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I am using a MCP73844 dual cell Li-Ion charge management controller to charge a pair of LIR2032 cells, which are rated at 45mAh.

I have breadboarded the circuit according to the schematic in the datasheet, however I am using a sense resistor of 2.8 Ohms to achieve a constant current of ~40mA so I do not exceed the 1C rating of the cells. The only other difference is that I am using an NTD2955 as the driver MOSFET.

I have connected a 10v input to the charging circuit, and a depleted battery, and I am getting unexpected behavior. When powered up, I experience the following.

1 - The circuit draws ~2mA for a couple of seconds. As expected in the charge qualification phase.

2 - The status LED illuminates and the circuit draws ~40mA, (sometimes, and only for a few of seconds)

3 - This quickly decreases to ~20mA, and then continues to slowly decrease down to ~12mA (Presumably as a result of having transitioned into the constant voltage charging phase)

4 - After some time has passed, the current draw drops to 0mA, and the status LED turns off, indicating the charging has finished, but after a few seconds, the current jumps back up to ~12mA, and the LED comes back on. This cycle seems to be repeating indefinitely.

5 - The battery voltage ends up being only 7v

So, the questions that I have are:

1 - Why is the constant current phase not always happening, and when it does, why does it only last for a few seconds?

2 - Why does the charge cycle keep restarting?

3 - Why does the battery not reach the expected 8.4v? Cheers.

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  • \$\begingroup\$ It is very likely that the phenomena you see can be attributed to the high parasitic resistances of your breadboard. \$\endgroup\$ – Laszlo Valko Jun 14 '15 at 18:29
  • \$\begingroup\$ @LaszloValko - extra resistance would need to be very high at those currents. At 40 mA to get 0.1V needs 2.5 Ohms. Possible but seems unlikely. \$\endgroup\$ – Russell McMahon Jun 14 '15 at 20:40
  • \$\begingroup\$ How have you connected the external FET. The diagrams in the data sheet are misleading - verging on wrong. The source connects to Vin, not to battery, despite the diode around the FET suggesting otherwise. This is a P Channel FET (both their one and your replacement.) For FET to act as a FET the body diode must be as shown of diagram but to do this drain is to battery. If d & s were reverse FET could not be controlled as Dds conducts always. As Vin is 10V and Vbat max = 8.4V & less if depleted then charge I may lift Vbat due to IR drop & ... \$\endgroup\$ – Russell McMahon Jun 14 '15 at 22:00
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Best guess is that the misleading / verging on wrong example circuit diagrams in the data sheet have lead you to reverse the FET drain & source connections.

The FET source should connect to Vin, not to battery, despite the FET-symbol on their diagrams suggesting otherwise. This is a P Channel FET (both their one and your replacement.) The symbol used incorrectly implies an N channel FET with drain to input.

For the FET to act as a FET the body diode must be as shown on their diagrams but to do this the drain must be to battery when a P channel FET is used. If d & s were reversed the FET could not be controlled as the d-s diode always conducts.

As Vin is 10V and Vbat max = 8.4V & less if depleted then charge I may lift Vbat due to IR drop & ...

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