Thoughts on sources of small, cheap Li-ion batteries

I think I’m on a “make it smaller” bent again.

I’ve ended up building a laser tachometer to assist in testing the motor speed controller code (which is going to end up using a PID loop for speed control, more later). After I finished it, I realised that I’d ended up designing what effectively amounted to a reusable test/measurement hardware platform. All I needed was a power source.

Farnell stock a decent range of Li-ion batteries, but it seems all the reasonably high-capacity ones are either horrifically expensive, or just plain too big for the Maplin “T2” plastic cases I’ve grown fond of.

Enter the Hahnel HL-5L, a clone of the Canon NB-5L. Truth be told, I wanted the Jessops NB-5L clone (it’s 1100mAh, the Hahnel is 950mAh), but said battery wasn’t in stock at the time…

The measurements of the battery are:

* 45mm long
* 32mm wide
* 8mm tall

So it’s a perfect fit for the T2, among other things. The near-1Ah capacity also makes them very useful for portable gear. I suspect they’re probably Lithium Polymer rather than Lithium Ion, but they’re much better than the Samsung mobile phone batteries I’ve been using recently…

Connecting them would seem to be the hard part – there are three pins — +, T and -. + and – are the positive and negative power supply (actually +3.7V and 0V, but that’s an academic point). The T pin appears to connect to a 10k thermistor inside the battery pack.

There don’t appear to be any magic-handshake procedures involved in getting these batteries working, unlike most laptop batteries (I bought a few Compaq laptop batteries a while ago and ended up stripping them for cells, and adding a custom safety circuit). Charging should be a piece of cake — no doubt the good old [Maxim MAX1811](http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2536) or the newer [MAX1555](http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4002) should be suitable. Both are specced to charge Li-ion and Li-polymer packs.

Next step will be to build a connector for this thing – the Ixus cameras that use this battery (860IS and such) use three thin brass (?) spring contacts mounted in a plastic body. I’m probably going to end up using epoxy putty to make the base, and some brass sheet to build the contacts…