NiMH caving lamp and charger

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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 final product

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%.

Battery construction

Materials and equipment

For each battery you will need the following:

  • Old Duracell LR3 'flat-pack' battery - as commonly used in Petzl Zoom torches and cycle lamps. It may be possible to use Energizer or other moral equivalents but the construction details will probably differ from what is described here.
  • Three 7/5Af size NiMH or NiCd cells. Here I'm using 3600 mAh NiMH cells. I also have reports from Nathan Gabbott than 4/3Af cells may also be used - although this make the completed pack very slightly longer.
  • Futuba male race-pack connector - Also made by Kyosho. Commonly used for battery interconnects on radio controlled models. Note the male connector has female electrical contacts but a 'female' polythene housing.
  • Polyurethane foam. You only need a small amount, but even the smallest can I've found contains a phenomenal volume of this stuff. Its definitely more cost-effective if you can make more batteries in one sitting. Note that these cans of foam have a limited shelf-life.
  • Plastic Padding filler or any other generic car-body plastic-based filler should work.
  • Wire capable of carrying 2 A to connect the cells and the connector. High current carrying capability for use with fast chargers. If you're sure you're only ever going to use a trickle-charger then substitute less substantial wire.
  • PVC insulating tape
  • Soldering equipment
  • A very small self-tapping screw and/or amazing neoprene adhesive
  • Optionally, some paint for finishing

Dismantling the Duracell LR3

Taking the old Duracell apart can be a little tricky. If you're not careful you'll need two LR3s in order to obtain undamaged black and copper ends. First of all straighten the two spring contacts on top of the battery to be perpendicular to its upper surface. The best method to open the case is to prize the two components apart using a small flat-bladed screwdriver. The two pieces are only spot-glued together.

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.

Preparing the case

In order to create some extra headroom inside the case three plastic cross-ribs within the copper top can be removed with a sharp chisel or screwdriver. Severing the joint between the end of the ribs and the sides of the case, and then bending the ribs over sideways to snap them off works well. The picture on side side-bar shows the interior of the lid before and after removal.

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.

Wiring the NiMH cells

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.

Battery assembly

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.

Foam injection

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.

Power connector attachment

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:

  • Mechanical attachment using a small self-tapping screw. It is possible to drill and countersink a very small hole through the most solid part of the connector and into the battery case underneath. A tiny self-tapping screw can then be used to attach the connector. Note that the length of screw selected is critical - one does not wish to come into contact with the cell and the screw head should be slightly recessed below the surface of the connector for the purposes of waterproofing. Also, do not over-tighten the screw as it is easy to strip the thread from the hole in the battery case - which is difficult to deal with.
  • Neoprene adhesive. I'm not talking about the stuff used to fix wetsuits here, but rather a very viscous, sticky, brown substance which I found in a French supermarket in one of those enormous syringes of the type that usually contain bathroom sealant. I've never found an equivalent in the UK. At one point the majority of the external components of my Astramax van were secured to the body using this stuff, so its suitability for fixing anything to anything else is assured. A small dab of this is sufficient to attach the connector to the battery.

Body work

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.

Finishing off

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.

Headset construction

...coming soon!

Charger construction

...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.

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The finished product. These batteries have proven to be robust and simple to use.



See also:

  • item1
  • item2
  • item3


Internet links:


UK Suppliers:



The materials needed to construct the battery

Removing the copper top from a Duracell LR3. Note the small amount of damage sustained by the flange on the lid.

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