Recharging Cable:
Charging cable fits the terminals on the HP classic calculators. The cable is well made with 0.81" diameter brass tubing, very sturdy. I used shrink tubing and the USB cable is 3 foot long. Note that the tubing part is connected to the upper terminal and the other to the left side lower terminal.
For your reference, the middle pin is
negative. The lower left pin is positive and is wired to
the battery pack and the third pin is wired to the calculator circuitry. Note a gold clip connects the two lower pin
allowing the battery power to connect to the circuitry and run the calculator when the adapter is not plugged in.
Since the USB voltage is 5 volts DC you can directly connect to the pack and it will charge the cells. When the HP adapter is connected, there are two voltages 5 volts and 14 volts. Second pin gets 5 volts and the circuitry gets 15 volts. My design allows the two lower terminals to remain connected and thus the 5 volts travels also to the calculator, you can turn it on when plugged in.
I tested how long it would take to recharge the 3000 maH pack. I plugging it into an Apple cube at 2.1 Amps for this test. I tested the charging time for this battery pack with a voltmeter also attached to the charger and battery. It took 55 minutes to completely recharge the battery pack. It fully discharged back to no display, in 60 minutes with all "888" digits turned on. I checked the voltage as it discharged and it goes blank when the voltage hits around 3.4 volts, when it is charging it his 4.5 volts and the pack gets warm at one hour charging time. Even though the voltage is 5.0 volts unloaded, it can only reach 4.5 volts while charging. That is what my test showed. I've since recharged a battery pack three times and it slightly increased in use-time to about 60 minutes. The 3000 maH cells use time frames are fairly close to the 2000 maH cells, but they charge faster.
NIMH and NiCAD charging circuit with Auto-Off: This option is so much better than my previous design. Set between the USB power plug and the calculator battery pack is a circuit board that regulates the amps delivered to the battery pack. The circuit is designed to detect both NiMH and NICAD battery packs. While charging the LED light flashes every one second. Upon completed charging the circuit stops charging the battery pack, and the LED light goes off.
First Time Recharging: Be careful the first time you recharge the battery pack. Charge it for 30 minutes and see if the battery pack is warm. If it is warm then only recharge it for 30 minutes in the future. Different USB cubes have different amperages so be careful.
Battery Storage: Please protect your beautiful HP calculator! All battery cells will eventually leak a corrosive acid that may harm your calculator. If you are not using the calculator for a long time, please consider removing the battery from the calculator. These devices are just too awesome to risk potential corrosion damage. Note that the dates on the battery packs are 2022 and 2023. The date on your battery pack will be the purchase date plus 3 years.
If you are unhappy with it, mail it back, and I'll refund your money.
Shipping: Shipping costs are listed below. For multiple battery pack purchases contact me for actual shipping costs. I will refund your extra shipping charges and only charge you actual shipping costs. BTW, 93.9% of my sales are mailed in one day.
Also check my other HP calculator battery packs for sale especially for the Spice HP 33-34-35 calculators and for the larger HP 95-97 calculators.
Waterhosko (Mark Hoskins) is also on the HP Calculator Museum site