On Wed, 5 May 2004 16:21:59 +0000 (UTC), "Stephen Haley"
wrote:

I thought that some LIon are actually quite good at high current draw
applications. They are certainly making inroads into the model plane market
now their price is dropping. The main problem with them is their pontential
to explode if shorted out through thermal runaway and they require
specialised charging requirements. A secondary problem is that you cannot
measure the battery state through voltage.
The main advantages of lion are the light weight per amphour BUT they are
bulkier than lead acid by about a factor of 2.

Are you sure? I made some calculations of Li-poly vs NiCd, comparing
two Kokam 145 mAh Li-poly cells delivering 5 v through a 7805 v.reg
with a five cell pack of Sanyo N50-AAA (50 mAh) cells. The Kokam pack
gives three times the capacity for 1/2 the weight and 2/3 the volume
of the NiCd pack. It's power/wt ratio is about 4.5 times better and
its power/volume ratio is 4 times better than the NiCds.

On a quick and dirty comparison using a Kobe 12v 7 Ah gel cell and
5500 mAh D-size NiCds an equivalent NiCd pack (10 x D cells) would be
very similar in power/volume ratio to the gel cell - certainly within
+/- 10%. A cross check for Sanyo 7Ah F-size cells gives the same
answer, but the super-F (10 Ah, F-size) will have a 30% better
power/volume ratio than a gel cell. I've no idea about power/weight
ratios: I haven't got anything to hand that can weigh the 12v gel
cell.

Li-poly cells are better and safer than Li-ion cells: the Li-poly seem
to be more popular with the indoor RC crowd than Li-ion. I think both
will spontaneously combust if the thin plastic cover is pierced.

As others have pointed out, you MUST have a special (and relatively
expensive) charger for Li-Poly cells.

For real usable results in battery technology I suspect we should look to
the new Hybrid Petrol/Electric cars being developed by Toyota but as with
all things it wil take time for them to be affordable/usable.

What sort of battery technology do these use?

I suspect a real driver in Glider battieries will be the requirement in
Europe for Mode S transponders with some countries adopting them before
discrete iterrogation is switched on meaning that in some areas transponders
may be interrogated many times a min. This will require some real
improvement in battery technology for existing gliders where retrofitting
with solar charging would be difficult .

Although available NiMH cells of AA cell size or smaller have double
the power density of NiCd there's nothing in Maplins or RS catalogues
bigger than 2.3 Ah C and D-size cells while NiCds go to 10 Ah per
cell. Has anybody spotted an NiMH with a capacity of 5 Ah or bigger?

So, it looks like the answer will be Li-poly, then. Expensive, and
requiring crash-proof containers to prevent the outer membrane being
pierced in a crash and causing a fire.


--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :