Measuring charge current

Normally I'd use my EVision to measure the charge current, but it needs more voltage than a few cells provide. Last year Vik gave me a few Alegro ACS754LCB-100 hall effect current sensors, so I dug them up and now I'm measuring the charge current. A hall effect sensor uses physics to measure the magnetic field created by a current passing along a wire -- this one is calibrated to produce 20mV on it's output per amp flowing in it's sense wire. I tried 3 sensors and only one of them matched the datasheet. After I'd soldered that one with my too-small soldering iron (which means the part was kept very hot for a long time) it wasn't all that close to specification either. It's response is still linear so I've just adjusted the mV/A value in my code. I'm measuring the signal with my LabJack U3. The code will be in svn soon.

A hall effect sensor is good because it isolates the sensed current from the signal wires. This way I can tolerate one isolation fault between my computer and the battery. If I used a regular resistive shunt, the battery would be connected to the computer and an unexpected second isolation fault could cause potentially damaging current to flow. The low insertion losses of the hall sensor also make the charger's voltage regulator more accurate. The disadvantage of hall sensors is they produce more noise in the output signal than a typical resistive shunt.

Below we see 3 cells being charged. The charge current starts out high, I adjust the voltage control down and we see the current drop as the cell voltage rises. Then I adjust it up a little. It becomes obvious that the green and blue cells are nearly full while the purple cell is not, so I connect a separate 3A supply to that cell only. This increases the total voltage and the charge current drops. The PFC30 voltage regulator seems to cut the charger off completely below about 300mA.