Recently I have been involved with several side conversations with members revolving around the topic of portable rechargeable batteries.
WalMart sells a 4 pack of 600 mAh 3.2V LiFePo cells that are AA size. $10. Yes, WalMart. They sell these cells as replacements for the little solar rechargeable lawn lights that stop working when the neighbor's dog pees on them.
Lithium Ferro Phosphate. Li-Fe-Po. Many people have never heard of it before. Now you are no longer one of them.
The story is, you can easily charge these inexpensive and lightweight cells to provide a quick and cheap 12V+ power source. Yes,the pack of four cells I am referencing is just 600 milliamp-hour rated. But, you can put these four cells in a common AA holder and get 12.8V out of them for a LONG time compared to other types.
Also, hams are often about portable stuff. It would take ELEVEN series connected NiMh cells to get a similar voltage. Who makes an eleven cell holder? Nobody. And that's kind of big anyway. And decent quality NiMh cells cost too much. Meh.
If you get these LiFePo cells I am talking about, do NOT EVER mistake them for regular AA cells and put them in something that takes regular AA cells. Poof. You have been warned.
RadioShack still sells this 4 AA cell holder item, #270-391:
or you can get similar from a variety of other places on line if you don't already have one.
You can charge this battery pack with a 13.8V current limited source, like the famous OCRA charger kits of some years back. I still have four of those kits remaining at $8 each if you need one. Or, just use your existing 13.8V regulated supply with a suitable series resistor. Since you know Ohm's Law, I will let you do the math.
Here is a link to learn more about the modern battery chemistry that makes this possible:
Here is a link to the WalMart item: