Survival Tech Nord has an open project running for making a small external battery bank for radios that operate at low output (5+ watt).
The basic principle is using 18650 lithium-ion (Li-Co or Li-Mn) batteries that have big energy density and match their voltage to that of the radio.
STN could have opted for 18650 LiFePO4 batteries but they pack half the energy and the project is about making a (small) portable pack.
Granted the idea have been tried before by me, and there are improved and commercial facsimiles, but STN, aka OH8STN, is going for the proper (complete) setup, integrating quality materials at reasonable cost.
Furthermore, all parts are easily available to anyone and this is what makes an open project, doesn’t it?
Last, the Ultrapack is scalable as more 4S battery modules can be added.
Here is the project page.
The Ultrapack in prototype stage
In short STN uses a Drok DC-DC converter that takes power from a solar panel and any other similar source (car battery) and charges the batteries.
Batteries are connected 4 in a series for a voltage of 16.8V max, and –most critically- a Battery Management Board is used to control charging, and perform the critical battery cell balancing.
Then the battery voltage is stabilized by a switching regulator and given to the radio.
And here are some more photos from STN.
The project is far from finished, as more testing must be done and a case found for it, so stay tuned for updates either by me or by following STN in Google+.