Battery choice was mainly influenced by the gravimetric energy density available : the lighter the battery for a given capacity the better. It was decided to use a rechargeable (secondary) technology, because the amount of experimental run time would incur massive costs if a non-rechargeable (primary) battery technology were used. Three types of rechargeable battery were considered: Nickel Cadmium (Ni-Cad), Nickel Metal Hydride (Ni-MiH) and Lithium Ion (Li-Ion).

Ni-Cad was the first of the rechargeable technologies and is still used to a wide extent today, mainly for high current drain applications such as power tools. Ni-Cads are relatively inexpensive but have half the capacity of Ni-MiH or Li-Ion, typically 300mAh for a AAA battery. The terminal voltage of NiCad and Ni-MiH is 1.2v compared to the 3.6v of Li-Ion.

Battery Technology Comparison, Panasonic Corporation

As illustrated by the graph above, Li-Ion batteries have superior gravimetric and volumetric energy densities. After much searching it was found that the most appropriate, easily available battery was a Li-Ion 'Sony Minidisc' battery (LIP-12B), which gives 1500mAh at 3.6v for a weight of 50g. The internal cell itself only weighs 39g, the remainder being made up by the protection circuitry and outer casing. The weight of the battery pack was minimised by discarding the outer plastic casing which gave a weight saving of 8g. The protection circuit was incorporated onto the control board with a small lightweight socket to plug in the battery cell. A further protection circuit was mounted on a charging adapter to allow the battery to be recharged with the safety mechanism in place.

The use of this new battery gives a huge increase in the operational time of the blimp (from 10 minutes using two Alkaline (famous make) batteries to two and a half hours). There is one small drawback with the new batteries in that the payload of the blimp has been pushed very close to the limit, with only a further 8g of payload that can be added. This has several implications:

·Any further sensors to be added will require an increase in the envelope size;
· The envelope will need to be kept full of relatively pure Helium to keep it aloft. The Helium leaches out of the envelope at approximately 1% a week, given that the envelope is sealed properly, so the blimp will lose buoyancy over time;
· With an extra payload capability, ballast can be added to make the blimp neutrally buoyant, then slowly taken out as the helium seeps out, thus maintaining the neutral buoyancy without constant refilling of the envelope.