Whats the charging voltage? If its 5V yu can cut the lead in between and connect the oral B base with a 5V USB cable.
Like cut after the base, cut a usb cable and isolate the red and black wires and attach them to respective wires to the base. Even if its 3.7V you can attach a IN4007 diode in series with 5V charger to drop the voltage and use it.
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If you are not a DIY person take help of someone.
