I must apologize, I have two DC-DC converters and I've measured their efficiency at 83-87%, I use them on a 24v system.. The 93-97% for the MPPT is a whole new way to look at it. At that efficiency using the 24v Panel to charge a 12v battery does has some merit.
On Mon, May 18, 2015 at 9:31 PM, Carter Quillen twowheelinguy@yahoo.com [electricboats] <electricboats@yahoogroups.com> wrote:
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. CarterOn 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. CarterOn 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-sheetNote 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
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Posted by: Kenneth Reese <kcr@kcrproducts.com>
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