I've just been through a lot of frustration with my Link 20 having the above described slightly inaccurate current
readings, but in the end I've succeeded in calibrating it properly. Let me share the solution with those who have the same problem and wish to correct it.
Since day one my Link 20 had an error in current measurement of -0.2A on both battery banks. My starting battery reading was -0.2A even when the starting battery positive was isolated. On the house bank, with everything switched off (including the main electrical
distribution board), the reading was fluctuating between -0.2A and -0.3A, because the Link itself was using a bit of power.
-0.2A does not sound like much of an error, I can hear you say, but in just one week this adds to -0.2 x 24 x 7 = -33.6Ah of false discharge readings. In my case (just got new AGM
batteries), no amount of charging would get the Ah reading back up to zero, because the charging current would eventually drop to 0.0A - 0.1A and the only way to "start over" was to manually reset the Ah to zero.
Needless to say, I double and triple checked the wiring
. During the installation, I'd used the original Xantrex supplied cable packing 4 twisted pairs, the length of the wiring
was not excessive (23 feet) and the wire did not run along any AC cables
or similar sources of interference
. It was obviously a calibration error, or more precisely a "zero offset instrument error".
How to fix it:
Before you go any further, you'll need a stable and precise source of 40.0V DC. That seems to be the safety
Xantrex built into the unit to prevent accidental calibration. I found an electrician who agreed to lend me a variable DC power source from his shop. You may know someone at a computer shop, electronics
lab, university, etc. You'll also need an accurate digital voltmeter to confirm that you're supplying exactly 40.0V DC to the unit during calibration, but these days it shouldn't be a problem - any reasonable digital voltmeter will do.
1. This procedure has not been obtained directly from Xantrex, but it worked for me exactly as described. I hope it works for you too, but you're doing it at your own risk.
2. Any temporary wiring connections you make during the calibration setup should be firm and the electrical
3. This procedure will completely reset your Link 20, so you'll need to re-program any user defined parameters (e.g. battery bank capacity & type, Peukert's exponent, etc.) back into it. You will also lose the history
from the unit.
Connections to make:
1. Pull out the fuse on the Link 20 power supply red wire and check that the unit is off.
2. Make sure the variable DC power source is switched off.
3. Connect the negative of the DC power source to the small screw in the middle of the load side of the shunt where the Link 20 black ground wire is connected (screw them together into the shunt).
4. Disconnect the blue wire (Bat. 1 voltage sense) from the house bank positive and the violet wire (Bat. 2 voltage sense) from the starter battery positive and connect them together and to the positive of the DC power source. Leave the fuses
in those two wires.
In steps 3. & 4. you have effectively rewired the Link 20 to measure the DC power source voltage instead of the voltage of your batteries.
5. Disconnect the orange wire (Bat. 1 current sense battery side) from the side of the shunt and connect it to the green wire (Bat. 1 current sense load side) on the side of the shunt (screw them together into the shunt).
6. Disconnect the yellow wire (Bat. 2 current sense battery side) from the side of the shunt and connect it to the brown wire (Bat. 2 current sense load side) on the side of the shunt (screw them together into the shunt).
In steps 5. & 6. you have effectively shorted the current sensing circuits on both battery banks, thus simulating zero amps conditions. This is necessary because you will be calibrating the instrument for zero amps.
7. DO NOT CONNECT THE RED LINK 20 POWER SUPPLY WIRE TO THE DC POWER SOURCE, leave it connected to your house bank.
8. Connect an accurate digital voltmeter to the positive and negative of the DC power source.
The calibration itself:
1. Ensure Link 20 is switched off (fuse out on the red wire).
2. Ensure the voltage control on the variable DC power source is turned down to zero or minimum.
3. Turn the DC power source on and slowly increase the voltage while observing the reading on your voltmeter - adjust the voltage to precisely 40.0V. DO NOT EXCEED 50V!
4. You may need someone to help you with this step if your Link 20 is mounted far from the batteries. Push both Sel and Set buttons on the Link 20 and replace the fuse on the power (red) wire while those buttons are still pressed, only release them when the unit powers up. In other words, power the unit up while pressing those two buttons, then release them.
5. You should now be in the "calibration mode": the "V" and "A" lights on the Link will flash alternately, the "t" light will also be illuminated and the display should be showing the voltage from the DC power source.
6. It is not possible to proceed from here no matter which buttons you press until the voltage on the display is equal to or exceeds 40.0V (40.0V - 40.4V). If necessary, further fine adjust the voltage from the DC power source to exactly 40.0V DC and when ready hit the Sel button on the Link 20. You're now done with the voltage calibration.
7. You should now be in the current calibration mode, indicated by the "A" light being illuminated on the Link 20. The display should now be showing the small current offset error.
8. Using Bat1 (offset -) and Bat2 (offset +) buttons, fine adjust the display reading to 0.00 and press the Set button when ready.
9. The previous step needs to be done 4 times: Bat. 1 low amps range, Bat. 1 high amps range, Bat. 2 low amps range and Bat. 2 high amps range - every time fine adjust the display to 0.00 using the Bat1 (offset -) and Bat2 (offset +) buttons and when ready confirm by pressing the Set button.
10. Press Set again to enter the display test. All LEDs except the two on the right for each bank should be illuminated. The display sould be showing ±188.8
11. Press Set again to exit the display test and the calibration mode.
12. The Link should now be in the "normal" mode, displaying the 40.0V from the DC power source. Press Sel and Bat2 to confirm that the current readings are now 0.00A for both battery banks.
13. Remove the fuse from the red power wire to turn the Link 20 off.
14. Remove all temporary connections and the DC power source and connect all voltage and current sensing wires to their proper place, as they were before. Protect them from corrosion.
15. Replace the fuse in the red power wire to turn the Link 20 on and check for proper operation.
That's it, you're done. It sounds complicated and perhaps a bit daunting, but in reality it only took me about an hour to do, including the mop up in steps 13 - 15. To repeat the calibration, you will need to repeat steps 1. to 12.
My Link 20 now works accurately, as per Xantrex's published specs. After recharging, the batteries now show a small positive Ah reading, which gets wiped off once the discharge begins.
Hope this helps someone, it certainly cost me a lot research and frustration to get there.