Amp reading on multimeter significantly varies depending on port

Question

I'm using a rapid 318dmm multimeter, and I am measuring the short current of a 5x5 inch solar cell which has a power rating of 2.7 W and a nominal voltage of 0.5 V. I'm measuring the current outside on a sunny day using the 10A fuse port, but I keep getting very strange readings. For each multimeter amp range the figure seems to change, and it's giving me a readable figure at the micro amp scale instead of a "1" which is to signal me to increase the range. I have numerous 2A fuses at my disposal, so I decide to try out the 2A port; it gives me a reading of 1.2A and when I change the manual range down to the microamp scale it gives me a "1", signaling me to increase the range. The reading of 1.2A stays consistent throughout the 2A and 20A range.

I tested both the 2A port and the 10A port with a continuity checker, the 10A port beeps, while the 2A port does not beep but does give me a reading that isn't '1'. I then check the resistance of each port, the 2A port is giving me 200k ohms, while the 10A port is giving me close to 0 on the side of the port and around 10k ohms in the center right at the back of the port.

Is it normal for the continuity checker not to beep when checking a port, does this mean the 2A port is also faulty? Or is the 10A port faulty.

Answer 1

A single solar cell only generates about 0.5 V (as your rating indicates). Many DMMs have a significant resistance in some of the current ranges -- needing 100-500 mV or higher voltage drop across the current-measuring terminals to operate. In addition, you may have non-negligible wire resistance to connect your solar cell to the meter.

The meter has different internal resistances on each current range and this is the source of your inconsistent readings.

Your cell is capable of generating over 5 A, but at only 0.5 V. even 0.1 Ω of wire series resistance will prevent it from delivering 5 A.

To read the performance, use the 10 A scale and use short thick wires with good connections (soldered or well clamped) to the solar cell.

Answer 2

You didn't post a link to your multimeter user manual so I'm assuming it's a 199.9 meter type. The meter module is full scale at 200.0 mV. That means that the shunt resistor value for each range is as follows:

 20 A    0.01 ohm
  2 A    0.1 ohm
200 mA   1 ohm
 20 mA   10 ohms
  2 mA   100 ohms
200 uA   1 kilohms

This may help you debug any problems with the meter and understand the relationship between voltage and current readings.