class for solar

[John Hilley]

We are actually talking about two wire runs. The run from the panels is more like 18 VDC, from the controller to the batteries is the 14.8 VDC run and usually only 3 or 4 ft, so there isn't any reason to go with 10 ga there. So in actually the voltage drop between the controller and batteries is very low.

[Dave78Chief]

When I recommended 6 to 8 guage wire it was based on panel to controller wire sizing calculators from several solar sites:
Wire sizing calculator for Solar Panel Arrays
Solar Wiring Size calculator
Wire Size Calculator | SolarDropShipping.com

Using the values
Volts: 12VDC (a 12 VDC system using max 18VDC panels)
Amps: 20 amps
Length: 20ft

I feel the solar community is using these calculators to pass on information they have learned the hard way by recommending you use the 3% maximum loss specification. In effect, I also feel that is what HandyBob is trying to convey also.

My 4 gauge recommendation was based on the need to ensure that the controller see's the actual battery voltage. If using a controller that can monitor true battery voltage, the 6 gauge wire (0.1VDC loss @ 4ft for 20amps) would be acceptable. Better controllers do monitor the battery voltage seperately.

Here is another handy wire gauge calculator (not solar oriented) which, when used from the perspective you want 3% or less voltage drop, still supports what I recommended.
Wiring Size Guide

I suspect the confusion comes from whether you feel 5% maximum loss is acceptable or do you want 3% maximum cable loss. It is that differencs that I feel is driving the questions. I feel that for solar you have to work at the more stringent specification due to losses from daylight length and effects nature imposes (clouds, angle of sun, etc.). Nothing is free and I am only recommending you look at and consider all aspects for this topic in order to maximize your investment. Remember, at 12VDC the impacts of just 1/10 of a volt have more of an effect. 12VDC battery working range (100% to 50% SOC) is only 0.6VDC (12.6VDC to 12.0VDC). In order to overcome self discharge you have to have a float voltage of 13.2VDC. Gassing voltage is around 14.4 volts. That range is only 1.2VDC. Again, not much of a window voltage wise. So, effective management of voltgae losses is more on a concern when wanting to extract maximum energy from a solar setup.

Dave

[bluepill]

Good discussion, good thoughts. Thanks.

[Perry White]

I must be nuts, but isn't solar just a big battery charger? I think you guys are nuts with the wire sizes. Just my opinion.

[Dave78Chief]

LOL - Yes, it is another form of charger. Problem is that you do not have the power of edison (generator/shore power) behind it to prop it up therfore you have to approach things a little differently so you can tease (ok squeeze) the energy out of the PV array. We get spoiled by edison until we suffer something like a brownout (low voltage) which is something solar is always fighting.

Dave

[Grog]

Can someone point me to Handy Bobs thread on the solar Charging? I am new to the forum and new to the RV way of life but I am interested in installing a good solar system.
Thanks

[Dave78Chief]

Here is Handy Bob's web site: The RV Battery Charging Puzzle « HandyBob's Blog

Here is another web site of a person (ex-installer) with simular thoughts about how to most effectively put together a system: RV Electrical

Dave

[John Hilley]

Has anyone installed the 18 volt panels in parallel and then changed them to series to see if there was a difference in charging?

[Dunner]

Good info.

Subscribed.

[bluepill]

Originally Posted by John Hilley

Has anyone installed the 18 volt panels in parallel and then changed them to series to see if there was a difference in charging?

I think that series connections producing 36 volts would be appropriate only for 24 volt systems. Unless MPPT technology can recover that much extra voltage for a 12 volt system, and I don't recall seeing any for that application.

Anyone else?

[John Hilley]

Originally Posted by bluepill

I think that series connections producing 36 volts would be appropriate only for 24 volt systems. Unless MPPT technology can recover that much extra voltage for a 12 volt system, and I don't recall seeing any for that application.

Anyone else?

Here is a quote from the Morningstar Tristar MPPT controller

"High Voltage Strings and Grid-tie Modules
Another benefi t of TrakStarTM MPPT technology is the ability to charge batteries with solar arrays
of higher nominal voltages. For example, a 12 Volt battery bank may be charged with a 12-, 24-,
36-, or 48-Volt nominal off-grid solar array. Grid-tie solar modules may also be used as long as
the solar array open circuit voltage (Voc) rating will not exceed the TriStar MPPT 150 Volt maximum
input voltage rating at worst-case (coldest) module temperature. The solar module documentation
should provide Voc vs. temperature data.
Higher solar input voltage results in lower solar input current for a given input power. High voltage
solar input strings allow for smaller gauge solar wiring. This is especially helpful and economical
for systems with long wiring runs between the controller and the solar array."

[Dave78Chief]

First let me point out that PWM controllers are typically limited to the array size and system they are designed for (12, 24, 48). It is the MPPT style controller that typically is designed to be used with higher voltage array on a 12VDC battery system. It is a function of the MPPT DC-DC Power Supply design that allows the current array power (measured in watts) to be converted to a different voltage level at nearly (small efficiency loss) the same power.

Allowable array voltage is dependent on the given MPPT controller. As Mr. Hilly indicated the Morningstar MPPT Tristar (Morningstar Corporation » TriStar MPPT) can accept a 12, 24, or 48 VDC array input.

The Rogue MPT-3024 (Rogue Power Technologies) can accept a max 24VDC array (60 VOC).

Schneider (formally Xantrex), "XW" series 865-1030-1 (Renewable Energies - Schneider Electric) can accept 12/24/48 Volt arrays (150 VOC). "C" Series Xantrex controllers are PWM based.

As you can see there are controllers available but you have to read the specs for the one you are considering.

Using a higher voltage array allows you to reduce the array to controller wire size. The controller to battery wire size would still have to be larger because it is still based at 12VDC. Even though you can reduce the size of the array to controller wire size at higher voltages, in order to use all the power available you should ensure the wire size used still meets the 3% rating for the array voltage selected (http://www.freesunpower.com/wire_calc.php). Voltage drop do to the resistance (thermal radiation) of the wire = lost power available to the controller. Even a MPPT controller still as to follow the rule of max watts out cannot be any more than max watts in. Even then, it is not perfect (efficiency losses)

Dave

[John Hilley]

Morningstar lists 125 VDC as the array voltage for both the PWM and MPPT controllers. In the PWM controller manual they even mention 36 VDC in and 12 volt charging voltage. This would make it easy to compare series or parallel array connection of the Kyocera panels I have. When I get to Quartzsite I will give this a try. I will have to get one more panel though as I only have 3 135 watt panels.

[KJINTF]

John

I'm curious and looking forward to what you find after you get the 4th panel installed.

I currently have two of the 135W Kyocera panels in series and have seen as high as 40Vdc at the input to the Tristar MPPT 60. This winter I will be installing two more panels hooking them in as a series pair to the original two sticking with a max of about 40Vdc.

Do you have the remote control panel? lots of good status
Have you connected your PC and used the MsView / MsLoad applications, if not suggest you give it a try.