Making NiMH Battery Packs
Introduction
'Consumer' type rechargeable NiMH batteries are widely available from high street retailers and on-line merchants. As raw material for battery packs, they offer several advantages over ready made battery packs:
- Cheaper
- Choice of configuration
- Individual cells can be checked for voltage
The only slight hurdle is that 'consumer' cells are not tagged. Nevertheless, if you're careful with the soldering process, the result is just as good if not better than ready made packs.
Choosing a Brand
It's worth sticking with the established brands like GP, Sanyo, Kan and Panasonic. I'm currently using GP cells as they have a good reputation at cold temperatures. Cells I've tried are 2500 and 2700 in AA size, and 1000 AAA. So far very promising. I obtained my cells from BatteryPlanet and I've also seen then at Currys Digital.
Check the Voltage
After removing the cells from the packaging, the first thing to do is check the voltage (off load), as this will give an indication of their condition. Around 1.1 - 1.2 V is normal. Anything less will indicate a sub-standard cell due to overlong or inappropriate storage.
Unfortunately bad cells are a fact of life, all you can do is return them. If your supplier makes a fuss about replacing them, then find another one. Remember, one sub-standard cell wlll drag down the whole pack...
As an aside KAN cells have their date of manufacture prominently displayed on the outside - it's a shame that other manufacturers don't follow their lead (GP have a datestamp, but it's coded and you have to remove the heatshrink to see it. If anyone understands how the coding works, I'd be interested to know).
Less Heat = More Reliability
Since consumer cells are untagged, it's important to apply as little heat as possible for as short a time as possible when soldering. Paradoxically, the secret is to use a hefty 25 Watt soldering iron, so the heat from the iron will transfer quickly to the pads. Conventional wisdom says that soldering paste ('flux') should also be used, but I've found that polishing the terminals with 800 wet/dry alone is OK in conjunction with cored lead-based solder. Your mileage may vary.
Construction Sequence
These photos should explain all:
12 March 2007 Nigel Potter writes: "One extra step we eflight boys take is to put a piece of masking tape between the cells, this is because if you short the pack or charge it aggressively it can get hot enough that the heat shrink splits allowing the pack to short itself out."
19th March 2007 Roger Robbins writes "If you crimp up your own connectors, the connectors usually come attached to a thin gold plated strip. These make ideal terminal bridges." |
