I have several external batteries that I need to keep my toys, um I mean electronic tools working when I am away from commercial power. Instructions I have read on the Goal Zero site and otter places suggest running the battery on the device down and then charging it back up with the external power supply. I believe the tinkling is that the device such as a laptop knows it’s on battery power and it will switch to a mode that uses less power. That theory has some merit but in my opinion is not goo advice. Here’s why:

• Charging one battery with another never results on full power transfer. The loss result in wasted amp hours.
• Cycling any battery will eventually wear it out. if you charge the device battery with the external battery both batteries need to be cycled. If the device is powered by the external battery only the external battery will be cycled unless it’s necessary to run both down. I that case you still gain the efficiency of not having to charge on battery from another. Also it’s easier to replace the external battery than the device battery.
• In the case where you will be using the power form both the eternal battery and the device battery, running down the external battery and then using device power allows you to charge the external battery with the device used it’s internal power. If you have enough solar panels it should be possible to charge the external power before the internal battery is depleted.