Wireless charging for a HP-25 calculator

[Maximilian Hohmann]

Hello!

(05-14-2021 07:53 AM)jwr Wrote:  That's not likely to work well — LiPo voltage varies a lot, the usable range is roughly 3.0-4.2V. With two diodes, you'd be getting 3.0V when the battery is fully charged (or charging) and 1.8V when it's nearly depleted.

Unfortunately... So a voltage converter is a must.

(05-14-2021 07:53 AM)jwr Wrote:  Please be careful with those "wireless charger receivers" from AliExpress — the ones I saw have USB connectors, so they produce 5V. Definitely not what your calculator wants to see on its battery terminals, and this will not charge a LiPo, you need a charge controller for that.

So the diode and maybe a resistor will have to go between the wireless receiver and the LiPo battery instead. In one of my shoeboxes I found a bag with battery protection boards (BBP), so while I wait for the wireless receivers and LiPos from China I tried out this (the original (dead) Woodstock battery pack is not connected to anything, it is just there to show the size):



A single cell LiPo battery is connected via a BBP to a small buck converter which I have set at 2,5V. The BBP should protect the battery from both overcharging/overvoltage and deep discharging and the calculator will never see more than 2,5V, no matter what the output of the wireless receiver may be. It should all be small enough to fit inside an original battery case.
I tried it with my Panamatik LP25 and it worked well. The only problem is that my cheap buck converter draws 5mA even when the calculator is off, which means that it will drain the battery within three or four days. Not good. On the other hand it will have to stay on all the time when one of the "C" Woodstocks is going to be powered.

(05-14-2021 07:53 AM)jwr Wrote:  To be clear, the circuit I built is not rocket science, but power management is tricky, especially with LiPo batteries. I built this because it turned out that I couldn't easily assemble it from available off-the-shelf components.

Well, rocket science is more my background, but I am afraid that I can't design a circuit like yours nor do I have the tools and skills to assemble it :-)

Regards
Max