Bafang Ultra, Frey EX - Battery hack


100 mW
May 23, 2020

Frey EX comes with twin battery as optional. This creates issue, what happens when one battery is empty and someone connect external fully charged battery? Since both batteries connected in parallel to the motor they would equalise, but rather rapidly at the rate limited by battery BMS. In the process possibly damaging connectors due to arcing on connection.
Battery manufactures solved this issue by installing two 50A, 58V diodes to prevent batteries to cross-feeding each other.

While this is convenient safety device, it create an issue, as diodes have forward voltage loss of 0.75V. This means, fully charged battery will show on display as 90% full and will be switched off by Bafang safety cut-off earlier due to lower voltage at motor.
Apart from losing some 10% of battery capacity, this diode is also causes 10-20 Watts of loss to heat (load dependant), due to resistance across it.
Reversible work-around it is to unscrew battery terminals and relocate them, bay-passing diodes.

While testing the bike, it actually performed noticeably better on the road. There was reduction in voltage sink under the load, usually over 3V at full load to just over 2V. Bike gained some 15% range increase.
The drawback, external battery must be at the same level of charge as bike inbuild battery, when connecting to the bike.


100 mW
May 27, 2020
To put classic diodes, it's totally wrong.
To couple batteries, you have to put "ideal diode" circuit, done with Mosfets.
Incredible error