If I use an external battery pack to recharge a device's battery through the regular power port (so, NOT the USB port, but the 3 mm jack; the battery pack delivers the correct DC voltage and remains under the amperage indicated on the AC charger), is it more energy efficient to recharge the device when it sits idle in standby/deep sleep mode, or to keep using the device while plugged in to the external battery pack?
Given the limited amount of energy available and knowing that charging from battery to battery can be a complicated matter, I thought I'd give it a shot and throw this into the forum. So, anyone knowledgeable enough to give me a convincingly-sounding reply?
And on a related note: I did build myself a connector for another battery pack, by using a switching regulator to reduce the output voltage from 16 to 5 volt (the battery pack was meant for laptops, hence the high output voltage), but is 5 volt really the most economical input voltage, or merely the easiest for the manufacturer? After all, LiIon cells are usually 3.7 volts, so maybe the device's charging circuitry would do better on an input voltage of 3.7?
Questions, questions, eh?