hello and question - 24 Volt panels into a 12 volt batter bank?

[44toy]

Hello everyone!
I am a new to forum and getting back into solar. We experimented with some home brew 12 volt stuff 10-12 years ago but have been out of it for the most part since. I am starting to plan some stuff for our 30 class A we are going to full time in the next few years.

The question revolves around a guy I ran into who travels with 2 (Amerisolar 250 Watt 24Volt UL Polycrystalline PV Solar Panels) wired into a cheap looking charge controller I did not catch name feeding 3 telcom batteries wired at 12 volts.

The question is - If the panels are 250 Watts at 24 volt lets say approx 10 amps out output as that POWER was passed through a decent charge controller into a 12volt battery bank 400-500 ah would the output from the charge controller still be 250 watts with the output to batteries being roughly 14V charging voltage at 17ish amps? I have been running this around in my head and don't know if the 24 panels are appropriate for a 12volt bank, never ran anything but 12 volt panels.

I am sorry if this is maybe a stupid question but just cant wrap my mind around it. This is link to ebay auction with the same panels he is running http://www.ebay.com/itm/Brand-New-Am...QAAOSwubRXK48d . Thanks in advance for and help understanding this and sorry if it's a stupid question.

[SunEagle]

You can certainly use "24volt" panels to charge 12volt battery system. What would be a waste is to use a cheap charge controller (like a PWM) which will waste a large % of the panel wattage.

With an MPPT charge controller you basically get Watts in = watts out. Which calculates out to 500 watts / 12 volts = 41.7 amps of charging.

If you use a PWM charge controller you get Amps in = amps out. Based on that 250 watt panel, Imp = 8.26amps. So 2 of them = 16.52 amps of charging. That is a loss of over 25 amps.

[44toy]

Thank you for the explanation, I was positive that a MPPT controller would handle the conversion with minimal loss. I never had a MPPT but a Outback flexmax 80 is top of my list right now. Thank you again I just felt like I was missing something. Sorry for double post

[SunEagle]

Thank you for the explanation, I was positive that a MPPT controller would handle the conversion with minimal loss. I never had a MPPT but a Outback flexmax 80 is top of my list right now. Thank you again I just felt like I was missing something. Sorry for double post

The Outback is a quality CC. There are others. Also unless you plan on going up to 1000 watts for a 12v battery system you do not need to go with an 80amp CC. They make 60 amp ones that can accept up to about 750 watts of panels but you must read the specifications so you do not exceed the input DC voltage and wattage rating or you may cook the CC.

[44toy]

The plan as of right now is to have 4 of the 250 watt panels mounted on the passenger side roof with tilt. We also may have a fold up set that can be deployed if we need them on the ground with tiltable rack if we needed a little more power for long term boon docking in the future. I am just trying to get a idea of cost and feasibility right now.

I do have another question. If the 24 volt or even higher voltage panels with 2 or 3 times the watts seem to be a better buy why does almost everyone I see have 12 volt panels to charge a 12 volt bank? Do they just not want to get on board with a decent charge controller to handle the power? Thanks again for your time the solar world has changed so much in 10 years I feel lost

[Logan5]

Lack of information, and cognitive dissonance. We call it "stuck in the 12 volt box".

[SunEagle]

The plan as of right now is to have 4 of the 250 watt panels mounted on the passenger side roof with tilt. We also may have a fold up set that can be deployed if we need them on the ground with tiltable rack if we needed a little more power for long term boon docking in the future. I am just trying to get a idea of cost and feasibility right now.

I do have another question. If the 24 volt or even higher voltage panels with 2 or 3 times the watts seem to be a better buy why does almost everyone I see have 12 volt panels to charge a 12 volt bank? Do they just not want to get on board with a decent charge controller to handle the power? Thanks again for your time the solar world has changed so much in 10 years I feel lost

Well they may have fallen into the 12volt box (like I did) and did not realize that; 1) the higher wattage panels are cheaper / watt and 2) while 12volt inverters are easy to find the 24volt ones are also easy to find if you know what to look for.

I have a small system which consists of 5 panels in the 80 & 90 watt range, a 30Amp PWM charge controller, 4 x 12v 50Ah AGM battery (wired in parallel) and a 12v 600W Xantrex Pure Sine Wave inverter. Including racking, wire, fusing, combiner box, hand cart, battery box, connectors, etc. I spent about $2500 and can get maybe 600 watt hours a day out of it.

Now I could have gotten 2 x 200 watt panels, a 30A MPPT CC, 2 x 6c 232Ah batteries and the Xantrex PSW for about $1500 which would have gotten me a 700 watt hour per day system. But I did things my way and did not listen to the experts here. Even that system is still low on the scale of being good. Attached are a few pictures of it.

So I try to warn people not to be as stupid as I was and design the system before they spend their money.

[ewarnerusa]

I have not heard of the term "12V box" but I think that it sounds pretty accurate. An RV requires a 12V system to operate appliances, lighting, and emergency trailer brake systems so 12V becomes the mindset. Add to that, an MPPT controller often costs much more than a comparably rated PWM one. For example, my Morningstar Tristar PWM 45 costs $165 and the MPPT 45 costs $470. That's why for smaller systems PWM is often cheaper and adequate but MPPT becomes the better choice as the size of the system increases. If you're only buying a panel or two, then the more $/watt 12V nominal ones aren't much of a big deal since the PWM controller costs much less. But if you're buying several panels, then the savings from the cheaper $/watt higher voltage panels offsets the higher priced MPPT controller that's needed.

[Logan5]

An RV can be served with a nice step down converter and minor wiring modifications. I found huge savings by buying the MPPT CC before I purchased anything else. This allowed me to chose a smaller less expensive 250 watt panels and AWG10 wire leading from the array.

[SunEagle]

An RV can be served with a nice step down converter and minor wiring modifications. I found huge savings by buying the MPPT CC before I purchased anything else. This allowed me to chose a smaller less expensive 250 watt panels and AWG10 wire leading from the array.

That was a good decision to make. Think first. Spend second.

[Sunking]

Add to that, an MPPT controller often costs much more than a comparably rated PWM one. For example, my Morningstar Tristar PWM 45 costs $165 and the MPPT 45 costs $470. That's why for smaller systems PWM is often cheaper and adequate but MPPT becomes the better choice as the size of the system increases.

Once you cross the 200 watt line, MPPT is a lot less expensive.

A 200 watt MPPT system is going to cost you $400 to $500 for a 200 watt panel and 20 amp MPPT Controller. With a MPPT you can use much less expensive GT panels for about $1/watt, plus another $200 for the MPPT Controller.

To have an Equal PWM system requires you to use very expensive battery low voltage/power panels that cost 2 to 6 times more than GT panels. It would take 300 watts and a 20 amp PWM controller to equal a 200 watt MPPT system. You would have to use at least 2 (150 watt) if not 3 panels (100 watt) which requires much larger wire and a lot more hardware. Ignoring all the additional cost and just looking at panels and controller an equal PWM system will cost you $600 to $800.

Do you know how many fools, I mean people that come here with a 300 watt or more PWM system?