Charger Immunity

A project log for Classic HP Calculator LiPo Battery Pack

Safe and convenient power for HP 35/45/55/67 Calculators

tomcircuittomcircuit 11/26/2020 at 00:530 Comments

I’m retagging this project as “ongoing” now. As Mike Szczys pointed out, there’s the possibility of someone connecting an HP AC charger to a classic calculator with this pack inside. The HP AC charger produces a ~50mA constant current source output into the battery pack when it’s connected. Such long term ‘trickle charging’ is not recommended for LiPo chemistry. So, need to prevent this current from flowing into the LiPo battery  

A simple Schottky diode could be used to block the reverse current into the cell. The downside is the 0.3V drop that it imposes when the LiPo is powering the calculator. That would bring the nominal voltage to 3.4V, which is a bit on the low side. 

What’s needed is an “ideal diode”: zero voltage drop when forward biased, and zero current flow when reverse biased. Back in college this was a pipe dream. Fortunately, that’s “a thing” nowadays! The circuit idea is pretty neat: manage a P-channel MOSFET gate depending on the direction of current flow between drain and source.  There’s a good writeup here on this. It’s a cute application of a differential amplifier biased at a miserly operating point. It turns out that you can get the whole shebang integrated into a friendly SOT-23 sized package. In the GitHub repository you can find my LTspice simulation of the HP AC adapter, the ideal diode, and how well it performs in the two usecases (charger connected and not connected)

So, ‘version 2’ of the classic HP calculator LiPo battery pack will (probably) feature a MAX40203 “ideal diode” to make the LiPo immune to damage from long-term connection to the HP AC adapter.  Oh, and I’ll correct the &$@%# polarity of the JST LiPo connector while I’m at it!