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Old 27-01-2010, 18:57   #1
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24vdc Solar to 12vdc Battery Bank

It seems that with 24v solar panels you can get much higher wattage ratings then with 12v. My system is 12v so I would need to convert as well control the charge if I went with 24v. Are there arguments for or against this I should be aware of? I presume the conversion occurs before the controller?
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Old 27-01-2010, 19:28   #2
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Just use MPPT controller. Conversion from higher to nominal voltage is its main job.
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Old 27-01-2010, 20:10   #3
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If you use a conventional controller you will get very poor results, as BambooSailor says, use an MPPT controller.
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Old 27-01-2010, 20:28   #4
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Even with so called 12 volt panels, a MPPT controller will help.

I'm in the middle of adding an MPPT controller and a 120 watt panel to my 2 existing 32 watt flexible panels.

So far, I have added the MPPT controller (I don't live aboard) and even though it's January, the MPPT Bluesky controller goes into the MPPT phase during the day.

I just got the 12 volt 120 watt panel mounted this afternoon, and will get it hooked up in a day or so. I'll post the results.
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Old 28-01-2010, 00:32   #5
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brand recommendation

Is there a particular brand anyone would recommend?
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Old 28-01-2010, 03:45   #6
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Quote:
Originally Posted by hummingway View Post
It seems that with 24v solar panels you can get much higher wattage ratings then with 12v.
Is that due to the cable cross-section issue?
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Old 28-01-2010, 06:24   #7
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Blue Sky Energy has some of the more inexpensive controllers but what you will find is that you loose features and capacity as you go cheaper. The less expensive regulators may have only a two stage charging process instead of a three stage, may not have different settings for different battery types and have more limited amperage capabilities. Without knowing the design of your system including batteries it's really hard to tell you what the best fit is for your installation. There are an number of existing threads discussing various MPPT controllers that you should read through.

Idpnd, No it does not have to do with the cable cross section, but the surface area, number, and arrangement of the cells.
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Old 29-01-2010, 22:30   #8
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Is there a particular brand anyone would recommend?
Outback and Bluesky both have comparable controllers at the same price point when it comes to high-amperage MPPT controllers. Personally I'll go outback when I build out my system the next few months - the price is the same, but I think outback is better with more features at the same input maximums (60amps).
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Old 30-01-2010, 01:19   #9
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I have Outback (MX-60, though there is now a newer model) and the system integration features are excellent... solar MPPT controllers, inverter-chargers, and monitors all plug into an Ethernet hub and talk to a single display (which also has a serial output for datalogging).

I have no experience with Bluesky, so can't compare them. But Outback makes solid stuff...

Cheers,
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Old 30-01-2010, 11:47   #10
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" the MPPT Bluesky controller goes into the MPPT phase during the day."
MPPT controllers in general, and the BS for sure, don't "go into" a special phase at all. They are not 3-stage or 4-stage, etc. chargers or regulators. Rather, the BS controller is programmed with your battery type, voltage and capacity, and then the controller looks at the system and sets the *voltage* level slightly above the battery voltage, converting the power from the solar panel into the maximum amperage that can be accepted for that battery voltage state.
It is a clever and efficent way to do things, but anyone expecting a traditional acceptance-bulk-float charge voltage from an MPPT controller will be baffled by how it actually works. The output voltage will almost always seem terribly low--until the battery is nearly fully charged--but that's actually a better way to do it.
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Old 30-01-2010, 11:56   #11
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An additional thought when purchasing the controller. Use the manufacture recommendations for sizing (controller and wiring)and then look to future expansion. If there is room to expand the array look for a controller that will take more power added. This way it will be easier to add panels without having to rebuild your system and throw away a good controller and wiring just to upsize.
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Old 30-01-2010, 12:28   #12
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We have two 24v, 120w panels on Jubilee. The controller is a BZ 500hv, which can take 12-24-48v input and output either 12-24-48. It has worked well, is reasonaby inexpesive, and I would recommend it.. But, we have some RF interference issues. Based on the recommendation of a radio expert, I installed a capacitor and some chokes. It helped, but only removed maybe 25% of the noise. I then placed a call to the manufacturer, who suggested a much higher rated capacitor, 1mf vs .001

I have not installed the new capacitor yet.

I think 24v has a lot going for it. You can use smaller connecting wires, or just get less current drop then you would with a smaller gauge wire. I have 6ga wire from the panels to the controller, because I actually got a better deal on the 6ga then 8ga. Go figure. Also, if the voltage drops because of shading, I think you will end up getting more power to the batteries starting at a higher nominal voltage. Finally, I got 24v because we have a Torquedo electric outboard for our dingy, which is 24v. I might rig things up to charge from the panels some day.

Chris
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Old 30-01-2010, 12:52   #13
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Most newer (not the Blue Sky) MPPT controllers can take some pretty high voltages (150VDC) on the input side, such as the new Morningstar Morningstar Corporation » TriStar MPPT which allow - in theory - up to 6 or 7 12 volt panels in series. I say in theory because on a sailboat you can get shading, and if just one panel in a series string is shaded, it cuts down the total output of all panels in the string.
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Old 30-01-2010, 13:50   #14
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So ... does that mean if shading can occur parallel will be more effective then series?
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Old 30-01-2010, 15:07   #15
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By putting the panels in series, you end up with a higher voltage and lower current. Lower current means less loss in the wires between your panels and the inverter. Downside for in series is if any of the cells in either panel is shaded you lose the output of both panels. In parallel only the panel with a shaded cell stops working.

Series Wiring of Different-Wattage Solar Panels?
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