DC Voltage loss

[Shadow Catcher]

This is also posted on one of the solar forums, but I think we have more "understandable" folks here

OK I am having real difficulty in understanding the "voltage loss" question. I am aware that DC differs from AC thank you Mr. Tesla, but the only apparent difference (limited understanding) is the ability to step up/down the voltage easily through a transformer.
The problem with voltage loss as I currently understand it (subject to change with more information) is having enough voltage to charge a battery at 14. what ever V necessary for that battery, requires that you have enough more than that so that any loss is less than that which allows 14.4V at the battery terminals.
Given the voltage from the panel(s) is in my case 43 volts under load and the controller can handle that, Morningstar MPPT can, and that the output can be regulated to match the voltage the Concord AGM battery requires at the battery terminals adjusting for the voltage loss in the wire from the charge controller.
Is there a reason to use more than a 10ga wire?

[jimqpublic]

I think 10 gauge would be fine.

Where you see voltage drop issues is with long runs where the source voltage is close to the battery's resting voltage. For instance a 20' cable run from a single-phase RV converter at 13.5 volts to a battery at 12.8 volts. The voltage differential is essentially the "pressure" available to charge the battery. Try to push many amps through a thin wire and you end up not getting much charge current.

Once the MPPT voltage regulators came on scene the obvious choice was to boost panel voltage way up.

With your system you have a huge voltage difference so not a big issue. Figuring 10 amps and 20' of cable you're still under 1% drop.

The only concern is in the run from MPPT to battery. Keep it short and appropriately heavy.

[S. Heisley]

I can think of one good reason (but it is not because of voltage loss) and that is if your converter stipulates it...: preservation of converter warranty.

[Engineer Guy]

'Handy Bob' has some great Solar/wiring insights between the rants. He's gained a lot of believers. See his writeups linked from here:

http://www.irv2.com/forums/f93/solar-that-really-works-66571.html

My take away was:

1. Measure and control Voltages at the Battery, and ensure they're set appropriately for the Battery being used.

2. Use 'large' gauge Wire from the Panels.

3. Avoid even minor shadows on the Panels.

I've used large, parallel wire runs at Research Facilities to 'beat' ordinary resistance loss limitations. For example, 2 parallel runs of #10 ga. come very close to being the same ohmage/run as 1 #7 gauge. That is, parallel wire runs drop ~3 wire gauge sizes. But, #10 is easier to handle. See the 'Ohms per 1,000 ft.' here:

http://www.powerstream.com/Wire_Size.htm

My preference was to clean up all connections, then crimp. Flux, then Silver Solder with a Plumbing Torch on low flame. Vise Grips can be clamped on as a Heat Sink to protect Insulation, or heat sensitive 'stuff' near the soldering region. Clean off Flux with Isopropyl Alcohol. Install appropriate Fuses.

[Shadow Catcher]

In reality I will use 6ga wire, but I am trying to fully understand and so far it aint happening. I need to know why something works as it does. My suspicion is that there is a certain rather high level of the end product of hay through a male bovine in the solar community/business or myth and magic and that is frustrating me no end.

Sharon the Morningstar controller has a great deal of flexibility and can monitor performance and is programmable using a computer interface. The controller will take over voltage with out harm and regulate output.

EG I am aware of Handy Bobs Blog and have read it a number of times but there are interplays using higher voltage panels and MPPT controllers that he does not cover, plus he seems to exceed the theoretical needs on wire size.

[Engineer Guy]

SC ~

I got into this whole topic via reading up what it would take to go off Grid in our new Solar House. Fer now, we're on Grid. I'll put up some PV Panels and sell back to the local Utility when a pal's Company hits the magic number of '$1-/Watt' for generating PV power, as they believe they're about to do. That's all on the QT for now. I did not want my Wife to someday hassle with Battery maintenance and watching current drawdown, etc.; stuff I could do in my sleep.

This Site below had some good Science on it re: wire size implications. The Solar 'Bovine byproduct' you allude to is sometimes the practical [vs. theoretical] truth, but that's due to the higher power levels large Solar applications deal with. At those higher power levels, just a little bit of incremental resistance, from wire size or corrosion, matters a lot.

http://www.bcae1.com/wire.htm

One fellow, whose U.K. Site I cannot find now that I'd like to post it, made a good, Math-based case for even having equal length Cables between multiple Battery installations. At first, I thought this was all over-the-top thinking too, but small Resistances can matter when trying to equalize larger charge levels amongst Batteries. In his layouts, Cables came in between 2 Batteries, and was then jumpered by equal length Cables to each + Terminal, and so on. He made a believer out of this skeptic, since his Math was impeccable.

I thought you were referring to Handy Bob's Posts prior, but he is one 'end point' to the extreme of the discussion in which the 'correct Wire size' truth for you likely lies somewhere in the middle. I'll post insightful Links as I run across them from my extensive Archives. The good news for you is that you're working at 'higher' voltage, so current is inversely less. Current is what causes power loss [concept simplified], and was the heart of the Tesla vs. Edison debate last century [power loss over transmission/distribution distance]. This is why Hydroelectric power output is stepped up to higher voltage, despite Transformer losses, to cross the Desert from Hoover Dam to L.A..

[Shadow Catcher]

I have a section of line from Hoover and it is hollow. I do not know if it had something inside but it lead to my thinking concerning flux fields.
I am familiar with magnetic flux theory and in an AC generated flux field it is for all intents on the surface of the conductor. A DC field however is to some extent below the surface.
The electrical theory exceeds my level of incompetence with math

[fromeo]

I have a section of line from Hoover and it is hollow. I do not know if it had something inside but it lead to my thinking concerning flux fields.
I am familiar with magnetic flux theory and in an AC generated flux field it is for all intents on the surface of the conductor. A DC field however is to some extent below the surface.
The electrical theory exceeds my level of incompetence with math

SC, can you give us some idea of what your system will look like? As in wire distance from the panels to the morningstar, and then to the batteries? With that information and a chart like EG linked above, it's pretty easy to run some numbers and see the difference made by various wire combinations. If you provide the numbers, I'll take a stab at a drawing and some figures.

- Frank

[Engineer Guy]

Here's some resources to distill ideas from. One can always simplify a complex RV System; no need to copy it exactly, of course...

http://www.happy-wanderers.com/technical-articles/109-rv-solar-system

http://www.rvstuff.ca/battery.html

http://www.backwoodssolar.com/catalog/batteries.htm

http://www.backwoodshome.com/articles2/yago108.html

http://www.phrannie.org/battery.html

[Shadow Catcher]

The 180W solar panel will go to the rear and the waterproof connector comes down through the roof and between the rear wall and the rear cabinet.


The Battery was supposed to be in the rear however I think it ended up in front due to space considerations, and will end up in front of the platform or inside the tongue box which will be on the platform.

The Sunsei controller is already installed inside

this will be replaced with the Morningstar meter. The lower cabinet seen in this picture is probably the best location for the MPPT controller despite the long run (12' to14') to the battery.

[bdosborn]

I have a section of line from Hoover and it is hollow. I do not know if it had something inside but it lead to my thinking concerning flux fields.
I am familiar with magnetic flux theory and in an AC generated flux field it is for all intents on the surface of the conductor. A DC field however is to some extent below the surface.
The electrical theory exceeds my level of incompetence with math

I think you're talking about skin effect. However, there is no skin effect with DC so its not a concern in this instance. I've noticed that some guys go absolutely bonkers over voltage drop and oversize conductors a ridiculous amount. Rather than guess on the wire size or oversize like crazy, you can use a voltage drop calculator to figure out what size you need. 3% or less is typically considered acceptable.

http://www.nooutage.com/vdrop.htm

You can minimize your voltage drop by locating your MPPT controller as close to the battery as possible. That way the longest wire run is at the higher panel voltage. It doesn't help with a PWM as the panel runs at the same voltage as the battery. However, the controller needs to have an accurate reading of the battery voltage for the best charging results so its always a good idea to locate the controller close to the battery.


Bruce

[Shadow Catcher]

Bruce
I really do not want to have the controller in the tongue box if I can avoid it. With the Moringstar I can get the correct voltage at the battery by programing the controller.

[bdosborn]

I've tweaked mine a little and I know you can program the battery charge voltage but I didn't think you could adjust what voltage the controller is reading at the battery. But that's not a big of a deal, just make sure you size the wire between the controller and the battery for a minimum of voltage drop.

180W/14.3V=12.5 amps, probably the most current you'll realistically see from your controller.

That's a 3.3% voltage drop over 14' on a #10 awg. It's probably fine but I'd be tempted to go to a #8 awg as that would be a 2% drop. I've got my controller located about 8' from the battery with a 2% voltage drop and it works well.

Bruce