johnr's diy phone battery

Mobile Phone Battery
Made with wood, tin, cardboard and patience

see copyright notice. Page created 28-Sep-2006

Obsolete my a*se!

I bought my present mobile phone, a Philips C12 on a Botticellinet^* "Go Away"^* tariff, on 1 January 2000. On 7 June 2006 an SMS text message arrived from its [renamed] service provider: "Your 2Bad^* mobile and SIM won't work after 30 June. Please visit your local 2Bad^* store...". This I did the following week, and was informed that my phone had been declared obsolete. If for some reason I didn't wish to spend fifty pounds on a new phone, then they graciously offered to sell me a new SIM card for a fiver - but with no guarantee that it would work after the cut-off date.

A few days and several indignant phone calls later, I secured a free new SIM card and a promise that it would be ok, as indeed it was. So on 1 July, although I cursed 2Bad^* for requiring me to re-enter my entire contact list, and commit to my own memory several cryptic codes that had replaced convenient hot keys, I felt pleased to have averted what I saw as an attempt to make me throw away a perfectly good piece of kit. Apart from anything else, I feel safe carrying it, because it's old enough not to be an attractive target to thieves, whilst not yet being old enough to have antique value. A bit like me?

There was one snag. Over the years, the rechargeable battery had become progressively less so, and was now reluctant to work at all in cold weather. Tests showed that its capacity wasn't very much down on the specified 700mAH, but it had developed an internal resistance of about an ohm, which was enough to make the phone shut down when on full transmit power. When it got to the stage that it couldn't even struggle through network registration unless I switched it on in one particular upstairs room, the time had come to act.

Like the other articles in this corner of my website, what follows is not intended as a complete set of instructions. Rather, I want to record some of of the decisions I made, and techniques I used, to get results. I hope this will be of interest to any other electronics boffin who is foolish enough to try something similar!

^* Some names in this document have been slightly disguised.

WARNING: a short circuit almost anywhere inside a battery like this could cause overheating and even a fire or explosion. Make such a battery at your own risk, and use an external slow (C/10) charger, not the phone's built-in charger unless you've properly replicated the original battery's protective devices.

Assault on battery

Having tried unsuccessfully to recondition the battery, and met with scorn and derision on asking in several mobile phone shops if they could supply a replacement, I began to think about making one, and set about finding out why there were 4 terminals instead of the more logical 2. The casing felt as though it contained 4 cells, but with a terminal voltage of 3.6 I knew there must be 3. With the aid of a multimeter, I established that terminals 1 and 2, at the "+" end, were electrically common. Relative to these, Terminal 4 carried -3.6V, while terminal 3 measured about -2.8V on my (20k/V) meter. I'd already determined that the phone wouldn't "start" if I just fed it 3.6V on terminals 1,2 and 4, so I instantly jumped to the wrong conclusion - terminal 3 must carry a logic signal from some form of internal battery monitoring device, probably with an impenetrable serial protocol to exchange data with the phone! When I finally bit the bullet and cut off the casing, then removed a plastic cover clipped over the terminal connections, I found what appeared to be a 33k resistor welded between terminals 3 and 4. Actually I suspect it's a thermistor, to monitor the temperature of the battery. But I got the phone to work fine with just a resistor in place, so I decided to stick with this approach and, for safety's sake, use a separate charger rather than the phone to recharge the new battery.

The original cells were slightly longer than AAA, but about the same diameter. This was good news. I was puzzled by the thin device that occupied the "fourth" slot, but eventually identified it as a PTC resettable fuse, type LS180L. I now had to make a decision: should I try to replace the cells in the original battery, or assemble a completely new one from scratch? I chose the latter approach for two reasons. First, I didn't want to risk running into problems during conversion and ending up without a working phone. Second, I still had a faint hope of reviving the original battery by some sort of violent charge/discharge regime, but again this risked wrecking it completely.

Precision carpentry

The 4 terminals of the old battery are separated by insulating ridges to guard against short circuits and incorrect insertion. The terminals are offset to one side, so normally the ridges lie in the gaps between the phone's contact springs, but if upside down, they fall in the middle of the springs and prevent contact. To mimic this, I found some high quality 3-ply wood, about 3mm thick, traced round the end of the old battery (it's shaped a bit like a 25-way D connector) and cut and filed/sanded the block to shape. Having marked the positions of the separator ridges, I made hacksaw cuts either side of each one, to the depth of the first ply, then, with the edge of the file, filed away the wood until I reached the glue layer bonding the next ply. The terminal strips were then cut from 5mm wide strips of tinplate (an old tobacco tin from a family friend, as it happens), folded over the edge of the block, and glued in place. This photo shows the block from the rear, you can see the terminal side in the finished item below.

The three 700mAH Ni-MH cells are connected by straps of fairly thick copper wire, bent to clear the rims and soldered directly to the cells. This will probably horrify the manufacturer, but I was careful to minimise the heating time and the result seems to me to be perfectly sound. The negative ends needed light abrasion with wire wool to make them accept solder. Thus prepared and tinned, I taped the cells together with a short plastic tube (if anyone's curious, it's cut from a rocket-launching tube supplied with a box of fireworks!) before making the wire connections. Inside the tube is a 2.5A, fast-blow 20mm fuse, also soldered in place. The resettable fuse in the original battery was specced at 1.8A holding current, 3.8A trip current, and although a blown "conventional" fuse would render the battery useless, I decided I ought to include this miminal form of protection. I inadvertently tested it during assembly, by dropping a pair of tweezers across the contacts, and cursed my cautious approach as I de-soldered and replaced the fuse - but so far there haven't been any failures in use.

Best for Baby, Best for Battery

After soldering the terminal connections, I made a base plate by cutting a similar shape to the terminal block out of thick cardboard, and covering both sides with PVC tape. The "outer" tape had sufficient margin to fold onto the ends of the cells as in the photo. Six plastic rods space the base plate from the terminal block, also helping to flatten the front and rear of the battery. After careful consideration, I chose the shafts of some "Cotton Buds" to make these. They are just the right size, and easily cut to length with a blade. Those abutting the resistor and soldered terminals were cut slightly shorter to fit snugly.

Before final assembly, I interleaved a piece of polyester film (cut from a roasting bag) between the cells, as a precautionary insulating layer in case abrasion damaged the cells' outer covering. Also, a length of double-folded cardboard, 5mm wide and wrapped with PVC tape, was slid between the two sets of plastic rods, to pad out the gap between the cell ends and the terminal strip. Two more narrow strips of cardboard widen the profile at the sides of the battery. With everything temporarily held together with rubber bands, I covered the assembly with three strips of PVC tape, starting at the base.

Mass production

Once I'd established the battery was AOK, and as I had one AAA cell left over from a pack of 4, I immediately bought another pack (same shop, same batch I think!) and made a second battery. So now I can take a spare with me when the phone's getting low, and I have a pair of left-over cells which I'm sure will find a use elsewhere!

As I mentioned earlier, I decided it would be unwise to use the phone to charge these batteries, so instead I use a 5V DC "plug-in" adapter, originally supplied for a digital camera, with a 15ohm, 1W series resistor. This delivers a 1.5C charge (1050mAH) in about 18 hours.