Replace the battery with a 48V battery and directly connect a 12V lithium battery in series. Simply disconnect both and then reconnect them.
But it is better to buy a new one. If you really need it, you can only buy a 12V lithium battery with a protective plate. The capacity should be the same as 36V. Then charge them separately and use them together. The chemical properties of lithium are very active and easy to burn. As the battery is charged and discharged, the temperature inside the battery continues to rise and the gas generated during the activation process expands, increasing the internal pressure of the battery.
The easiest way to reduce voltage is to use 4007 rectifiers to reduce voltage. It is enough to connect three in series, but the unstable effect is not very. GOOD. You can go. You can buy a modthe online voltage reduction for 2/3 yuan. It's very cheap and the effect is very good. Look at the photo, I use this kind of module.