USb Charging & rechargable batteris HP41

[jebem]

As far as I know, the HP 41 machines does not have a built in charger.

Therefore an external power supply applied to a I/O Port will not charge batteries installed in a Battery Pack at the back of the calculator.

HP released the 82120A Rechargeable Battery Pack (and an AC Charger) as an alternative to the standard non rechargeable Battery Pack using dry N sized batteries.

What I see people doing since long time ago is to either rebuild these 82120A packs with new NiCd batteries to keep using them, as usually NiCd batteries will soon or later die in leaky way, corroding everything around.

Other alternative is to take a standard Battery Pack, remove the plastic separators inside to have free space (or use a old 82120A cabinet), and do some DIY to build a modern LiPo rechargeable battery pack with just a few of-the-shelf components (LEGO style).

An USB female connector (USB-C for 0.8 EUR), a 3.7V 300mAh LiPo battery (6 EUR), an intelligent LiPo charger with booster 5VDC power output (4 EUR), two "L" shaped strips of spring metal sheet as connectors to the calculator, some wires, glue, and a steady hand to solder tiny pins on the USB connector.

The 5VDC from USB and the 3.7V LiPo battery are connected to the Charger on dedicated pins. The calculator is connected to the Booster 5VDC output of the Charger/Booster.

So the calculator never see the battery.

Therefore this basic solution will not give early warning on battery low to the calculator, as the charger/booster will try to maintain 5VDC at its output until the battery falls bellow the low battery threshold level, and then it is too late as the charger/booster will simply shutdown the 5VDC output power.

Not a huge problem for me, as the calculator have constant memory and I try to charge the battery on a regular basis.

Let us say that 300mAh capacity is huge for such a modest current consumption calculator (7.5mA when pressing keys and running, less than 1mA (typ. 0.3uA) idling (standby), and between 5 and 75uA when powered OFF (sleep).
Unless a card reader is used, that will consume 10 times more.

But even with a card reader, 300mAh is a larger capacity than what the original rechargeable NiCd battery pack could offer (I recall it was about 150mAh or so).

We can always go for a larger LiPo battery, I believe that a 500mAh will fit inside the Battery pack together with the above electronics parts.

Some remarks on the Charger/Booster choice:

- It must be preconfigured for ONE 3.7V LiPo battery.

- The booster output must be 5VDC.
This allows the charger to switch off the booster and bypass the USB 5V to the output when it senses power applied to the USB port.
In this way, the calculator will run from the USB power while the Charger keeps charging the LiPo Battery.

- Choose a lower as possible standby current under no load, to save battery power when the calculator is powered off.
Many modules looks good, but they consume way too much current when idling.
I found a Charge/Booster module using a single IC solution based on a IP5306 intelligent power management IC, that features just 100uA of standby power.

- And choose the highest operation efficiency as possible. The above mentioned IP5306 offers 96% on boost and 97% on charge.