Yes. It's not ideal connecting to the charging port.
First I did it connecting directly with a power resistor in line to lower the current.
Later I changed the resistors to a portable step-up charger from aliexpress.
But still it's not good because I think the battery disables charging when it heats up so the charging is slow.
Now I did it like I unscrewed the motor and pulled out my own cables with connectors where I can simply unplug the main battery(I just leave the ground) and plug my external battery basically directly to the motor.
The bike motor still thinks it's plugged to the original battery and it always shows a full charge if I use the external one, so I monitor the discharge myselt with a lipo buzzer. When the external one is discharged I stop and reconnect back the original one and drive with that.
I can use any external 36V battery. But I just reuse my old lipos from my model airplanes since I have a lot of them. I must have the original always on the bike because it sends a signal to the motor that the battery is there. Without it I would need to put a microcontroller to generate that signal.
You do not need any diodes to stop the external battery charging internal battery? I've read in some other topics that diodes are necessary. Did you have any problems with it?
Ahh I see. You are not using them both at a time. Just switching between them. How did you unplug + from internal battery? Do you have a switch for this?
Yes I left the batt ground to the motor as it is, just peeled 1cm of insulation an soledered my own - onto it. And then splitted the + and left it outside with the - so I can connect the external one. I'm doing more in a DIY style and use a small piece of black american tape so hold it on the side.
First I did it connecting directly with a power resistor in line to lower the current.
Later I changed the resistors to a portable step-up charger from aliexpress.
But still it's not good because I think the battery disables charging when it heats up so the charging is slow.