On many devices, one is fine if the volts are close, and the amps really just need to be higher.

Some devices are designed to do their own voltage regulation, and can take up to 20V or so... But one never knows, which is why I sent the note to Samsung technical support, and was just blown off.

I presume most of the 12V ==> USB adapters actually have voltage reduction, and hopefully voltage regulation circuitry.

I think the 8.4 V light battery packs were just being cheap. Trying to give an extra option, but without adequate circuit protection.

What might be nice would be a super-sized battery pack, capable of say 5x the power of a standard battery pack, but would otherwise seamlessly fit into the back of the phone (custom rear cover).