I work on a wearable, battery powered device. When it is connected to an external power source it's supposed to charge the battery and switch load to that source.

I use LiPos 1000-2500 mAh. I've experimented with two different new ones, with the same result.


simulate this circuit – Schematic created using CircuitLab

But actually, it doesn't charge battery (not completely). Voltage on mcp's vbat pin is 3.9-4.01V (or even less). Not 4-4.2V as must be according to the datasheet. Also D1 is flickering.

I've tried to connect battery (2500mAh) though an amperemeter and it showed 300mA at max, instead of 500, and after several hours battery was even discharged up to 1 Volt! Without ampermeter (quantum effects? :) battery charges up to 3.8-3.9V after more then 8 hours.

I don't have special precise power source, and tried macbook's USB port and iphone's power adapter. I've tried with and without load.

UPDATE I checked 2.1A power source - nothing changes.


2 Answers 2


The datasheet states on page 3: Electrical Specifications: Unless otherwise indicated, all limits apply for V DD = [VREG(typical) + 1.0 V] to 6 V. For 4.2 V output voltage you need at least 5.2 V input voltage.

  • \$\begingroup\$ Because I found myself here 10 years later... the datasheet p11 states "A supply voltage of [VREG (typical) + 0.3V] to 6V is recommended." so you only need at ~4.5v for a 4.2v battery, so 5v supply should be plenty. \$\endgroup\$
    – John U
    Nov 30, 2022 at 17:12

USB and MacBook you say...Your USB can deliver 5[V] but can it supply the needed current?

First of all, USB 2.0 can deliver at most 500[mA]. You need to supply current to both circuits, which is the battery charger and the load. Your current is split in two, 300[mA] to the charger 200[mA] to the load. I won't go into the details of how current works.

Second of all that DC-DC circuit with the transistor is redundant in my opinion and should be discarded.


My remark about discarding DC-DC is wrong, I was thinking about something else.

  • \$\begingroup\$ Tell me please, why do you think it is redundant? My load needs 3.6-4.2V, its possible to use diodes.. But they are current dependent.. \$\endgroup\$ May 6, 2021 at 12:36
  • \$\begingroup\$ Yeah sorry, it's not redundant, I was thinking of another charger. Anyway, the current supply stands, and I recommend finding a power supply that can source at least 1A. \$\endgroup\$
    – andrew
    May 6, 2021 at 12:47
  • \$\begingroup\$ I've found 2.1A usb adapter! But it didn't help 8( \$\endgroup\$ May 6, 2021 at 13:06
  • \$\begingroup\$ ok, I'll think again about it... \$\endgroup\$
    – andrew
    May 6, 2021 at 13:18

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