Mod an HP50G calculator to use rechargable lithium batteries

First let's start out with a short rant:

HP50G is an awesome calculator; RPN is the greatest thing ever. After using RPN, I'm trying to enter stacks into a normal calculator. If you're an engineer and you haven't used an HP calculator, there's probably something wrong with you.

AAA Batteries are terrible; they are usually more expensive than AA's in retail stores which makes no sense, when they have a third of the capacity vs AA's. I hate it when devices are powered by AAA's especially when they're higher drain devices. The only devices that should use AAA's are things like clocks, radios, and remote controls. Which motivated me to do this hack. The HP50G chews through AAA's really fast. I've been in more than one exams where it ran out of batteries half way. Therefore here's the goals of this hack:

1. Replace AAA battery with Rechargeable lithium battery
2. Improve battery life significantly
3. Enable charging over USB

The HP50G already has a Mini-USB port which it can power itself off of, so it just makes sense to do this mod.

Materials:
I COULD design my own USB charging circuit and output regulation circuits and buy the optimal lithium cells. BUT that would take time and effort. A real smart person buys a product with all the features, and then cannabalize it. The perfect candidate for this is an external battery pack for the PSP available from DealExtreme. It includes all the components I want, all for less than $10 shipped! A keen observer will notice that DX advertises this as a "12000mAh" battery pack. This is partially BS ofcourse. What they mean is it has approximately 12000mWh of energy storage. When I opened it up, I found 2 Li-Po cells marked "103447R". A quick search on Google revealed that this is a 3.7V 1400mAh cell. The picture below shows the disassembled battery pack.
Now let's open up that HP50G:
There's an awesome tutorial on Youtube for this, so I won't write a whole thing on it. The Youtube video shows the guy using a suction cup to pull the LCD cover, but I just pried it carefully with a screw driver since I don't have suction cups laying around. Next, we will gut the HP50G case to make room for our new battery:
Lucky for us, the Li-Po cells that come with the external battery pack from deal extreme is the same thickness and height as AAA cells! however it is wider than 4 AAA cells, which means we need to gut some of the case away to fit it. There will be no exterior aesthetic damage since we will only be removing ribs and the battery holder; the area which needs to be gutted out is shown in the picture below.
We will also need a space for the PCB which has our charge regulation and output step up circuits, but lucky for us, there's a perfect spot near the bottom of the case (marked in the picture below) The PCB has an LED on it that indicates charging status, and it could be a good idea to drill a hole so that the LED can peek outside. The LED is actually really tall, so you will either need to re-solder it to have a shorter profile, or bend the legs or else the whole thing won't fit together in the end.

You will notice that I completely destroyed the battery holder and pulled out the battery contacts to avoid short circuiting anything. When you are doing this, you will also notice that the power wires from the battery holder is soldered on to a large capacitor. Since our PCB already have capacitors on them, the removal of this capacitor is optional; I left it there since its legs made it a lot easier to solder on the subsequent wires.

Putting it all together:
Put everything in their place. Wrap things in electrical or packaging tape to avoid shorting stuff. Pretty self explanatory so far. I used double sided tape to hold things in place but that's optional.
You will need to take the 5V output wires from the PCB and cut off the connector and strip off the black jacket. Solder the output wires onto the capacitor that's glued on to the case. Obviously black goes to where all the black wires go, but you'll need to solder the red wire on the leg that has the blue wire on it.

Getting USB power to charge your battery:
The USB ground is already connected through the common ground, so the only thing you have to do to get power from USB to charge your battery is to connect pin 5 of mini USB to the external power input of the PCB. Pin 5 of the mini USB is the pin that's closest to the IR emitters on the HP50G motherboard, and it is VERY hard to get to. There is a metal casing that covers the mini USB connector that does really nothing at all, so I used a plier to snap off one of the walls (be very careful!) thus exposing the mini USB pins. I tinned pin 5 with some solder, and tinned the tip of the wire, and soldered them together very carefully. I guess an alternative is to probe around for an easier to solder pad that connects to pin 5, but since I was comfortable with my soldering iron, I chose to solder it directly on the pin.

When everything's in its place, it should look like this:

If you did everything correct like me, you can simply close the case back up and everything will fit. Remember to solder the two red wires on the speaker(buzzer) back on to the motherboard when you close it back up!.

Results:
The calculator weighs a bit more but otherwise looks exactly the same. The LED sticks out the bottom for me which makes the calculator wobble. I will need to get a taller rubber feet to compensate for this.
When I plugged it into the USB port of my laptop, it immediately started charging. Great success! And ofcourse, I can use the calculator while it's charging as well. After charging for 6 hours, the LED finally turned green. I unplugged it from the USB, turned it on, and everything works the way it should :) The day is mine...

I haven't tested the battery life improvements, but theoretically it should be about 2X longer.
4XAAA Alkaline = 1.5V x 1000mAh x 4 = 6.0Wh
2X 1400mAh Li-Po = 3.7V x 1400mAh x 2 = 10.36Wh