Maker Pro
Maker Pro

e scooter power switch oddity

Pharaday

Jan 18, 2016
138
Joined
Jan 18, 2016
Messages
138
Hey guys I built an e scooter and everything works great but there's one thing I was wondering about. I bought a 48 volt 3 phase DC motor controller and in order to power on the scooter, there are 2 wires that must be connected using a switch. The thing is that when I connect my multi meter to those 2 wires, I see that they are positive and negative system voltage wires. I see a full 48 volts. How does connecting positive and negative not blow up the battery???

Until I figure this out, I'm legally insane.
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,763
Joined
Nov 17, 2011
Messages
13,763
How does connecting positive and negative not blow up the battery???
It would if these wires were directly connected to the battery poles. But they aren't, they are connected to a controller and the voltage you see is just the sense voltage the controller uses to detect the switch position. Current is internally limited by a high value resistor.
Set your multimeter in mA mode, measure current between the two wires. You will see a low current, probably a few hundred µA only.

Compare this to a light switch in your home. When you measure across the open switch you will see the full mains voltage 115 V - 120 V. But when you close the switch you won't short circuit your mains but turn on the light instead. The light bulb (or equivalent CFL, LED) limits the current.
With the switch off there is no current so consequently there is no voltage drop across the lamp and the full mains voltage is apparent across the switch.
 

Pharaday

Jan 18, 2016
138
Joined
Jan 18, 2016
Messages
138
hmmmm ok. that makes sense, it just seems odd that the positive wire of the power switch is spliced directly onto the main positive wire that goes straight battery positive terminal (no battery management system). so basically to turn it on, I COULD just plug the negative wire of the power switch straight to the battery positive terminal and cut off the red wire of the power switch completely.

Anyways interesting! I gotta turn my gears on that one a bit. Thanks a lot, Kapp!
 

Pharaday

Jan 18, 2016
138
Joined
Jan 18, 2016
Messages
138
guys what do you suppose would happen if you put a resistor between that positive and negative
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,763
Joined
Nov 17, 2011
Messages
13,763
guys what do you suppose would happen if you put a resistor between that positive and negative
You'd have a resistor in between...

Since we do not know what's behind these connections, we cannot say for sure what will happen with a resistor instead of a switch.
Most likely you will find that for some high resistance values nothing will happen whereas for low resistance values it will work like a closed switch.

Why don't you just follow the instructions?
in order to power on the scooter, there are 2 wires that must be connected using a switch.
 
Top