[tsac]

What I'm saying is to directly charge the battery with a charger over rated for the device. can cause problems. Overcharging a battery causes excessive gassing .On sealed batteries, the buildup of gases could cause the battery to burst. most devices ( like laptops )have a charge limit to prevent problems. when you read about cell phones catching fire I would suspect they do not have the circuitry to limit or shut off the charging
Using a battery pack to supplement the internal battery does not cause overcharging.
It's allowing the phone to draw from another source. Generally the external battery pack gets its charge from an external connection and has a limiter to prevent problems. The units I checked all have protection circuits on them on both the input and output ports.