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.