Inconsistent Charge Termination Voltage with battery charger - Page 1
Forum section: electronics / beginners

author: Freshman
author_rank: Regular Contributor
timestamp: June 17, 2025, 04:40:53 pm
content: I'm using the MCP73871-2CCI/ML (https://ww1.microchip.com/downloads/en/DeviceDoc/MCP73871-Data-Sheet-20002090E.pdf) Li-Ion charge management controller and have built 10 fully assembled PCBs with this IC. As per the datasheet, the expected charge termination voltage is 4.2V. However, during testing, I observed noticeable variation between units—some boards terminate charging around 4.05V, others around 4.15V, and a few correctly at 4.2V.I'm also using a fuel gauge IC to monitor the State of Charge (SoC). When the termination voltage is lower (around 4.05V), the SoC typically tops out at 93–96%, whereas units terminating at 4.15V or 4.2V show a full 100% as expected.I'm trying to understand what might be causing this inconsistency in termination voltage. Could it be due to tolerances in passive components, PCB layout differences, or something else entirely?As I prepare for larger-scale production, I would really appreciate any advice on how to achieve consistent termination behavior across all units.

author: thm_w
author_rank: Super Contributor
timestamp: June 18, 2025, 01:03:50 am
content: Schematic?Are you using the same battery cells to test each IC?Purchase from reputable seller?The datasheet says 4.18 to 4.22 is the specified charge voltage, so something is up.Though if the cell is discharged, it can reach as low as 3.97V before the IC will start charging it again. So remove any load if there is one.

author: Psi
author_rank: Super Contributor
timestamp: June 18, 2025, 02:06:48 am
content: Are you reading the battery voltage at the exact moment it terminates charging? or some time later.Because lithium cells have internal discharge and lower quality cells have much worse internal discharge and the rate is much higher the closer you get to fully charged. So it might have reach 4.2V, then charger shut off, and then dropped to 4.1V over some number of hours, or days even even with no load.Also consider the PCB track/wiring layout and voltage drops.

author: paulca
author_rank: Super Contributor
timestamp: June 18, 2025, 12:07:27 pm
content: The voltage relaxation is HIGHLY dependant on the "taper current" and it's cut off.In the upper portion of the 4.10>4.20V region the charge curve is nearly vertical. Tiny amount of charge produce large voltage swings.Therefore if the IC brings the cell to 4.20V +-0.2V at 1 Amp the "settled" termination cell voltage might be 4.10V.If you do the same with a 100mA taper you might get 4.15V. With a 10mA taper a 4.18V and so on.As the voltage in that region is very highly reactive to current it very hard to be accurate. The flip side of the is, there is sod all charge up there anyway and the first time you put it under any real load it will drop way under 4.0V.The trade off you make is risking voltage measurement errors exceeding the +-100mV threshold in service and giving the cell an early retirement holding it at or above 4.20V. That's again the 2-5% of charge you are losing just doing what most "sane" people do and stop at 4.10V.Further a cell at 4.20V will have nowhere to go to, except over voltage if you warm it up a few degrees celcius. Charge it to 4.20V outdoors on a cold day and put it in your pocket and it will likely badly overvolt above 4.3V ( don't recommend this).This is why most high volume lithium chargers with build in pouch or 18650 batteries don't go anywhere near 4.20V.(assuming most common lithium cell behaviours and not considering the dozen caveats).