new solar install - Page 2 - Fiberglass RV

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Old 03-16-2012, 04:58 AM   #15
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Originally Posted by Jon Vermilye View Post
I feel it is wise to think about voltage drop differently when looking at recharging batteries, particularly from solar. ...........
Right. It took me a while to wrap my head around this, as we spend most of our lives thinking about sizing wires such that they don't overheat and start a fire. It really comes down to the voltage that the battery sees and how that voltage correlates to the battery charging rate.

I've mentioned this before, but I buy heavy wiring at my local metal recyclers (aka junk yard). Electricians sell the "short" ends off huge rolls for scrap, so the wire is new and may be 10 or 20 feet long. You can find any gauge you want within reason - I picked up some #6 to wire my TV for charging the trailer battery. The price by the pound is a fraction of retail.

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Old 03-16-2012, 05:18 AM   #16
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Nice neat install.

Norm, John,
Wire size is less critical when connecting panels in series and using an MPPT charge controller. The series connection means that you're feeding the charge controller with a higher voltage and lower current for a given wattage (power = voltage x current). Panels connected in parallel should use larger wire to the charge controller.

The wire size for the connections between the charge controller is very important. These wires should be as large and short as possible to minimize voltage drop. Many solar charge controllers provide staged charging for your battery bank, which extends the life of your batteries. The controller determines the state of the charge by reading the battery voltage and the difference between fully charged and 50% discharged is only about 0.5 volts. A tenth of a volt drop can easily affect when your charge controller switches to a float charge rate. Overcharging will boil away electrolite and destroy a battery just as easily as overly discharging.


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Old 03-16-2012, 08:26 AM   #17
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I carry distilled water on our trips, check my battery water monthly and see very little loss of battery water. I assume this means I'm not over charging.

Yesterday I measured the controler's input voltage, it was near 18 volts; the controller's output voltage was 13.4 volts (about the same as the battery) indicating no current flow.

This suggests that as the controller sees a high voltage at the battery, current flow is reduced, further suggesting that wire size is not important (to a limit) because when there's no current flow there's no significant voltage drop.

It seems the controler charges to 13 plus volts, not unlike my intelligent Converter. In the morning after running off the battery the battery voltage is typically around 12.6 volts.

I hope this makes sense and further hope it indicates that the panels and controller are working correctly.

I have my panels wired in parallel. When I started I had a non-MPPT controler.
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Old 03-16-2012, 03:23 PM   #18
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Jon, thats a great tip about going to a scrap yard to get cheap large gauge wire. Even at retail prices though large wire is the best investment you can make in your system. It also makes adding panels or batteries in the future much easier.

Norm, I highly recommend reading the link for Handy Bob that Jon posted in this thread. The Battery Charging Puzzle does a great job at explaining how less than half a volt can make a tremendous difference in system efficiency and how wire size plays a role.
Bob also shows how MPPT charge controllers are not cost effective on small systems because the controller costs much more than adding a panel and the boost that MPPT controllers can provide is only available under certain conditions.
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Old 03-16-2012, 07:17 PM   #19
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Handy Bob


I did read all of Handy Bob's post on Solar Panels and RV electronics.

I definitely agree that Trojan 105s are great batteries. My motorhomes 105's were 15 years old when I sold the rig and still going strong and, though he suggests other wise, were charged by an inverter with a charge wizard on it that only produced 14.1 volts.

My solar controller never outputs over 14 volts as far as I've seen but the battery, even after being used all night for lights and so on are still near 12.6 volts in the morning. I think they fully charge but I don't have a hydrometer to really check.

Though I don't have super large wire for the panels everything seems OK. Of course, our panels are a lot smaller than his by more than a factor of four.
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Old 03-16-2012, 09:51 PM   #20
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The peak voltage depends on your controller. My Go Power PWM-25 controller can be set for battery type. If it is set for flooded, it will produce 14.1V absorption charge when the batteries are low, 13.7V as a float charge once the batteries are fully charged, and go to 14.8V for a couple of hours every 28 days to equalize & desulfate the batteries.

Oh, by the way, getting cable from a scrap yard (as long as the insulation is in good shape) is a great idea, but it was Thomas G's idea, not mine...
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Old 03-25-2012, 12:53 PM   #21
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Fully Charged

I have read all of Handy Bobs report on solar panel and his reccomendations for large wire and the voltage required to fully charge a flooded wet cell battery.

I have a pair of 40 watt panels wired in parallel.

In my case I use 16 gauge wire for all my wiring and my solar controller never produces more than 13.9 volts as an output.

My battery fully charges usually by around noon time in this AZ sun. I measure it with a digital voltmeter that during sunlight hours measures around 13.6 volts.

Since Handy Bob said that a hydrometer is needed to measure the state of a battery's charge I bought one. The hydrometer shows it's fully charged.

I believe the 16 gauge wiring works in our case because 80 watt panels are relatively low current devices compared to Handy Bobs 375 watt panels.

On a seperate topic we have now completed over 5000 miles of towing and almost a half year with our taped down flexible solar panels without any issues.

Norm and Ginny

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