Presented here are designs for a simple and highly convenient system of caving lamp and charger. The arrangement described here has been in use for over four years by the author in the caves of Britain, Ireland and France and by Andrew Palmer in the caves of New Zealand. However, after three years of heavy use, the time has come to build some replacement batteries in preparation for a trip this year to the French Pyrenees.
Below you will find a step-by-step description of how to build a battery similar to mine, and also some photographs of the charger that goes with it. I find that a battery can be made in two evenings. In fact, making two or three batteries simultaneously on a production line is probably the most effective use of resources as much of the time is spent waiting things to set, and you will tend to waste less material.
The batteries that have been in use for four years are based on nickel metal hydride (NiMH) chemistry charged by a simple constant current technique. All of the batteries used are of 7/5Af size (66 mm x 17 mm). The original battery used 2500 mAh cells (the best available at the time for a sensible price) and by the time the second was constructed only a few weeks later capacity had jumped to 2800 mAh. Andrew Palmer's batteries were later constructed using 3000 mAh cells. The units presented here use the latest 3600 mAh technology - available for the same price as the original 2500 mAh cells - a capacity increase of 44%.
Materials and equipment
For each battery you will need the following:
As you withdraw the batteries be careful that the cells do not short out! Its probably best to disconnect the cells from each other before you responsibly dispose of them.
The lid should be further modified by the drilling of two small holes with the same diameter as the wire chosen for the flying leads from your new cells. These holes should be drilled horizontally adjacent to each other as close the the top of the enclosure as possible.
To permit injection of the foam into the case a hole must be drilled into the copper lid - the easiest location for the hole is to drill out the pimple present in the centre of the lid. It is a good idea to match the size of the hole to the internal diameter of the nozzle from which the foam will be injected, for maximum efficiency and minimum leakage.
Three holes should be drilled into the centre of each cell compartment in the lower black part of the housing - these will allow air to escape from the compartments as the foam is injected, and will prevent large air bubbles being trapped.
The NiMH cells need to be wired together into a battery before insertion into the larger black part of the case. All three cells are orientated the same way - in this instance with the positive terminal towards the top. Here the interconnects between the cells use thick single core copper wire taken from mains cable between the solder tags that are on the cell terminals. If your cells do not feature solder tags then you will need a slightly different arrangement, taking care to minimize the profile of your soldering on the cell ends. The fly leads on the ends of the battery are standard automotive multi-strand copper cable. It is a good idea to leave these leads at least 150 mm long as they can easily be trimmed to the correct length later.
Carefully insert the new cells into the black part of the case. Thread the flying leads through the two holes you drilled in the copper top and fit the top over the upper part of the cells. It may take some time to achieve the optimal routing of the flying lead over the top of the cells to minimize the separation between the upper and lower parts of the case. Looking at the outside of the case the positive wire should come out of the left-hand hole, and the negative wire out of the right hand hole. Mark the positive wire appropriately. Carelessness or confusion at this stage could lead to an expensive and possibly dangerous blunder!
Having threaded and identified the wires correctly no attach the two parts of the case together with insulation tape. The broad 3 inch variety works best but the narrow 1/2 inch tape will do. You should aim at this stage to minimize the separation between the two parts, whilst attempting to align them as precisely as possible - experience has shown that it is almost impossible to achieve exact alignment owing to the amount of 'give' in the tape.
Warning! Be aware that this stage of the process can be incredibly messy, and that expanding polyurethane foam seems to be one of the most adhesive and robust substances around. It will bond to kitchen work-surfaces to such an extent that a chisel will be needed to remove it - probably damaging your kitchen for good. Perhaps more worryingly it will bond to your fingernails with greater adhesion than your fingers do, and then when it finally grows out to the end of your nail, it will break your scissors or clippers. The brand of foam I used was supplied with disposable polythene gloves - it is excellent practice to wear them.
At this stage, the partially completed battery needs to be secured firmly in a vice or by another pair of suitably protected hands. You will get best results if you can do the whole foam injection in one single action, rather than incrementally, so carefully plan the process before you set to. I found the flexible tube supplied with the canister a hindrance, and did not use it, preferring to press the can nozzle directly against the battery.
The foam should be injected continuously until it emerges from all three holes in the bottom of the black part of the container - only then can you be sure to have no air bubbles in the system. As soon as foam is emerging from all three stop injecting! It is known as expanding foam for good reason and any excess will only contribute to the magnitude of the clean-up operation.
The foam should now be left to cure for twenty-four hours. Resist the temptation to remove any excess foam until setting is complete, as you will only make more mess - it is far easier to remove it later. Whilst the foam in the battery is setting, it is a good idea to clean the nozzle on the aerosol - otherwise, you may find yourself in the situation of having a large and curious stalagmite appear on your worktop overnight.
When the foam has fully set, remove the surplus - which has probably grown into a large and bulbous form over night. Often the hard foam can be removed simply by snapping the blobs cleanly away from the battery - otherwise simply use a screwdriver to prize them away.
Satisfactorily securing the Futuba race-pack connector to the side of the battery is probably the most difficult part of building of these batteries.
First of all, the connectors sport a raised lug on the opposite face to the catch which needs to be removed. This is best done with a very sharp knife.
First of all, to provide a sound foundation for the connector, file a small flat onto the side of the battery immediately below where the two wires emerge. The flat does not need to be as large as the face of the connector, just large enough to rest the connector against the casing and prevent it rocking from side to side. Temporarily secure the connector in place by using some tape and estimate the correct length for your flying-leads emerging from the battery. It helps to have a pre-assembled race-pack connector to hand, so you can see precisely where the terminals are located inside the plastic housing. Cut the wires to length and solder the connector terminals to the ends of the wire, then push the terminals into the rear of the connector until they are home with a reassuring click.
You should now have the connector dangling from two very short flying leads. Attaching the connector to the side of the battery case is usually troublesome. The problem arises because it is difficult to find a glue that will adhere successfully to both the tough plastic of the LR3 casing and the polythene of the race-pack connector. Several techniques have been tried, sometimes in combination, the two most successful of which are described here:
The final stage is to waterproof the housing whilst adding to its robustness and strength. Remove the tape connecting the copper and black parts of the LR3 case to reveal the hardened polyurethane foam beneath. The gap between the two parts of the case is going to be filled with car body filler (I use Plastic Padding, because that's what my Dad used, and probably his Dad before him - but feel free to substitute your own family favorite), so you may need to carefully remove some of the phone from the gap with a blunt screwdriver.
Mix up some filler and using the supplied spatula neatly fill the gap in the housing. Then carefully fill around the two sides and the top of the power connector to enclose the wires and further secure the connector to the battery. Finally fill the holes in the top and bottom of the battery which were drilled for the foam. Allow the filler to cure for the requisite time.
When the filler has set finish off the battery with a rasp and then a file (the beautiful thing about plastics is that both wood and metal working tools can be appropriate), and finally sandpaper.
I have painted some of my batteries, but with others I haven't bothered. Its really only a question of aesthetics. I've successfully used aerosol car paints, although I suspect Hammerite or the smooth equivalent applied with a brush would provide an excellent finish.
...coming soon! But for now - here are some pictures of the internals of the charger:
The finished unit. Note the two battery shaped sockets and three LEDs - one for 'power on' and a charging indicator for each battery
The charger with two batteries. The pink battery is my oldest 2500 mAh unit which has sustained quite a lot of damage over the years - including being dropped down several pitches. Its still works though!
A plan view of the insides of the charger. The electronics and wiring are on the left and consist of two Maplin Elecronics L200 circuit boards in constant current mode charging at approximately C/10 rate. There are some extra resistors too for LEDs etc. The socket on the right connects to the front-panel LEDs so the lid can be totally removed.
A closer view of the battery holders. The aluminium holder is recycled from an older charger design - this aluminium box used to house the old charger. However, it had just the right dimensions to be modified into this device. See how the race-pack connectors are attached to the aluminium with cable-ties.
The aluminium housing doubles as the heatsinks for the two L200 regulator ICs. Again the cable ties securing the race-pack connectors can be seen.
The original cells from the Duracell LR3. Take care not to short out the cells at this stage.
Note how the replacement cells are slightly narrower and longer than the original LR3 cells
To create maximum room inside the case, modify the inside of the lid by removing the three plastic ribs with a chisel or miniature screwdriver.
The three new 7/5Af NiMH cells, ready for insertion into the modified LR3 case. The two intermediate interconnects are single strand wire from domestic mains cable; the two red flying leads are multi-core.
The three cells inserted into the bottom portion of the housing. Note how the positive flying lead has been carefully routed over the top of the batteries.
The lid of the battery secured with adhesive tape into the desired position
Injecting the polyurethane foam into the hole in the battery lid
The foam after expansion and during curing
After the lid has been added, tin the wire ends with solder for attachment to the race-pack connector
The battery with the race-pack connector added, but not yet secured to the side of the battery casing
The battery after being sealed with car body filler. Note the liberal covering over the race-pack connector - it can be removed easily with a file once it has hardened
The finished battery after finishing with a file and sandpaper
Last updated 2001-12-05 © R J Smallshire