If your charger is getting so hot that the lettering is melting, you have some kind of a severe problem.. it could be the power supply or the charger (as pointed out) or it could possibly be the battery pack shorting out on you, drawing a lot of current - more than the charger can provide.

May I ask what kind of a "12V Charger" you are using? Is it a battery charger or a real power supply?

Battery chargers aren't power supplies, regardless of what you may have heard. A battery charger puts out a 'pulsed DC' (for want of a better description, while a power supply puts out a smooth output. The battery charger could possibly wreck your charger, if it hasn't done so already.

If it's a power supply, is it large enough to put out the current that you need, and not a wall-wart? The wall-wart would get very hot also because it's working too hard.

Just curious....

Bob