Community Forums

woodly1069 · 02-18-2011, 02:40 PM

Since we know that running a higher voltage system is the way to go for ease of installation, initial cost and overall efficiency how do you know when you have gone far enough? I guess I am curious to know how many folks out there are using 48V and up? Is this something even worth considering or do we usually stop there?

BryanL · 02-18-2011, 04:41 PM

A key issue is the charge controller. For this sort of voltage conversion, it has to be an MPPT controller and the maximum input voltage is limited.

Another factor is that of diminishing returns. Many tend to oversize wiring as it is (larger is better, right?) when it comes to cost efficiency concerns. The wiring power loss difference between 10 and 20 amps with, say, 10 gauge wire starts to fade into losses from connections and other sources - and that is for peak power for a 500 watt solar array, which is rather large for most RV's. Average power is a lot less and, therefore, so are resistance losses.

There are also shock and short hazards.

Then you get into the cell arrangements and considerations for shading and other such esoterica. It can be fun!

Ed-Sommers · 02-19-2011, 11:41 AM

There is some evidence that having incoming voltage higher than battery voltage gives the whole MPPT conversion principal maximum benefit.

I have 24 in and out. Have been meaning to increase incoming but have not gotten around to it. When I do I'm going to 48 in, 24 out. I don't really need maximum efficiently as I've got close to 1200 watts on the roof, but I like to tinker.

Ed

ralper · 02-20-2011, 03:35 PM

ED,
Your wattage is the same no matter what voltage you use. Higher voltage is only good if your wire is grossly undersized. The higher voltage only has to be lowered back down in your controler and so some efficiency is lost there.

w6pea · 02-20-2011, 06:00 PM

I have a system from http://www.amsolar.com, I have 3-100w. panels and I added 6 Interstate U2200, 220ah 6v.battery's . I am looking to add three more panels and maybe 2 more battery's when I need to replace the battery's I have now.

Midniteoyl · 02-20-2011, 06:21 PM

Quote:

Originally Posted by ralper

ED,
The higher voltage only has to be lowered back down in your controler and so some efficiency is lost there.

yep, but as BryanL was trying to say, I believe, is that the higher voltage will make all the connections more efficient (to a point) due to having to pass less amps...

TechWriter · 02-23-2011, 01:22 PM

Quote:

Originally Posted by Midniteoyl

yep, but as BryanL was trying to say, I believe, is that the higher voltage will make all the connections more efficient (to a point) due to having to pass less amps...

It's also the charge controller efficiency. For example, the Apollo T80 charge controller manual says:

"While it is possible to input 72 Vdc and output 12 Vdc, it is not the most efficient configuration for the controller. A system which had 36 Vdc input and 12 Vdc output would run more efficiently from the T80HV’s perspective. The most efficient configuration is with 60 Vdc input and 48 Vdc output."

Engineer Guy · 02-23-2011, 10:07 PM

As an 'end point' to the discussion, PV Panels for Residential Roof installs going up all around me use standard Panels connected in series. Wiring for these ~2 kW -> 10 kW 'high' voltage/low amperage Systems is barely the size of a #2 Pencil diameter. Xantrex, Frontius and other Controllers are spec'd to handle inputs of up to 600 VDC to handle this very-standard wiring protocol. However, Installers I've talked to stay a conservative bit below that upper Voltage limit when determining how many Panels total to connect in a series string. Edge effects on Clouds and Snow bounce effects can cause even-higher spiked voltage outputs from Panels, and they don't want to blow the Controller input stage.

See the 600 VDC series connection Spec on a nice Sharp PV Module linked below. The other pix are from a ~1/2 megaWatt install near me. The Panels are flat North - South. Large, geared, 90 degree Drives rotate them from East to West to follow the Sun's transit. The red Vehicle-type Shock Absorber dampens movement.

In that a RV Rooftop full of PV Panels connected in series would generate some sobering voltage - say, 6 Panels in series @ ~40 VDC each - there's no argument from the Peanut Gallery here that parallel Panel connections using 'large' AWG Wire is the safest way to go. Note, however, that there are substantial I/O voltage differentials in these very efficient Systems.

Solar PV Panel Pix

ralper · 03-30-2011, 04:56 PM

The rule of thumb is to have 1 battery for each 100 watts of solar panels.

wa8yxm · 03-30-2011, 08:38 PM

First, you almost never get the full output on a solar system.. I would think however, that anything over 24 volts at the panel is just plain wasting sunlight.

srzak · 07-25-2011, 05:57 PM

I new to the solar world, and would like to get a complete system, my first question is attaching to the roof? leaks? promblems at all, next finding a person or company that does a good job, and then the cost , i was thinking 4 100 watt panels, new batteries, and can it be set up to also charge the engine battery when in storage, thanks for any tips, i live in northern calif near sacramento. steve

c4b2on · 12-04-2011, 11:08 AM

Quote:

Originally Posted by ralper

The rule of thumb is to have 1 battery for each 100 watts of solar panels.

The thing about "rule of thumb" it depends on the size of the thumb.
I have 400 watts and one 245 AH battery that weighs 158 lbs. four of those would take most of the allowed weight in some RV's.
Mickey

kketterling · 12-04-2011, 11:25 AM

w6pea: We currently have 8 coach batteries and 6 solar panels each are 135 watts and it works great. If we have sun when dry camping we don't need to run the generator and can power everything in the coach. You won't regret adding to your system.

BryanL · 12-04-2011, 12:29 PM

re: "rule of thumb"

a more refined rule of thumb for those who just can't stand it and need to find fault is that you need a minimum of one watt of solar per pound of battery.

They typical RV battery that most folks use weights about 60 - 70 pounds so a typical 100 watt solar panel for that battery is about right.

What are the criteria behind this rule of thumb?

First is the energy density by weight of lead acid batteries. This comes down to about 12 usable (50% SoC DoD) watt hours per pound.

Second is the typical RV battery bank reserve rule of thumb - you need sufficient battery to last 2 or 3 days without recharging in a nominal situation.

Third is a charge rate. You aren't going to get a proper vigorous charge rate from an RV solar system but you should be able to get something more than just a maintenance charge.

Watch out for claims. Off grid RV energy availability is quite constrained. That means adjustments and accommodations must be made. You can either trim your energy use, use a genset, or use more dense energy forms such as propane. What anyone chooses to do to get what they want is going to depend upon their particular tastes and capabilities.

Just think, those guys in their Conestoga RV a hundred years ago spent a couple of months off grid with no solar, no propane, and missing many other conveniences and did just fine, thank you. That doesn't mean you should use that experience as a model.