Common negative wire for solar panels?

Rick D · Oct 24, 2017

galynd said:
I currently have 2 - 100 watt solar panels connected to a 20 amp mppt controller. I'm planning to add two more 100 watt panels and another 20 amp mppt controller. Can I use a common negative cable for the two set ups? My thought is to run a Pos (+) cable from the new panels to the new controller, connect the neg's at the panels, then run a Neg cable from the older mppt controller, where the neg cable is connected, to the new. Would this work?
My reason: less intrusive installation through cabin top.

Thanks,
Greg.

Well, I did. In fact, I had seven neg & pos together before the controller. Ran it like that for nine years. Don't know why it's bad or good, just my way.

jeepbluetj · Oct 24, 2017

Maybe. Maybe not. I'll outline a few reasons why not, if none apply, then maybe.

1. Some/many/most? MPPT controllers do not have - input connected to anything other than the input stages of the controller. I.e. it 'floats' - not connected to the negative bus in the boat. These controllers definitely do not want anything else connected to the negative input other than the negative side of the panels that are connected to the positive. Midnight Solar is an example of this kind of controller. Bad things happen if the - input is connected to the - output for example.

2. A much more common issue - your wire is likely undersized. You need that single negative cable to handle 2x the current the positive cables are carrying. And since you've got 200w there now I'm guessin the wire was sized for that. Put 400w through it you'll end up with more voltage drop (power losses) and at worst case possible bad things that smell like smoke.

Paralleling all panels together (like RickD mentions) then running to the controller works fine, as long as the wires and the controller can handle the maximum current.

I'd just run another negative cable.

walt · Oct 24, 2017

Example of a possible issue.. The Genasun controllers connect the solar panel negative directly to the battery +

I.e, if have the Genasun controller running and the solar panel producing 18 volts, if you took a volt meter with the negative lead to the battery ground, you would measure the battery voltage on the solar panel negative and battery voltage plus 18 on the positive lead.

If your solar controllers were identical, you probably could get by with your common ground. If the controllers are different such as one that biases the panel negative at ground and a second that biases the panel negative at the battery voltage, all you are going to do is blow a fuse. No fuse and wires burn up.

Also, the point about running all the current on a single ground that was originally set up for half the current is important. You power losses in the wire by doubling the current will not just double but go up four times. That power loss directly correlates to a power loss in what is delivered to the batteries.

I would just run the extra ground wire from the new set of panels... Higher delivered power from the panels plus you dont have to worry about some screwy incompatibility.

Sumner · Oct 24, 2017

I agree with the others. Two controllers, two negative feeds and not a common feed from two different sets of panels that might be charging at different rates.

Sumner
==================================================================================
1300 miles to The Bahamas and Back in the Mac...
Endeavour 37 Mods...
MacGregor 26-S Mods...http://purplesagetradingpost.com/sumner/endeavour-main/endeavour-index.html
Mac Trips to Utah, Idaho, Wyoming, Canada, Florida, Bahamas

Brian D · Oct 24, 2017

Here is how I see this. If you had an Ohm Meter and tested for continuity between the input ground (where the negative wire from the solar panel enters) and the output ground (the point from the controller to the battery) lug you will find that they are indeed connected and not isolated. One cannot have separate grounds (in the controller) because that can cause a ground loop which would/could destroy the controller.

However, for the purpose of keeping everything in an order in which one can easily troubleshoot, it would be wise to treat each controller separately. Once all the grounds are connected to the batteries or distribution panel they will all get combined anyway. However, it is better to have them combined either at the panel or the batteries than to have separate ground points.

Now, you can bundle your panels if you want and run one pair of wires as long as the total power of the panels does not exceed the input power of the controller. Also, if you do that, make sure the wiring can handle this load and put fuses in line to protect the wiring.

JMHO

walt · Oct 24, 2017

If you had an Ohm Meter and tested for continuity between the input ground (where the negative wire from the solar panel enters) and the output ground (the point from the controller to the battery) lug you will find that they are indeed connected and not isolated.

FYI, I made a mistake in what I said about the Genasun controllers. The Positive of the solar panel is directly tied to the battery positive on the controller. The panel negative is NOT connected to the battery ground terminal. If you measured the voltage while the Genasun is running (MPPT), the solar negative side would likely measure around -5 volts. Genasun likely did this to simplify the circuit part count / cost. The quote above is NOT true for a Genasun controller. But the quote is often true. For example, it is true for a Victron MPPT controller I have. You just can not assume its always true.

Sumner · Oct 24, 2017

I have different size Blue Sky MPPT controllers on both boats and both sets of instructions warn you to NOT connect the panel negative to the battery negative on the controller. They mention that the controller won't work properly. The connection needs to be made by the controller internally. There are one set of pos/neg posts on the controller for the solar array's output and another set for the connection to the battery.

I also have an additional 12/24 volt Morning Star controller on the Mac and it is wired the same as the two Blue Sky controllers. It is wired to two 12 volt batteries in series (24 volts).

Follow the instructions of your controller, since they don't all seem to be the same,

Sumner
=======================================================================
1300 miles to The Bahamas and Back in the Mac...
Endeavour 37 Mods...
MacGregor 26-S Mods...
Mac Trips to Utah, Idaho, Wyoming, Canada, Florida, Bahamas