the end of the plug.

If you live outside the USA, your electrical cords will be different, so you’ll want to find a similarly overbuilt cord for your project.

Aside from two heat shrink butt-end connectors we will need, thats it for the parts. There is a complete tutorial on how to make reliable crimp connections here. You can refer to it if needed.

Lets Get To Work

Step 1 is to figure out just where on your power cable you want to place your timer box. For my project, since I was mounting the box in my garage I had to measure from the wall plug, to the place I was mounting the timer. I also had to be sure I had enough cord to reach from the timer to my ebike’s charging cable.

So… Using a sharp and sturdy cutter (12/3 cord is not going to cut cleanly with ordinary scissors) cut your power cord completely in half at your desired spot.

Next, cut back the cord insulation without slitting the insulation on the inner three wires. I used a utility knife blade held in my hand, and cut very carefully. Just as important to not slitting the insulation on the inner wires is to not slit yourself, either. This process requires considerable caution. There are specialized cord stripper tools out there to take the danger out of the process.

After this, strip a short length of each inner wire on each side of the cord. The end result should look like this:

With your two stripped ends ready, thread the strain-relief connectors over each end of each cord. Now run the two cords thru each side of the box, one from the top and one from the bottom. Screw down the connectors into the box, but do not tighten down the outer rings, so it is still possible to move the electrical cord back and forth thru them. When you are done it should look like this:

Now its time to re-connect the cord and put the switch in between. Here’s where things get tricky if you intend to mount the box vertically on a wall, as I did. This is where the deep box giving extra room comes in handy.

You only need to plug the hot wire (black) into the switch. The neutral (white) and ground (green) wires can be connected right back together again (note to folks outside the USA, your wire colors will be different and will match an international standard that – of course – we don’t use here in the States).

The neutral and ground wires are reconnected with simple heat-shrink, adhesive marine butt-end connectors. The hot wire is attached to the switch via its simple teeter connectors, which work fine on stranded wire even though they are meant for solid copper. This connection is a big part of the reason I chose thick 12-gauge electrical cord.

When deciding what side goes where, the Line side (male plug end) goes to the power source (the wall). The Load side (female plug end) goes to your charger.

Because of this Line and Load business, and the fact that I was mounting this switch vertically (notice the directional Top markings on the front of the switch) I had to run the top cable to the bottom of the switch (Line) and the bottom cable vice versa to the top (Load). Thats why everything is running around in a circle inside the box.

If you look to the right side of the switch you’ll see Line (2) and Load (2) plugs are there and reversed, which would solve this alignment issue. However, this second set of connectors are dummies lacking the hardware to be usable. They’re just blanks not used by this Single Pole, Single Throw switch.

Once you get to this step, all the real work is done. Carefully move the switch into the box, ensuring the connected wires are not subject to any pulling stress. You’ll want to be tugging back the still-loose electrical cord a little to help make room, but again don’t overdo this. You don’t want to cause any strain on the switch connections, and you have a deep-version of the box so there’s room to fit the cable behind the switch.

Screw it down carefully once its fit into place. The screws to do this came with the switch.

And now screw down the decor cover. Take care not to torque it down too hard as it is a delicate part and you can crack it if you go too far. Next, tighten down the two strain-relief connectors, which fixes the cord in place so you can’t tug it apart.

Those last two steps took only about a minute, and we’re not going to need much more than that to finish the job: The switch comes with a little cheapie hold-down ‘nut’ that is little more than thin bent metal. It needs to be that thin as there are almost no threads available for it to grab. Drop the inner timer plate onto the opening. Tighten down the nut so the inner plate is now fixed to the timer face.

Don’t overdo the tightening. You will figure this out as you see the plate bend inward as you tighten. I used needlenose pliers as fingertips don’t quite have enough grip to do the job.

Next, press on the timer knob. You’re done. You just created your heavy duty timer, that you can use to plug in between your ebike battery, or anything else you want to provide time-limited electrical power.

Now What?

Plug that sucker in and make use of it! This is the part where it becomes handy to have the lighted plug end. Turn the timer on and you’ll see the power come on via the little light. Come back a few hours later and the presence of a light at that plug will instantly confirm the power is on before you even glance at the timer dial.

Author: m@Robertson

I'm responsible for the day-to-day operations at my place of business: Leland-West Insurance Brokers, Inc. We do classic and exotic car insurance all across these United States. I'm also an avid auto enthusiast, a born again cyclist (i.e. an ebiker) and participate in medium and long range CMP and NRA sanctioned rifle competitions.

7 thoughts on “Ebike Battery Charge Safety: Heavy Duty Cutoff Timer”

  1. Or you could just buy a UL listed 15a smart power plug with built in timer functions for 12.00 on Amazon.

    1. We already explored the *intelligent* bottom-of-the-line in the previous post on this same subject, and it cost less than your smart plug. Relying on anything but the simplest, most fail-safe tech to shut down something as critical as an ebike charger creates additional ways to fail. Thats why the low end solution is mechanical, and the high end solution discussed here is mechanical.

  2. Sooo… I just use a feit smart plug adapter. Works great. I plug everything in and then turn it on from the app and set a timer somewhere between 4 and 8 hours depending on what kind of charge my battery needs before it shuts itself back off. I got a three pack of these from Costco for $15. The other two are currently being used as TV time or so my kids don’t stay up all night on TV.

    1. The simple and cheap solution was already demonstrated in the previous post, and its cost is US$9.99. The point of this article was stated right up front: To use a robust and more fail-safe solution that uses overbuilt equipment (the timer in particular). What you are describing is just another cheap solution, but it introduces added complexity and has more ways to fail. A dirt-simple mechanical dial is faster to use, immune to the wider range of mistakes possible using an app and generally more fault-tolerant. KISS principle applies. That 3-pack of Feit plugs is no longer stocked at Costco and the best price I can see is $32.95 at Home Depot. Single units are $9.99, So at best its the same cost as a mechanical solution that needs no electricity and less intelligence to operate.

  3. I’ve been thinking of this and for some reason never thought of using construction grade stuff and build it. I’m going to do this since I’ve already started tracking what goes back into my batteries with one of those watt meters already, now I can have it shut down automatically in case the charger malfunctions, which it kind of is. I have a LUNA charger that is putting out more amps than selected by 4x selected… so I have it set to charge at 1 amp but it puts out 4. It does this intermittently. Time for the Grin Satiator.

    1. I had the same problem and initially solved it with a Satiator. But you can get the same or better reliability for about 1/3 the price. I wrote up the whole process here: https://tinyurl.com/mevdr3p4 the article is entitled An Ultra Reliable Ebike Battery Charger, and it is also weatherproof and adjustable for current and voltage. I have one Saatiator but several of these at various locations, plus some mounted onboard my cargo bikes so I can just plug in like the bike is an ordinary appliance.

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