Gents,
I have been given one of these: http://www.civil-defence.net/t12.html as it has a buggered battery pack.
Taking it apart reveals a 10S D cell NiCad battery pack, beautifully made by the way, nestled behind what I have found out is a bona fide aircraft landing light bulb/reflector assembly!
The pack refuses to hold charge, never managing to extinguish the red light on the charger and setting off the low voltage alarm within moments of turning on the beam.
The low voltage alarm is apparently set for 10.3 volts so for a freshly charged pack to be dropping to that level almost immediately means that at least one cell is dead, probably two of them.
The cells themselves are 4000mAh D sized NiCds, probably due to their toughness in a military context and although a new battery pack is available at £65 it would be the 4000 mAh NiCds ( again presumably due to their resilience) rather than one of the more modern and capacious options.
http://www.civil-defence.net/12v_4ah_batteries.jpg
Whilst I have it apart and am going to be spending money, I am thinking that I may as well upgrade the NiCds to NiMH or maybe even Li-ion batteries.
The first question is which sort of NiMH batteries to use, looking around reveals a bizarrely wide range of capacities, ranging from AA battery capacity to 12,000 mAh or so. Can anyone recommend a good quality, ideally cheap as possible as I need ten of them, D cell NiMH battery for this purpose?
The lamp is rated for 100W at 13V so around 8-9 amps, would D cell NiMH cells be able to handle this sort of discharge rate comfortably?
The second question would be where can I get such batteries from most economically in the UK?
Using Lithium batteries is an interesting option, I have had good luck with D cell sized 36200 sized Li-ion batteries in my modded mag light but at £25 per pair, and I would be looking at a 4S3P configuration. I think there is space for the extra 2 cells but will check. A PWM voltge limiter could the bring the voltage down to the 12.5V for the bulb.
Comments, ideas and criticisms please chaps.