Thanks for the information about the sense resistors. I'm trying an experiment charging a lithium cell now. Getting somewhat strange results.
I have the supply limit set to 4.200V and 0.150 mA (in the 8V range). Then it switches to a live reading. My voltmeter and source agree on 4.200V with no load, with or without sense bars shorted.
Then I connect this circuit using wires under 50 milliohms:
The + lead goes to an ideal diode XL74610, then to the cell+
The + sense goes directly to the cell+ (so does the voltmeter)
The - lead goes directly to the cell-
The - sense goes directly to the cell- (so does the voltmeter)
The ground is not connected
The ideal diode prevents reverse the cell from discharging back into the power supply should something interrupt the power.
I'm connecting the sense wires this way because the XL74610 requires at least a 0.3 V initial startup before the MOSFET is turned on, then a small duty cycle steals a little voltage every so often to maintain the MOSFET. I was trying to get past the strange characteristics of the ideal diode and monitor the progress directly on the cell.
The strangeness is the disagreement between the voltmeter and the Agilent power source voltage readout under the following scenarios:
BOTH SENSE WIRES
4.198V Source, 4.149V at the cell, 0.149A current. Charging is proceeding as I want, but voltages do not match.
PULL OUT BLACK SENSE WIRE
4.163V Source, 4.149V at the cell, 0.149A current. Charging continues, voltages are somehow closer - opposite what I would expect.
PUT BLACK SENSE WIRE BACK IN, PULL RED SENSE WIRE
4.200V Source, 4.110V at the voltmeter-cell, 0.000A current
REMOVE BOTH SENSE WIRES
4.200V Source, 4.110V at the voltmeter-cell, 0.000A current. (Not enough voltage differential to turn on the ideal diode MOSFET)
After letting the cell charge a little while, the source is 4.200V, the cell is 4.150V, and the current is 0.099A & dropping. So now it is running in constant voltage mode. It's always 0.050V offset no matter the current. The only thing on the screen changing is a very quick 0.025A drop as the ideal diode tops off it's capacitor tank.
It will be interesting if the cell reaches 4.200V and the charging stops. That's what I wanted. However, I don't fully understand the discrepancy of the voltage readout on the source. It seems the source voltage would be reported higher because the source needs to add more voltage to leapfrog the diode and sustain the charging.