Tony Vandiver wrote:
> Does anyone know of a 3000mAH or better single cell Lithium Ion battery 
> (using LiPo now, but my peers wish that I would find something less 
> explosive) that comes in a flat rectangular package?  My envelope for a 
> battery is about 0.4"x6"x4".  I've found some that might fit in my 
> enclosure that are cylindrical, but I need more capacity than my 
> smallest dimension (diameter of the battery) allows.  As an alternative, 
> what are the down sides to putting multiple LiIon cells in parallel to 
> increase capacity, i.e. is there a way to maintain a balanced voltage on 
> them without a lot of danger/hassle? 
>
> Looking at the bigger picture, I've got a handheld device designed that 
> pulls 450mA at 3.7V when fully active & I'm charging via a USB port.  I 
> need about 5 hours or more of fully active operation on a single 
> charge.  I'm also concerned about the need to replace batteries in the 
> field with something off the shelf.  Cell phone batteries seem like a 
> possible fit, but it seems difficult to connect to them, and I don't 
> have documentation on the ones I've seen with 4 terminals (I'm assuming 
> that extra terminals are connections to thermistors for temp 
> monitoring).  Any recommendations?
>
> TIA,
>
> Tony
>
>
>   
We use li-poly batteries in combat robots, aside from the new rule, "you 
smoke you loose" being implemented in the states they are pretty ok really.
we have had 1 case of a lipoly cell "venting" (this is our whole comp) 
after being in use for ~a year with massive over current draw and 
getting mechanically shocked pretty dern hard.
The cell got fairly warm and put out copious smoke, which was pretty 
nasty and it only slowed down after being immersed.

We also had an OEM A123 cell explode in a far more exciting if less 
smoky way, it actually had the end explode out of the cell spraying 
copper foil around the place.

moral of the story is batteries have lots of energy in them, if you 
don't poke holes in them, don't over charge or pull 100A from a cell 
rated to 20A you should be pretty ok.

Anybody with a modern phone in their pocket more than likely has a lipo 
battery there, and they don't seem to be cooking off like hand grenades.

A smallish cell is probably safer than the ones we use with regards 
mechanical damage too, they need to get over a certain temperature (700C 
I think?) to liberate oxygen within the cell and most of the consumer 
cells are only good for 1-3C discharge into a dead short which is going 
to make it much harder to generate that kind of heat. The cells we use 
are in the 20-40C range

some photos of the lipo "event" are at 
http://www.robowars.org/forum/viewtopic.php?t=1361&postdays=0&postorder=asc&start=75
just below that is what the inside of an A123 cell looks like when its 
ejected at high speed from its casing.

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