I have a TW200, and I am wiring a usb port with a voltage read out. The only way to get a accurate reading of voltage is to connect to the battery directly. Testing with the low beam wire that comes on with key switch, the voltage is 1V lower than the battery. I don't want to wire directly to the battery and have it slowly drain the battery.

Is there a way to connect a switch that will only let the power wire to the battery draw power when the accessory wire gets power so that it is still connected directly to the battery but only on when the accessories are getting power.

idk if this makes any sense or a better way to go about it.

  • 1
    Yes, use a relay controlled by the acc switch feed. Several q&a on here about relays and they have a connection diagram on them.
    – Solar Mike
    Apr 30, 2020 at 5:22
  • Here is one with a diagram: mechanics.stackexchange.com/a/11429/10976
    – Solar Mike
    Apr 30, 2020 at 5:33
  • Thank you Mike, this will help al ot. Since I know nothing about this I was trying to google "electrical switch" and all that comes up is the switches that go on the wall for lights.
    – Xand
    Apr 30, 2020 at 22:08

1 Answer 1


You can add a Relay (Electronic switch) between your USB Module and Motorcycle battery and make your RELAY get power from accessory wiring/low beam/acc switch, that will turn your electronic switch to connect to battery.

Relay specs:

  • Coil voltage 12v
  • Has normally open (NO) pin (most common relay has it, 5-pin relay)
  • Contact current (2 Ampere or more, as USB does not output much wattage)

Works with 4 pin relay too if it is a Normally Open relay or else go for 5-pin relay and dont connect anything to NC (Normally Close) pin

CO = From Battery positive +, (probably red in color)

NO = To USB Module positive +

One pin of relay COIL (indicated by symbol) from common and another pin of relay coil from acc switch

Connect USB Module Negative - from common line (probably black in color)

Relay Pin Layout Diagram

  • I would agree with this with one caveat: don't use the light switch as your on/off for the relay. If you want to get a "true" reading, it will be with the accessories off. If you turn the lights on to get the reading, it will always read lower, just as the OP states in their question and as @solar_mike states in comments. Use the accessory feed to energize the relay. You can always turn the lights on after that to see what affect that has on the battery/charging system. Apr 30, 2020 at 20:45
  • Thank you.I'm sure I will figure it out now with your help.
    – Xand
    Apr 30, 2020 at 22:12
  • @Xand I recommend adding a 1N4007 diode in relay coil pins matching the correct polarity, forgot to mention above. This adds more protection to your battery and relay by restricting the flow of current in reverse polarity :)
    – Arun
    May 4, 2020 at 8:34

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