|Li-Ion powered WP34s|
Message #1 Posted by Mike Maiorana on 1 Apr 2012, 9:04 a.m.
In the spirit of tinkering, I purchased another HP30B and added a Li-ion battery. The battery is a 400mAH li-po rescued from a first generation Apple iPod nano that was headed to the trash. I designed a simple circuit that contains a li-ion charger and a low dropout 3.3V linear regulator, built on to a small circuit board that fits neatly into a slot in the rear cover.
The case required some small modifications. I put the battery directly behind the LCD display. In the 30B there is a small metal plate behind the display. Under the plate is a "honeycomb" of plastic which I had to cut away to make room for the cell. It only took a few minutes with a sharp pair of diagonal cutters.
I took the DC power supply and round connector from an old cell phone. I got a little sloppy in cutting the hole for the connector, but it doesn't look terrible. After I took this picture, I added a small LED next to the connector as a charge indicator.
The capacity of the new li-ion cell is a close match to the pair of CR2032 that are normally used in the calculator. As the battery voltage drops below about 3.5V the output of the regulator will begin to drop. When the battery is nearly depleted it should trigger a low batt condition on the calculator. The regulator contains a low battery detection circuit, but it simply provides a low signal on a pin. The WP34S would have to be programmed to acccept this and act accordingly.
The charging circuit will allow this to be plugged in at all times, just like my HP45 with no battery :)
There is still a chance that if left uncharged long enough that the li-ion cell may be damaged from over-discharge. A fully discharged li-ion cell is about 2.7V, which would present about 2.5V to the calculator, low enough to trigger a low-batt alert. Also, the charger has a "soft start" charge for extremely low cell voltages to prevent damage.
I would like to replace the round power connector with a small USB connector, which would allow charging from any number of USB devices and adapters I have laying around.
Here is a picture of the inside of the modified calc (minus the charge LED).