1

I’m using the diode pair solution from @james in this post with a MCP73831 and a 2000 mAh Li-ion pouch cell with this basic charging circuit:

enter image description here

And then one SS14 diode from Vin to the input of this 5 V Boost converter and the other SS14 diode from Battery+ to the same input of the boost converter. While powered from 5 V/3.2 A, when I flip the switch that is between Vo of the boost converter and a Teensy 4.0, which draws about 100 mA, the cell voltage drops and it actually goes down over time. How is this possible? I would have thought the load would be powered entirely off of USB/Vin… Can anybody shine any light on this?

Russell McMahon
  • 150,303
  • 18
  • 213
  • 391
Jeroen
  • 11
  • 4
  • 1
    Please provide a schematic that shows your diodes, your boost converter, your battery, your switch and your load (i.e. Teensy) – Math Keeps Me Busy Sep 09 '23 at 12:33
  • 2
    I edited my post with the requested schematic… – Jeroen Sep 09 '23 at 13:20
  • May I assume that you are testing this in a real circuit? Or just in simulation? FYI, there appears to be a short across the 4.7 uF cap in the lower left. Also, I don't know Fritzing, but it looks like there could be shorts across the diodes as well. Maybe that is just the symbol in Fritzing though. – Math Keeps Me Busy Sep 09 '23 at 13:36
  • I have made a PCB out of the schematic, so no simulation. And the shorts are my mistake I think. They do not exist in the PCB. Let me try and fix it. – Jeroen Sep 09 '23 at 13:54
  • 1
    The capacitor is fixed. The diodes are just how Fritzing displays them. – Jeroen Sep 09 '23 at 14:01
  • How quickly does the battery voltage drop? When the battery voltage is dropping, can you test to ensure that the battery diode is reverse biased, ie. that the voltage on the battery side is less than on the converter side? Similarly, can you test that the +5V is forward biased, i.e. that the +5V side is at a higher voltage than the input to the converter? Have you double checked all your connections / continuity comparing the board to your schematic? (I'm sure you have, but nothing simple is popping out at me as incorrect.) – Math Keeps Me Busy Sep 09 '23 at 14:22
  • It drops a bit quite fast and the re-stabilizes and then it slooowly goes down. The biasing was correct, but the differences where very minor. While checking the voltages as per your suggestion I noticed that the 5V input voltage measures as 4.1V and when I flip the switch to the off position it goes down even further! That is not good right!? But what can be causing it? As you suggested I did check contacts for continuity and everything checked out. – Jeroen Sep 09 '23 at 14:46
  • 1
    No, that is not good. Have you tested the boost converter independently? Or only in this circuit? I would test it. Sometimes the things bought from aliexpress are not what they purport to be. – Math Keeps Me Busy Sep 09 '23 at 14:57
  • I just disconnected the battery and now the input voltage measures @ 4.912V. Will check the boost converter now… – Jeroen Sep 09 '23 at 15:09
  • I said that I will check it but actually I’m not sure how except that I measured its Vo and that is correct at 5V. – Jeroen Sep 09 '23 at 15:15
  • 2
    Does Vo stay 5 V if you put a load across the output? For example powering your teensy apart from the battery charger etc. – Math Keeps Me Busy Sep 09 '23 at 15:34
  • The measurement of 5V I took before was with Teensy connected… – Jeroen Sep 09 '23 at 18:48
  • I took the boost converter off and the low input voltage situation did not change… – Jeroen Sep 09 '23 at 19:13
  • As described it is highly likely that "you are doing something wrong" OR charge current is coming from an unexpected source - possibly Schottky diode reverse bia current || . You say the 5V input remains connected, so the cell should be charged to maximum value 'after a while'. || An SS14 reverse bias current is 0.2 mA max at 25C. Q: Do you get the same result with a silicon diode temporarily used for the lower SS14? – Russell McMahon Sep 13 '23 at 11:29
  • 1
    Using unique component identifiers is very good practive. Having to say eg 'the lower SS14" rather than eg D2 is not good. – Russell McMahon Sep 13 '23 at 11:30
  • What sort of capacitpr is the 4.7uF (C1 :-) ). Is it leaky. How much current i flowing out of the battery when it voltage falls ? - Place eg a 1 Ohm in series and measure voltage drop. I = V/R. – Russell McMahon Sep 13 '23 at 11:32

0 Answers0