Electric Bike Battery / DC-DC voltage conversion Q

Discussion in 'General Electronics Chat' started by jayg867, Dec 29, 2014.

  1. jayg867

    Thread Starter New Member

    Dec 29, 2014
    Hi All,
    My homemade e-Bike 36 volt battery is about dead but I have a 48volt Li-On that I'd like to replace it with.
    The bike's DC-DC converter and motor are sized for 36volts.

    My question is what Power wattage of resistors do I get to create a voltage divider to power the 36 volt input of the DC-DC converter?

    My assumption for R1 / R2 would be a simple R1=250 ohm, R2 = 750 ohm.

    Both batteries are 10amps. The load is variable with motor speed but my assumption is the motor could draw up to or near the battery current capacity of 10 worst case.

    Thanks for any / all help. Also, any suggestions of where to purchase these large resistors would appreciated.
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    You cannot drop your 48V to 36V using a resistor. Firstly, the resistor would burn off a considerable amount of power as heat and secondly the output voltage would be entirely dependent on the load which would always be changing i.e. you would never actually get 36V and as the road load increases the voltage to the motor (and therefore the power and torque) will fall; the resistor, however, will set fire to your trousers.

    The only practical solution is to change the controller for one that works at 48V.
    Last edited: Dec 29, 2014
  3. jayg867

    Thread Starter New Member

    Dec 29, 2014
    LoL. Good to know. Ok, a new controller it is. Thanks for the input!