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The best suggestion that I can think of is to use the charger to batter the rats to death.
I feel that this has a greater chance of sending the creature to rodent heaven, than trying to electrocute it with 20 volts.
I have never seen a rat maybe because I live in a clean home in the city, not in the garbage dump where I hear there are plenty of rats. Maybe if you clean up your place then the rats will go somewhere else?
No, not turn it into. That would take a complete redesign of the internal electronics.
You can use it to power a boost converter that makes a high enough voltage to cause some harm. You don't say what the output current of the charger is, and that (plus the voltage) is what determines if you have enough energy to guarantee effectiveness.
If you have a 'Chinatown' in the area in which you, and the rats, live you will find they have some very effective remedies. One is what appears to be a box of school chalk sticks and the other is a white power in a package. (With the 'chalk' just make a continuous line around the rats favourite areas)
Both are very effective, so effective you should keep pets and small children away from where you use them.