# Accurate measurement with different ground reference

#### atanumukerji

Joined Apr 1, 2011
21
I have a system which measures the power generated by a solar panel, amount of power being fed into the battery through the charge controller and then the powersupplied to the load. In order to keep the current from the solar panels separate from the current being fed into the battery to charge it, I have three separate current loops - 1. for the solar current 2. for the battery current 3. for the load current.

The load and battery parameters - voltage and current being measured are referenced to the battery ground. However the solar voltage cannot be referenced to the battery ground since there is a charge controller is between that is a bought out device. Hence I am not sure of the ground connections. So essentially if I measure the solar panel voltage referenced to the negative input of the charge controller or the negative terminal of the solar panel, then how do I make sure that I can compare the voltage levels across the panel, battery and load.

Any suggestions?

#### crutschow

Joined Mar 14, 2008
25,980
Use a multimeter to check the resistance between the controller input and output grounds to see if they are isolated.
If they are, then you will need a differential amplifier to measure the panel voltage.

#### nsaspook

Joined Aug 27, 2009
7,850
The vast majority of charge controllers are non-isolated (meaning that the minus of the PV input is at the same potential as the minus of the battery output) have a DC ground circuit conductor (usually the negative) from solar panel input to battery charge output so all measurement points are at the same ground reference.

#### atanumukerji

Joined Apr 1, 2011
21
Use a multimeter to check the resistance between the controller input and output grounds to see if they are isolated.
If they are, then you will need a differential amplifier to measure the panel voltage.
Thank you crutshow, will check.

#### atanumukerji

Joined Apr 1, 2011
21
The vast majority of charge controllers are non-isolated (meaning that the minus of the PV input is at the same potential as the minus of the battery output) have a DC ground circuit conductor (usually the negative) from solar panel input to battery charge output so all measurement points are at the same ground reference.
So nsaspook what you suggesting is that I could use the same ground reference for all voltages. I will try using this method and see what results I get. Will cross-check with a multimeter and verify. Suggestions are appreciated. Thank you

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,369
Sometimes, you can do a voltage to current converter e.g. 0-20mA or 4-20mA as long as there is a ground reference that isn't too far off or varies all over the place.

one hat would vary all over the place is seven programmable power supplies operating at different voltages that us the (+) binding post as a reference for in and out.

With the 0-20mA, you use a 250 ohm resistor to convert to 0-5V locally.

#### nsaspook

Joined Aug 27, 2009
7,850
So nsaspook what you suggesting is that I could use the same ground reference for all voltages. I will try using this method and see what results I get. Will cross-check with a multimeter and verify. Suggestions are appreciated. Thank you
I normally use the DC negative to earth/ground tie point near or at the charge controller as the ground reference for voltage measurements. Usually the most sensitive operational measurement is battery current and voltage so the reference point should be selected to optimize those readings. If your system has sufficient precision and accuracy you should be able to easily see wiring related voltage drops in the tens of millivolts.

#### Lundwall_Paul

Joined Oct 18, 2011
227
Use a multimeter to check the resistance between the controller input and output grounds to see if they are isolated.
If they are, then you will need a differential amplifier to measure the panel voltage.
Yep. and if the are isolate, keep them that way. Tying them together will cause more problem.

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,369
There is always, the pseudo-differential technique. If they share a common somewhere, You measure A-C and B-C and add, assuming C is the lowest value.