Adding a Controller Pre-charge Switch

The Controller Pre-charge Switch: Protects the breaker-switch and controller from damage from sparking at switch-on



  • A crimper
  • A pair of wire snippers
  • A soldering iron and solder (optional)
  • Epoxy resin glue


So what is this for?

If you’ve been following my blog, you’ll know that I recently had to replace the breaker switch which began misbehaving a couple of months ago, and then completely conked out shortly after I put in a bodge to keep it going.

I originally assumed that the reason it broke down was that these switches were simply not designed for regular use, and that they would mechanically wear out fairly quickly if used with any frequency. However I subsequently heard a totally different explanation from members of the Electric Motoring Forum, and one which was to have implications for the way that the breaker circuit should be set up.

The reason was indeed to do with the fact that these switches are designed for a slightly different job, but has nothing to do with mechanical wear and tear alone. It turns out that while usage of these on an AC circuit poses no problem at all, DC voltages of any magnitude result in a pretty brutal spark every time you turn the thing on. Hence the loud “crack” that can often be heard whenever the switch is engaged.

The spark generated by the DC – particularly on bikes that have undergone the 72V upgrade – will, with constant use, quickly burn out the switch, or – even worse – damage the capacitors in the controller and send it to an early grave. Though a replacement switch is not expensive, modifications to the replacement and an extra hole in the mounting plate is often required to make it fit. In addition, the switch is awkwardly located, and dismantling the bike to get at it can be a tedious and time-consuming job.

Failures of the breaker switch that comes with this bike are therefore quite a common nuisance about which users frequently complain, but, as usual, more electronically literate forum members such as Mike have come up with a clever workaround that avoids this problem, and thus allows people to use this main cut-off switch without fear of damaging it or the controller.

How it works

The way it works is very simple. Usually when the breaker is switched on, power is rapidly drawn by some big capacitors in the controller (which is why there is a big spark and that ‘snick’ noise as it engages). The capacitors are charged almost instantly and ‘equalised’ with the supply voltage whereupon the current flow stops again just as quickly.

But the initial, sudden ‘jolt’ is what causes the problems. To avoid this, a switch and power resistor are connected in parallel with the breaker switch (that is, wired between the front and the rear contacts of the breaker switch). Before engaging the main breaker, this switch is activated, which allows current to be slowly drawn into the controller over a few seconds. Once the capacitors are fully charged, the main breaker can be switched on without any problems (as the voltages are now equalised). The pre-charge switch can then be reset until the next time that the bike needs to be turned on at the breaker.

This ‘soft-start’ to the controller’s main power supply is a great way of protecting both the breaker switch and the on-board capacitors, prolonging the life of both and preventing failures.

Building the switch assembly

Building the assembly is an easy job, requiring only two cheap components, a couple of bits of wire and a pair of spade connectors. It took me less than an hour to make a nice, tidy job of building one, and that was even with taking snaps as I went along. The main components are the 7W, 1000Ω resistor and a small ‘slim-line’ rocker switch (I got these from Maplin for under £2).

A couple of bits of red wire, a pair of spade connectors and some wire sheath for the resistor legs are required to put it together.

Just for good measure, I also stretched out some short lengths of heat-shrink, to cover the bare metal of the spade connectors. I removed the original plastic covers so that I could solder mine as well as crimp them, for a nice, secure connection.

First, I made a wire that would go from the rear of the breaker contact to one terminal of the rocker switch:

I used the red sheathing to cover the legs of the resistor and soldered and crimped one of them to the other spade connector. This will connect to the forward breaker contact.

Connect these to the switch. I’ve connected mine up so the ‘on’ position of the rocker will point the rearwards, like the breaker switch.

Next, you install the finished assembly, connecting it to either end of the breaker switch. You’ll need to remove the seat to access the breaker.

Once fitted, you can test it by reconnecting the batteries and attaching multimeter probes across the ends of the breaker switch (make sure the breaker and pre-charge switch are both in the ‘off’ position). The meter should read somewhere in the region of 72V with the capacitor fully discharged, depending on the charge state of the batteries. If the meter reads less than that, then the capacitors still have charge in them. They can be discharged by turning on the ignition for a few seconds, whereupon you should see the voltage rise and then settle at the proper level.

With the multimeter still in place, you can now turn on the pre-charge switch. The voltage should drop (exponentially) towards zero over the course of ten seconds or so. You can then engage the breaker switch, whereupon the remaining voltage will drop straight to zero. At this point, the pre-charge switch can be turned back to the ‘off’ position.

You can see the results of this test on my YouTube video.

The final assembly: The pre-charge switch is glued to the side of the breaker

Once you’re satisfied that everything is working fine, you can secure the switch to the side of the breaker, or whereever else you want it. I just used epoxy resin to glue it to the side of the breaker.

And there you have it! An excellent little safety feature that should prevent breaker burn-out and even extend the life of your controller!

Acknowledgements: Thanks to Mike and Flexy for the explanation and component details for this fix.


One Response to Adding a Controller Pre-charge Switch

  1. Thygadgets says:

    Thank you. This sorts out things for me..

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