DIY AA to D battery converter/adapter

A powerful torch light (well, it is flashlight for US guys) is necessary for a farmer to see things after it gets dark. There are 2 types of torch lights - conventional incandescent bulb and LED light. More than a century old energy inefficient incandescent bulb torch lights have been superseded by modern LED lights outperforming incandescent bulb in every department. Therefore LED torch light is the way to go. Fenix is a popular brand when it comes to high end LED flashlight. However, the price is insane - it costs several thousand Indian Rupees for a light! I need a practical solution, not an expensive "brand" I can flaunt. Therefore I started looking around for a lower cost option. Found some lower priced torch lights in the cost range of 1000-2000 Indian Rupees on Amazon and eBay. Still it looked too much to pay for a torch light. My search for a practical torch light continued. I found an unknown brand torch light in a small electrical shop in Sagara town. Best things of this light were throw of light and the cost. Cost was just 500 Rs., that too with 4 numbers of D size zinc-carbon batteries included. I thought I can very well risk 500 Rs. instead of risking 2-4 times of that money on some other torch light. Therefore I bought this torch light for experimenting. I pointed it to the road outside and switched it on after it got dark. Wow - it could light up several hundred meters distance easily. This was the practical torch light I was looking for - good light at reasonable price. Probably this torch light stands at the top position when it comes to light/price ratio.

No doubt this torch light is good for seeing outside things after it gets dark. It has also worked well as replacement for candle light when there was power failure in the night. Just switch it on and point it to the roof. Reflection from the white painted roof illuminates the room pretty well. Little bit of reflective paint on the roof can make it really nice.

This was going great until last month. For last few days I started feeling the light is not as bright as it used to be. Either the LED is going bad OR the batteries are getting weak. With today's LED technology it is rare for the LED to go bad so early. Therefore battery was the suspect. To test it I pulled out the battery and put a multi-meter across the battery. It read dismal 1.1 V indicating battery is dead. This battery is hopeless because open circuit voltage of 1.1 V means it will perform far worse when loaded.

D size battery (left) and AA size battery (right)

What are my options for the batteries? 4 numbers of D size Duracell batteries cost approximately 600 Indian Rupees - more than the price of the torch light. Therefore using rechargeable batteries is sensible option. However, rechargeable D size batteries cost several times more than conventional batteries. More over it is hard to find D size battery chargers in India. Therefore that is not a practical option for me. Only option I have is using widely available rechargeable batteries - which are AA size batteries. I have to somehow use AA size batteries in a D size battery torch light. Say, I have to adapt AA size batteries to a D size battery - either individual batteries or as a set of 4 batteries connected in series (as they are used inside the torch light).


There are AA to D size battery adapters available in the market. They are D size plastic shells inside which 1-3 AA size batteries fit in. After fitting the AA size batteries and closing the shell it looks like a D size battery - both in size and voltage (in theory). I am saying voltage is in theory because voltages measured with adapter may be slightly less than without adapter - simply for the reason that each adapter adds at least 2 more contact points, thereby increasing the contact resistance and resulting voltage drop. Using 4 such AA to D size adapters will make it possible to use AA size batteries in the torch light. Again, cost came in the way. On Amazon I found one such AA to D size adapter at 350 Rs. Paying so much for just a plastic shell does not make any sense. Making such an adapter does not need any complicated technology. Manufacturing cost of each adapter should not be more than 2 or 3 Rs. Selling price is nonsense even if I assume manufacturing cost is 10 Rs. Definitely seller/manufacturer (or both?) are making insane profit. I hate such businesses ripping off their customers. I will not buy it!

How about making my own AA to D adapter? It need not be a fancy thing like the ones sold online. It should do the job without major inconvenience for me to use it. A DIY adapter will do the job if it satisfies following requirements:

• Should have same outer diameter as that of D size battery - 33.2 mm +/- 1 mm.
• Should have same length as 4 D size batteries put together - 61.5 mm * 4 = 246 mm.
• Should have same voltage as 4 D size batteries connected in series - 1.5 V * 4 = 6 V. I have tested my torch light even with 4.5 V. It works well as long as around 300 mA current is supplied even at 4.5 V. Therefore voltage between 4.5 V to 6 V is fine.

I decided to use AA size Ni-MH batteries because they are widely used in digital cameras, it is easy to source them and it is easy to find a charger for them. Though a new fully charged Ni-MH battery gives out 1.4 V, average voltage is 1.2 V. Therefore they can be a good replacement for zinc-carbon batteries having 1.5 V. 4 numbers Ni-MH batteries in series are almost equivalent to 4 zinc-carbon batteries in series. Challenge is battery size - AA size batteries are much thinner and shorter than D size batteries. One way of solving the problem is:

• Add something around AA batteries to get outer diameter of 33.2 mm. Very high accuracy is not needed. Approximate size is enough. 1 or 2 mm tolerance is good enough.
• Put the batteries in series to reach same length as that of 4 numbers D size batteries put in series. AA size batteries have a length of 50 mm approx. 4 numbers AA batteries in series make only 200 mm (+/- 2 mm). We fall short of 46 mm when compared to 246 mm (see the requirement list above) length of 4 numbers D size batteries. How to manage that? Simple, use some kind of filler to take up 46 mm length.

23 mm inner (29 mm outer) dia uPVC pipe (above)
18 mm outer diameter PVC pipe (below)

Being a farmer I also have many pipes (used for watering the plants) and one them happened to be a uPVC pipe with 23 mm inner diameter and 29 mm outer diameter (though the label on the pipe reads 1 inch / 25 mm inner diameter). This is the closest dimensions I could manage to reach the goal of 33 mm outer diameter of D size battery. I cut out a 200 mm piece of the pipe - which is the length of 4 numbers AA batteries connected in series minus 2 mm (you will read later why 2 mm less). Next challenge is filling the gap between AA battery diameter and the 23 mm inner diameter of the uPVC pipe I chose. For that I chose 18 mm external diameter PVC pipe used as electrical wiring sheath. I took 200 mm length of this pipe and fitted it inside the uPVC pipe. Remember - this electrical sheath pipe should fit concentrically inside uPVC pipe; therefore their length should be equal: 200 mm.

Layer of foam tape on inner (black) pipe.
I have applied few layers of electric
insulation tape over foam tape for proper fit.

However, there is a gap between the outer pipe (white one) and the pipe (black one) fitted inside it. You have to somehow fill this gap so that the inner pipe is concentric when fitted inside the outer pipe. I used foam tape to fill the gap. One layer of foam tape was enough to fill the gap. I applied the foam tape on both ends of the pipe and in the middle. If you want feel free to apply the tape all along - does no harm but not needed, why waste the tape? I could source only double sided foam tape (are the foam tapes always double sided? May be ...). Be careful not to peel off the outer backing tape on the foam tape to avoid the foam sticking to the outer pipe when you are inserting the thinner pipe (black one) inside the thicker pipe (white one). Fret not if you peeled it off by mistake, just apply a layer of cello tape on it to cover up the glue layer.

Note: Depending on exact thickness and diameter of the pipes you are using there may be some slack between the inner pipe and outer pipe even after applying foam tape. In that case apply few layers of cello tape or electrical insulation tape on top of foam tape to fill the gap. Be creative and use what you have to reach the goal!

Now comes the challenge of achieving outer diameter of 33 mm. Foam tape comes to rescue again. I applied a layer of foam tape on the outer pipe (white one) to get 33 mm outer diameter. Well, in my case it turned out to be little more than 33 mm. But that is okay - torch light barrel has adequate tolerance to take slightly bigger diameter battery too.

Outer pipe of 29 mm outer diameter with foam tape to convert it to 33 mm outer diameter

Then I put 4 numbers AA size batteries in the newly made holder and tested it. Positive terminal of the first battery and the negative terminal of last battery extend 1 mm outside the 200 mm long holder. That is perfect fit because it is necessary for both the terminals to extend little bit outside the holder for establishing proper connection. This is the reason why I had cut 200 mm long piece of pipe instead of 202 mm needed for 4 numbers of AA size batteries.

Cut and split pipe pieces for fitting inside the
ends of 18 mm PVC pipe

However, there was a small problem. There was some gap between the batteries and 18 mm pipe containing the batteries. That much of little gap may not cause any problem. But I wanted to be more accurate. So I filled that gap using small pieces of 18 mm PVC pipe pieces. For that cut 15-20 mm long piece of 18 mm pipe and remove around 6 mm on its side. Then press it so that the cut ends meet and push it inside the pipe. You may have to remove more than 6 mm or less than 6 mm or even file the inside of the cut piece depending on the thickness of the pipe you are using. Just make sure AA batteries slide in comfortably after fitting in the filler pieces. Filler pieces have a tendency to slide inside the pipe. Therefore glue it to the outer pipe using PVC cement or any quick curing adhesive. Lightly file (use a circular or semi circular file) or sand the inside of the ends If at all you find that it is little too tight for inserting AA batteries.

Thin black layer outside the white outer cover of battery
is the filler piece

Just 2 pieces at each end of the 200 mm length of 18 mm diameter pipe is enough because any gap inside the pipe does not matter much. Filling the gap at the 2 ends of the pipe ensures positive terminal of first battery and negative terminal of the last battery are at the center of the battery adapter.

Filler battery to fill 46 mm shortfall

Now the challenge of making up for the 46 mm short fall between the total length of 4 numbers D batteries and 4 numbers AA batteries connected in series. For that I cut out a 30 mm length of the 29 mm diameter uPVC pipe. Then I harvested the top and bottom ends of the spent D size battery and attached them to the ends of the pipe. It should look like a 46 mm long mini battery. It should act like a filler battery to fill that space conducting the electricity through it. Solder a wire connecting both ends of this "filler battery" to conduct electricity from one end to the other end. In the picture you don't see soldered wire because I have put the wire inside.

Now open the torch light, drop the filler battery to the bottom, fill batteries in the battery holder pipe and drop it on top of the filler (make sure polarity of batteries are all correct). Close the torch light. Switch it on. You should see it lighting up fully bright :-) Don't worry when the brightness of the light goes down. Just pull out the batteries, charge them and put them back. It restores full brightness.

After this experiment I felt confident of handling D to AA conversion. Ordered one more much better torch light for further experiments :-)