I have custom-build e-bike which is basically a motor, battery, controller and an LCD screen with 3 buttons. When I push ignition button, the LCD sets voltage on one of it's signal wires (blue) to 51 - 67.2V (BAT+, depending on battery capacity) which is then wired to the controller.

I would like to sense voltage on this ignition wire and switch on/off an external dc-dc converter (8-72V to 5V5A) in order to charge my phone while riding a bike. One downside is that I can't pull too much current on this signal wire - dc converter need to be wired directly to the battery by mosfet or something else.

After 3 minutes of idle, LCD automaticly turns e-bike off (0V on ignition wire) which is okay, i want to charge my phone only while riding.

I need a solution that uses no power when switched off in order to not drain the battery. I'm thinking of using a MOSFET as a switch where this ignition wire would be connected to the gate of MOSFET. As far as I understand I need a MOSFET with Vgs of at least 67.2V? Could anyone point me at a right mosfet? Wiring schematic and any tips on this would be great too. Thanks!

PS. Added schematic of my wiring Wiring diagram

  • \$\begingroup\$ You may want to draw a schematic of it. \$\endgroup\$
    – Long Pham
    Sep 1, 2018 at 13:42
  • 1
    \$\begingroup\$ To save charge of the e-bike, the external dc-dc converter (8-72V to 5V5A) should be accitivated only when a phone is connected AND you ride the bike. \$\endgroup\$
    – Uwe
    Sep 1, 2018 at 13:58
  • \$\begingroup\$ I've added a schematic. I've tested the dc-converter and it's not using much of current when idle so I think I don't need to check if my phone is connected. It's okay to power converter at the time when bike is powered on, even if there is no load on the converter. \$\endgroup\$
    – Marek
    Sep 1, 2018 at 16:05

2 Answers 2


Use a relay, like this one. It can handle 72V.


I think you missed the point @Uwe was making. Unless your are using some kind of wireless charger, you have to plug your phone in order to charge it. So, your circuit should detect when the phone is plugged in, not when you start the bike.

This probably can even be done by some kind of mechanical switch. For example you can attach charging cradle to hold your phone securely and avoid dangling wires. Then a switch built into this cradle will connect DC-DC to the battery when you plug a phone. As simple as that.


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