24/3/2015 Ghetto Hax: Mod an HP50G calculator to use rechargable lithium batteries

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Wednesday, March 4, 2009

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. Afterusing RPN, I'm trying to enter stacks into a normal calculator. If you're anengineer and you haven't used an HP calculator, there's probably somethingwrong with you.

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

Replace AAA battery with Rechargeable lithium battery

Improve battery life significantly

Enable charging over USB

The HP50G already has a MiniUSB port which it can power itself off of, so itjust makes sense to do this mod.

Materials:I COULD design my own USB charging circuit and output regulation circuitsand buy the optimal lithium cells. BUT that would take time and effort. A realsmart person buys a product with all the features, and then cannabalize it.The perfect candidate for this is an external battery pack for the PSPavailable from DealExtreme. It includes all the components I want, all forless than $10 shipped! A keen observer will notice that DX advertises this asa "12000mAh" battery pack. This is partially BS ofcourse. What they meanis it has approximately 12000mWh of energy storage. When I opened it up, Ifound 2 LiPo cells marked "103447R". A quick search on Google revealedthat this is a 3.7V 1400mAh cell. The picture below shows the disassembledbattery pack.

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24/3/2015 Ghetto Hax: Mod an HP50G calculator to use rechargable lithium batteries

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Now let's open up that HP50G:There's an awesome tutorial on Youtube for this, so I won't write a wholething on it. The Youtube video shows the guy using a suction cup to pull theLCD cover, but I just pried it carefully with a screw driver since I don't havesuction cups laying around.

Next, we will gut the HP50G case to make room for our new battery:Lucky for us, the LiPo cells that come with the external battery pack fromdeal extreme is the same thickness and height as AAA cells! however it iswider than 4 AAA cells, which means we need to gut some of the case awayto fit it. There will be no exterior aesthetic damage since we will only beremoving ribs and the battery holder; the area which needs to be gutted outis shown in the picture below.We will also need a space for the PCB which has our charge regulation andoutput step up circuits, but lucky for us, there's a perfect spot near thebottom of the case (marked in the picture below) The PCB has an LED on itthat indicates charging status, and it could be a good idea to drill a hole sothat the LED can peek outside. The LED is actually really tall, so you willeither need to resolder it to have a shorter profile, or bend the legs or elsethe whole thing won't fit together in the end.

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24/3/2015 Ghetto Hax: Mod an HP50G calculator to use rechargable lithium batteries

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You will notice that I completely destroyed the battery holder and pulled outthe battery contacts to avoid short circuiting anything. When you are doingthis, you will also notice that the power wires from the battery holder issoldered on to a large capacitor. Since our PCB already have capacitors onthem, the removal of this capacitor is optional; I left it there since its legsmade 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 toavoid shorting stuff. Pretty self explanatory so far. I used double sided tapeto hold things in place but that's optional.You will need to take the 5V output wires from the PCB and cut off theconnector and strip off the black jacket. Solder the output wires onto thecapacitor that's glued on to the case. Obviously black goes to where all theblack wires go, but you'll need to solder the red wire on the leg that has theblue wire on it.

Getting USB power to charge your battery:The USB ground is already connected through the common ground, so theonly thing you have to do to get power from USB to charge your battery is toconnect pin 5 of mini USB to the external power input of the PCB. Pin 5 ofthe mini USB is the pin that's closest to the IR emitters on the HP50Gmotherboard, and it is VERY hard to get to. There is a metal casing thatcovers the mini USB connector that does really nothing at all, so I used aplier to snap off one of the walls (be very careful!) thus exposing the miniUSB pins. I tinned pin 5 with some solder, and tinned the tip of the wire, andsoldered them together very carefully. I guess an alternative is to probearound for an easier to solder pad that connects to pin 5, but since I wascomfortable with my soldering iron, I chose to solder it directly on the pin.

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

24/3/2015 Ghetto Hax: Mod an HP50G calculator to use rechargable lithium batteries

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If you did everything correct like me, you can simply close the case back upand everything will fit. Remember to solder the two red wires on thespeaker(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. TheLED sticks out the bottom for me which makes the calculator wobble. I willneed to get a taller rubber feet to compensate for this.When I plugged it into the USB port of my laptop, it immediately startedcharging. Great success! And ofcourse, I can use the calculator while it'scharging as well. After charging for 6 hours, the LED finally turned green. Iunplugged it from the USB, turned it on, and everything works the way itshould :) The day is mine...

I haven't tested the battery life improvements, but theoretically it should beabout 2X longer.4XAAA Alkaline = 1.5V x 1000mAh x 4 = 6.0Wh2X 1400mAh LiPo = 3.7V x 1400mAh x 2 = 10.36WhPosted by ghettohaxor at 2:47 PMLabels: Battery, HP50G, Lithium, Other Hacks, USB

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andre March 6, 2009 at 1:23 PM

this is like so pointfull

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Kyle March 8, 2009 at 4:32 PM

I'm going to use your method to take turn all sorts of normal"battery" power devices into Liion rechargeable ones! Thanks!

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sarah September 4, 2009 at 10:52 PM

I recently came accross your blog and have been reading along. Ithought I would leave my first comment. I dont know what to sayexcept that I have enjoyed reading. Nice blog. I will keep visitingthis blog very often.

Susan

http://carusbcharger.com

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