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Need help on weird issue,

I am riding a 8inch 36v 11ah escooter with external battery of 36v 25ah. Previously the batteries were running in parallel configuration. It was fine for very long until one day my 36v 450w 20a controller spoiled. So the shop owner says such battery configuration easily burns out controller so he change the batteries configuration to double switch and install an identical new controller.
Hence, the escooter runs well only when either battery is fully charge or near full charge. But when either battery drops a voltage after using for a while, the escooter runs slow and after another drop of voltage, it runs even slower and even slower. So what is the problem here?

I don’t have such issues when my batteries were running in parallel or even only with internal battery when I just bought the escooter. What is exactly the cause here? Why drop of speed and torque just after a few voltage drop when there is still alot of battery power left? Isn’t it supposed to run with the same strength from the beginning to the end of the battery?

The speed and torque will remain relatively constant while the battery pack has a good charge in it, however when the charge level starts to drop then the scooter will slow down and have less power. 

The useful Voltage range of a 36 Volt lead-acid battery pack is between 39 and 32 Volts and one the battery pack starts to drop under around 36 Volts then the amount of current that it can supply to the controller and motor will start to drop and the scooter will slow down and run in a state of reduced power.

If the scooter is going into a reduced power mode faster than it should be then replacing the old batteries with new ones is the only way to solve the problem. 

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