While considering portable equipment, one of the major considerations is always the power supply.
It has long been understood that portable devices are cheaper to run from a rechargeable battery. The most energy efficient technology, at the moment, is Lithium Ion.
Now, when working with lead-acid cells, charging and using the battery is simple - current in charges, current out discharges. The only real considerations are that overcharging reduces the life expectancy of the battery, and deep discharge can damage individual cells of a battery.
Nickel-Cadmium (NiCd) and Nickel Metal-Hydride (NiMH) require a bit more care, in that overcharging can do a lot of damage to the cells, deep discharge can do the same, and there is often quite a bit of heat involved with re-charging damaged cells.
Lithium Ion (Li-Ion) and Lithium-Polymer (Li-Po) cells are even more tricky in that they are extremely intolerant of any kind of charging/discharging abuse at all. Since they have an organic electrolyte, they often react to physical damage or overcharging by bursting into flames.
On the plus side, they have a particularly high energy density, are available in all manner of shapes and sizes, and they have an output voltage in excess of 3V (typically in the region of 4.1V at full charge.)
Just to make the Li-Ion cell more attractive, it is possible to obtain miniature charger modules for them, and tiny inverter units so that they can be charged with a standard 5V power supply (such as a USB socket on your computer), and can deliver whatever voltage you need at a fairly hefty current - and with a better than 90% efficiency.
The better charger modules even disconnect the load if the battery is close to going into deep discharge.
I found myself, some time ago, in possession of four thin, flat pouch cells giving a total storage of several ampere-hours. On their own, they were too delicate to use without a case, and I didn't have room in my project case to install them. Additionally, I didn't want my Li-Ion batteries to be enclosed in a plastic case ... just in case of fire.
I ended up using the bottom covers of a couple of old CD-ROM drives to make the PSU chassis, holding the batteries in place using neoprene foam tape, double-sided fixative for the circuitry and ordinary PVC electrical tape to hold the top and bottom covers in place.
This was still not sturdy enough, so I put together the top and bottom covers from an old (burned out) CB, using epoxy to join the two clam-shell halves, mounting a polycarbonate base on the resulting case, and a matching face-plate for the other end.
The face plate was glued to the battery chassis and the chassis slid into the clam-shell case - the whole was secured using that same PVC electrical tape.
The result is a fairly heavy battery pack that is armoured, pretty close to fireproof and can be opened if required (but is essentially permanently closed). It is also waterproof (always a good thing in field equipment)
And the output? 5.0V at up to 2A.
For the builders of home-grown portable equipment, I would always recommend putting your Li-Ion or Li-Po battery pack in a separate, steel or thick aluminium case, just in case of fire.
A final couple of notes on Lithium rechargeable cells:
Never, ever charge Lithium Ion or Lithium Polymer cells unattended unless placed in a fire-proof enclosure and away from other flammable materials - they can be unpredictable.
Regularly inspect the cells for inflation (bulging) as this is an indication of cell failure. Such cells should be disposed of properly.
In case of Li-Ion/Li-Po cell fire or uncontrollable overheating - place the entire, disconnected battery, complete, in a metal bucket filled with cold water (preferably with a little salt in the water) and put the bucket somewhere outside, away from flammable material, people and animals. This will cool the battery and discharge it. Leave it there for at least 72 hours, and assume that it is going to catch fire if you bring it out before then. Whatever circuitry is attached to the cell(s) may be assumed lost.
Likewise, if you have a Li-Ion/Li-Po cell that becomes punctured (they smell lovely), drop it in a bucket of slightly saline water and put it outside.
Once safe, it can be retrieved and disposed of through an appropriate route.
