24V Panel Charging 12V System

[gavinseim]

Thanks everyone. Great info. Ya it should fit nicely right on the roof. Excited to try it out.

[niel]

another cc option is this,
http://www.roguepowertech.com/products/mpt3024.htm
but it's doubtful you'll need to go to 30a out. you will find that in downconverting the voltage with an mppt controller that the current will go up. you won't quite double your current, but after efficiency factors you could get to to about 1.5x the current if at full sun (1000w/m^2).

[gavinseim]

Ya that looks pretty sweet, Though probably overkill in this small camper. Space is also an issue.

[waynefromnscanada]

another cc option is this,
http://www.roguepowertech.com/products/mpt3024.htm
but it's doubtful you'll need to go to 30a out. .

But definitely worth considering if there is the slightest possibility that you may, like most of us have, get addicted to solar and later add more PV, perhaps ground mounted.
There now, aren't you sorry you found this form?

[niel]

Ya that looks pretty sweet, Though probably overkill in this small camper. Space is also an issue.

no problem as i'm only giving you another option.

be careful that the premium space that you refer too doesn't interfere with the controller's ability to dissipate heat.

[gavinseim]

Hey all. Another question. I received the panel and it looks good. Voltage in sun comes in at about 39v which is just about right.

What I'm a bit confused on is the the short circuit test. If I put an ohmmeter directly to the pos and neg leads, I don't get a proper max amp reading because there's not enough current draw. Are you saying to actually run a separate wire between the panel outputs (truly shorting the connections) and then placing the meter leads inline with that short, so as to measure the amps being drawn? Won't that damage the panel?

What am I missing here.

[niel]

oh oh. not ohm meter. ammeter. it must read amps rated to at least the isc current. you could damage your meter trying to read amps in the ohms setting. see if the meter is working and if not check to see if it has a fuse internally protecting it.

when you use the current meter (properly known as an ammeter) it is acting as a short circuit to the pv and will pass lots of current, but it must be in full sun to give a good current output. you aren't hurting the pv at all.

[gavinseim]

Excuse me not an ohmmeter (that would be resistance I believe). I did mean an ammeter. I was in amps mode on the meter. But in all previous experiences and research with how to use an ammeter, it was necessary to have a load (ie a light bulb of resistor) with the meter in between. Not just use the leads. If I just place leads on +/- I'm only getting about 1.2 amps. But there' no load, so I don't know how that can be accurate.

[Cariboocoot]

Excuse me not an ohmmeter (that would be resistance I believe). I did mean an ammeter. I was in amps mode on the meter. But in all previous experiences and research with how to use an ammeter, it was necessary to have a load (ie a light bulb of resistor) with the meter in between. Not just use the leads. If I just place leads on +/- I'm only getting about 1.2 amps. But there' no load, so I don't know how that can be accurate.

Funny things, photovoltaic panels.
Current source, y'know.
Do as Niel says: set the meter to 10 Amps, hook one lead to the positive of the panel and the other to the negative. No loads; you're measuring short circuit current and the short is provided by the meter.

Put the panel in the sun. See the reading? Change the panel angle; watch the reading change! Outside in midday full sun you can get anywhere from a few mA to near full Isc rating according to how "square on" to the sun the panel is.

When you use an Ammeter with a load, you're measuring the current draw of that load. This is normal procedure for a Voltage-based system such as a battery. Never try to read short circuit current of a battery! It's instant fireworks to say the least.

[waynefromnscanada]

Hey all. Another question. I received the panel and it looks good. Voltage in sun comes in at about 39v which is just about right.

What I'm a bit confused on is the the short circuit test. If I put an ohmmeter directly to the pos and neg leads, I don't get a proper max amp reading because there's not enough current draw. Are you saying to actually run a separate wire between the panel outputs (truly shorting the connections) and then placing the meter leads inline with that short, so as to measure the amps being drawn? Won't that damage the panel?

What am I missing here.

Hi Gav, others have answered your questions perfectly, but I'll try to answer using somewhat different wording that may help you to more easily grasp the ideas.
Re the short circuit test. I assume what you actually have is a "multimeter", capable of measuring resistance (ohms); voltage, and current (amps).
A couple of cautions:
NEVER try to measure the resistance in any circuit which is energized (is connected to a power source) otherwise your multimeter will smoke and if you're lucky, the ohms function will be the only section of the meter that will be destroyed.
NEVER try to measure the current (amps) of any source capable of providing more current than your meter is designed to handling, otherwise your meter will smoke, or if you're lucky, your meter will have an internal fuse, which will blow and hopefully quick enough to save your meter. Your battery pack would be an example of a current source which would blow your meter, and for these purposes, you can consider your battery pack as a source of unlimited amperage, and putting a short circuit on that battery pack, would result in sever damage to something, or someone,, including your battery pack.
Solar panels on the other hand, can only supply so much current and doing so will NOT damage them. Therefore you can set your multi-meter to DC AMPS, connect it's leads to the + and - of the solar panel, and directly read the amperage with no damage to your panels whatever. The current will flow from one of the panels terminals, through the meter, then back to the other terminal, showing up on the meter face as amperage, all the while causing no harm whatever to your panels. And you can think of that amperage, not as being "drawn" as from a battery, but "produced" by the solar panel.
Hopefully this is of some help.