Classic HP Calculator LiPo Battery Pack

I received the V2 circuit boards from PCBWay earlier this week. These sport blue soldermask, to differentiate them from the red V1 boards (that are not immune to HP charger). The USPS package with components from DigiKey was delayed a bit (normal for this time of year, particularly for this year) but eventually made its way to my house.

I assembled the boards and did a brief checkout:

• First, I connected a LiPo battery + series current-meter to the board via J2
  
  
  1. Approx 100uA of static current is consumed by the MAX40203 (and perhaps Z1 leakage) with no-load across the PCB battery contact pads (those that mate with the calculator).
  2. The MAX40203 'ideal diode' allows current to flow from the LiPo battery, through the MAX40203, into a small resistive load (~50mA)
  3. Voltage drop across the MAX40203 while it is conducting is very low 
     
• Next, I applied a 10V source, current-limited to 50mA, across the PCB contact pads (to simulate what happens when an HP charger is present)
  
  

  1. The voltage across the pads clamped at around 5V. This is required to keep the MAX40203 voltage with specification

  2. There is no current flow 'into' the LiPo battery; the MAX40203 is indeed preventing charger current from entering the battery

During my testing, I found something interesting: With "no load" (just my Fluke DMM) across the contact pads, there was only a very small voltage (less than 150mV) present .  This seems very reasonable - I'm guessing the MAX40203 needs to see some differential voltage across the MOSFET before it decides it must be "on". I bridged JP2 to pull the MAX40203 ENABLE pin high, and the voltage across the contact pads matched the LiPo battery voltage, even with only a DMM load.  So, for now, I'm leaving JP2 bridged. Note that this behavior matches the simulation that I did previously, where the MAX40203 would not conduct when the simulated load was abruptly connected.

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!

This was a fun and easy project, and it went perfectly.  Well, nearly.  Had to swap the LiPo leads in the JST connector to match the polarity that I assumed on my PCB. Otherwise, no issues at all.