The problem we've seen has been when the mA output of the charger is greater than that of the battery. 510 battery chargers are rated 100mA to 300mA because of what the battery can take. Put a 510 on an eGo charger that puts out 420mA and you'll fry it. However, using a 500mA output charger on an eGo type batt should be safe UNLESS, it is a mini eGo. Here there are differing views. Some would say only use a 510 charger on a 350 mini eGo not a regular eGo charger. But others have said to use the eGo charger on the minis. I go with the 450 mAh mini and use an eGo charger without a problem, but if I got a 350mAh mini, I'd likely use the 510 charger (as long as the voltage was the same).
Because you can fry a 510 (where the top output of it's charger can be 300mA) on a 420mA output of an eGo charger, you can figure how high you might go with output mA's on an eGo charger. I've never seen one above 500mA, but if there was one, I wouldn't use it without knowing a bit more about the charger.
ok here we go.
the out put of any usb port is 5 volts whether its on your computer or wall charger.
now the amperage is the max recommended draw of current the circuit is designed for.
the amount of current drawn depends on the load.thats represented as the amount of resistance
of the circuit your plugging in has as the source(usb port) see's it.
so the source(usb port) has to have the capability to supply the current to the load (charger)
for it to run properly.
now we plug the battery into the charger.the chargers current out put depends on the load,(the internal resistance of the battery)and the voltage rating of the battery.
the larger the mah of the battery the lower internal resistance = more current flow.the lower mah of the battery means more
internal resistance= less current draw.
so batteries of the same voltage rating can be safely be charged on the same charger as long as the charger
can output enough current to properly charge the largest mah battery you have.
if the charger does'nt have the output current needed it will just charge the battery at a slower rate.
all chargers should as a matter of course be designed to out put there maximum current rating without
damaging themselves.
batteries of different voltages are a problem though you should be able to safely charge a battery
if its within 15 to 20% lower voltage than the charging voltage if watched closely.(i don't recommend this though)
of course if you throw a defective circiut into the mix,(usb port,charger,or battery) POOF!.
you just let the smoke out.
regards
mike