T-Pain Is On A Boat: Re: [Electric Boats] Re: Solar charging question: Panel voltage vs. battery bank voltage?

Tuesday, May 19, 2015

Re: [Electric Boats] Re: Solar charging question: Panel voltage vs. battery bank voltage?

Jerry.

Lucky you, property on an island in St. Thomas!

Most of the panels are rated to be strung together up to 600V max so wiring them in series to get 48V should not be a problem but I would stick to only stringing identical panels together.

Not sure but you might have some issues with a string using different sizes and manufacturers.

My panels on the Arc are from several different suppliers with slightly varying specs but all the series connections are only with identical panels and then I ganged them all together in parrallel to feed the charge controller. This has worked very well for me.

Like you said, it probably doesn't matter too much for the system you have described. I don't know for sure but my guess has always been that the farther you drop the voltage with your mppt charge controller, the greater the losses you might have in the conversion. So as long as your wire runs aren't too long, I think you're better off keeping the input voltage as close as you can to the battery bank voltage.

The other consideration however is the ampacity of the controller. You can control more Watts of input at higher voltages so the slight loss in efficiency caused by raising the voltage might be well worth the trade off if it would require another charge controller to achieve the desired capacity at a lower voltage.

P.S. Beware of going too far "off topic", the thought police are watching, lol.

On Tuesday, May 19, 2015 10:25 AM, "'Jerry Barth' shredderf16@sbcglobal.net [electricboats]" <electricboats@yahoogroups.com> wrote:

Capt Carter,

This is a little off topic as it doesn't involve electric boats, but it is near the water! I've got some property on an island in St Thomas harbour. In the future we're going to put up a prefab round house on the property. In the meantime, we're camping there for a week or so at a time. We've been using a goal zero camping battery/solar panel setup. We need more power, so I figured I'd do a homebrew system to leave there. Shipping to there is expensive, so anything we take needs to fit checked baggage limits. So that limits me to at the most 100w panels, although 50w would be easier. I'd like to eventually have about 400w of panels total, with two Trojan golf cart batteries (we can buy those there at reasonable cost). The goal is to run a small chest freezer with a beer brewing thermostat to make it into a refrigerator. Let's say that I figure out how to get the 100w panels out there, would I wire two strings of two in series for 24v each string? The wiring runs would be minimal since I can put the panels almost right next to the controller and batteries, although from what I've been reading if you are forced to make runs of over 10 feet or so the voltage drop forces some fairly large wires, so I agree with you the higher voltage the better. Is it possible to wire them all in series for 48 volts? I'm just wondering what the best way to do it is.

Thanks,

Jerry Barth

From: electricboats@yahoogroups.com [mailto: electricboats@yahoogroups.com ]
Sent: Monday, May 18, 2015 8:32 PM
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Re: Solar charging question: Panel voltage vs. battery bank voltage?

Kenneth,

The mppt (maximum power point tracking) charge controller will provide the maximum possible output of the solar panel by regulating the power to batteries at the optimum voltage for charging, thus achieving the maximum amperage.

So basically if you have a 100W, 24V panel it will regulate the output to 12V and still give you 100W, less a few percent conversion loss, or approximately 8 amps. Sounds a bit like magic but it really works.

Also consider that your typical 12V panel puts out about 18Volts in the full sun. When you hook it up to a 12 volt panel with a pwm or other type of controller it will be forced to operate at 12V. If you look at the power curve for the panel, you will see that 18V is NOT the maximum power point and you are not getting all the energy that the panel can produce. So even using a nominal 12V panel on a 12V battery, an mppt charge controller will give you up to 30% more power from your solar collector.

BTW, if you're feeding your electric motor directly with solar, the mppt charge controller will also dial in the optimum voltage that the motor needs and give you 30% more power too. It really does work, I've cruised almost 2000 miles on solar energy using it. MPPT controllers are well worth the extra expense.

Capt. Carter

www.archemedesproject.blogspot.com

On Monday, May 18, 2015 7:45 PM, "Kenneth Reese kcr@kcrproducts.com [electricboats]" < electricboats@yahoogroups.com > wrote:

Two 12v 50 watt solar panels... You are actually advising someone to charge a 12v battery with 24v x 4 amp (panels in series) vs using a 12v x 8 amp (panel in parallel) ?

On Mon, May 18, 2015 at 5:54 PM, Carter Quillen twowheelinguy@yahoo.com [electricboats] <electricboats@yahoogroups.com> wrote:

Kenneth is right if you don't use an mppt charge controller but dead wrong if you do. There are several good reasons for going with a nominal 24V panel and an mppt charge controller.

First, 24V panels are much cheaper. You can find them for as low as 50 cents a Watt as compared to 12V panels that range from $1-$3 per Watt.

Two, the higher voltage lets you use smaller wires.

Three, an mppt charge controller will get you up to 30% more power from your panels than a pwm controller.

The only downside to going with the standard 24V module is that they don't come in small sizes. They are all about 42"x56" or larger and can be a little too big to find a convenient mounting location on smaller boats. But if you've got the room to mount them, they are definitely the most economical way to go, even with the added cost of the mppt charge controller.

Capt. Carter

www.archemedesproject.blogspot.com

On Monday, May 18, 2015 10:15 AM, fitloose <no_reply@yahoogroups.com> wrote:

Well I'm afraid I must disagree with Kenneth. Using a 24V panel to charge 12v has benefits providing you are using a suitable MPPT solar charge controller. The easy way to see this in action is to experiment with this spreadsheet:

http://www.victronenergy.com/support-and-downloads/software#mppt-calculator-excel-sheet

Note you can also add panels of your own choice in the custom field. Higher voltages will be more efficient in terms of losses, the higher voltage also means you will also start charging earlier in the morning and continue later in the evening. For more information about matching panels to MPPTs see:

http://www.victronenergy.com/blog/2014/03/28/matching-victron-energy-solar-modules-to-the-new-mppt-charge-regulators/

If I had 12V batteries and say 2 x 12V panels of 50 Watts each I'd put them in series so 24V nominal @ 50 Watts instead of 12V nominal @ 100 Watts in parallel. Whilst the wattage for both (Volts x Amps = Watts ) is the same for both the ones in series have less losses due higher voltage and less current. Try it in the sheet. Note most controllers will need to be +5V over battery voltage to start charging. That decides the number of cells you need in a panel to be sure you have a suitable voltage in the first place. Another reason to use higher voltages, plus the charge earlier, charge later i.e charge longer times overall as mentioned before.

Personally I found the sheet a very useful tool, along with the graphs, to see what is happening.

John