Replacing 2 AA 1.5V batteries with 2 AA lithium batteries

Context

My digital camera (Canon A2100 IS) uses 2 AA cells as its power source, as can be seen on the photo below from the review site above :

This was one of the reasons why I selected this model, because I didn't want to have to discover that the battery is depleted when I need it, and AA batteries are easy to find. I replaced the batteries long ago with NiMH rechargeable batteries, but the camera is a bit picky on the input voltage and cuts off long before the batteries are depleted.

Failed attempts

NiMH - NiCd

I tried both NiMH and NiCd batteries just in case the discharge curve would be better on these last ones, but none really stands out of the lot. Below 1.2V approximately, the camera complains.

Lithium 1.5V batteries

I found long ago a very interesting type of 1.5V lithium AA batteries made by Kentli. These batteries are in fact made of a smaller 3.7V lithium battery inside, followed by a DC-DC converter to emit a constant 1.5V voltage. They're pretty good for what they are doing. But they are not suitable for this type of device. Indeed, their output voltage regulator suddenly cuts off once the battery's capacity is too low, and the regulated output prevents the camera from indicating it's going to stop working. Not only it's unpleasant to discover that the battery suddenly is dead when you want to take a photo, but it usually fails with the lens out and exposed without protection which is not fun at all. Ideally such batteries should lower the output voltage near the end so that the powered device detects the situation and can gracefully shut down or warn the user. But for torch lamps and laser pointers these ones are perfect.

Single LiFePo4 cell

I was thinking that there's always some margin in devices designed to be powered by batteries, so if two completely charged AA cells can provide about 3.1-3.2V, surely a single LiFePo4 cell (3.2-3.3V approx) would be fine as well. Such batteries are easily found on the net, sold with a spacer made of a simple wire, to replace the second battery without adding any extra voltage.

The problem with LiFePo4 is that the battery capacity is very low, 600-650 mAh on average so in the end I had an even shorter duration than with the NiMH ones.

Single Li-ion cell

I figured that since the LiFePo4 cells send 3.6V to the device once fully charged, there's probably still a bit more margin, so I decided to try a regular 3.7V Li-ion cell (which reaches 4.2V when fully charged), again with the spacer to replace the second battery.

The first difficulty with these batteries is that their advertised capacity is most always fake, so it's very hard to spot valid ones unless you're willing to pay the price. For this size in Li-ion it's reasonable to expect 800 to 900 mAh, not more. Also, at less than $6 per cell it's a low capacity one. But it's not a universal law since it's possible to find expensive ones which are fake as well :-)

Aside the difficulty to select the correct battery, it turns out that the voltage is perfectly suitable for this camera. The camera only warns once the battery is really depleted, and doesn't suffer from the extra voltage. I figured that I'd just have to spot a serious battery and would have an acceptable capacity with the ability to recharge it with a regular Li-ion charger. But isn't it too bad to lose half of the capacity in the spacer ?

Successful solution found

I spent two hours figuring the best type of batteries I'd need to go further and I ended up picking unprotected and flat battery cells. They are slightly shorter than the regular ones because they're designed to support installation of a small protection PCB if needed. I found this, made by Sanyo, which are apparently genuine given the capacity I tested (about 850 mAh each) :

Then I figured that I could install the two in the reverse direction, and connect them in parallel, negative to negative and positive to positive using wires, and isolate one side of each battery so that the resulting block is still two batteries with a single battery voltage. Thus I tried.

First, I prepared the batteries by cutting pieces of the isolation layer in order to solder them together. For this, the batteries need to be installed flipped and aligned. The minus pole should slightly stand out compared to the plus pole, which will receive some solder :

Then using my soldering iron at maximum power, I managed to solder them together without heating the batteries too much :

Finally, the two positive poles were connected together using a piece of wire. A bit of extra solder was installed on top of the wire to make a button which will help make good contact inside the camera :

And that's done! I now have a 3.7V, 1700 mAh battery that presents itself in the same form factor as a set of two AA batteries and which delivers its whole capacity before the camera shuts down. The main difficulty I had was that the space between the two batteries in the camera is very tight and I had to minimize the solder's thickness to let the battery block in. I also had to install some polycarbonate tape on one side to ensure that the metal part closing the battery holder doesn't short-circuit the positive and negative poles.

Unfortunately I didn't figure how to photograph my camera with the trap opened to see the batteries installed, but it's easy to imagine on this block on the photo at the top.

What is nice is that I can still charge it using a regular Li-ion charger supporting 14500 to 18650 batteries. Let's see how long it will last now.