The Power Booster Battery is an great affordable way to extend your stock 36V Sondors battery into a 48V powerhouse, giving you greater hill-climbing power, speed, acceleration, and even some extra range (since your stock battery doesn’t have to work as hard).
But what happens when each of the batteries run out of juice?
The Battery Management System (BMS) within each of the batteries will detect a low voltage and automatically shut off the battery to prevent the voltage from going too low, since if the battery voltage gets too low then the battery (and the cells within) could be permanently damaged. If this happens it would be dangerous to try to recharge the (damaged) battery – hence the reason why the battery is shut down by the BMS before the “point of no return”.
Assuming the motor is still running and trying to suck up power from the batteries, the battery that’s still on will still be trying to supply the motor and will be inadvertently charging the battery that’s off – but in reverse! This is known as reverse charging and it’s not good. (If the motor is off than nothing will happen.)
If the stock 36V battery runs out first, then the 12V Booster Battery isn’t putting out enough voltage to damage the 36V one. But if the 12V Booster Battery runs out first, then a positive 36V potential will be applied to the negative lead of the 12V Booster Battery (compared to it’s positive lead). This could overload the protection circuit in the 12V BMS and damage the Booster Battery unless something is done to alleviate the situation.
Fortunately, the Power Discharge Cable of the 12V Power Booster Battery is equipped with diodes to allow all that negative 36V power to bypass the Booster Battery and prevent a dangerous situation. But there is a very slight energy cost…
How to Maximize Power
If the 12V Booster Battery shuts down while you’re riding your bike, you’ll likely notice the drop in voltage from 48V back to 36V. But after that occurs, a tiny bit of that 36V power is lost going through the battery-saving diodes in the Power Discharge Cable. This happens only when the Power Booster Battery is fully drained. It might be such an insignificant amount of power saved you the wouldn’t even notice it, but for those who want every drop of available battery energy it could be worthwhile.
So if you want to keep the maximum power going to the motor once the 12V Power Booster Battery is fully discharged, stop the bike and disconnect the Power Booster Battery from the 36V Sondors battery. Simply unplug the two blue bullet connectors of the Power Discharge Cable and reconnect the two remaining bullet connectors without the Discharge Cable. That way you won’t be wasting those precious electrons in the diodes. Instead you’ll be able to use all the available power left in the 36V battery to get you to your destination!