Almost to embarresed to ask - Ohms Law & Solar Panels

[grazza]

OK, here goes, i have just signed up and could not see the forum search feature, I have encountered a mental road block and would appreciate any help;

From school days I have always thought i understand Ohm law, but.....

Take two 250w solar panels and connect them is series, this gives me I = P/V, 500/24 = 20.833 Amps

Take two 250w solar panels and connect them in parallel, this gives me I = P/V, 500/12 = 41.66 Amps

I get the equation bit, but from a input to a solar controller amps perspective i am confused, why would I hook up the rig to only give me half the amps @ 24 volts?

Embarrassing yours,

Graeme

[jflorey2]

I get the equation bit, but from a input to a solar controller amps perspective i am confused, why would I hook up the rig to only give me half the amps @ 24 volts?

Half the amps at twice the voltage is exactly the same amount of power (power = V * I.) Series vs parallel just lets you work with different voltages, but the power is always the same.

[Sunking]

Because you do not understand Ohms Law and series/parallel circuits.

10 volts x 10 amps = 100 watts
20 volts x 5 amps = 100 watts
100 volts x 1 amp = 100 watts
1000 volts x .1 amps = 100 watts
1 volt x 100 amps = 100 watts
5 volts x 20 amps = 100 watts.

1. In series and parallel ciruits power always adds.
2. In Series circuit voltage adds, and current remains equal.
3. In parallel circuits current adds, and voltage remains equal.

Remember those 15 laws, 12 Ohm's Law + 3 Series/Parallel Law

That is DC theory 101 1st week of school. A student will solve about 1000 equations in that week until it is drilled into them. You cannot proceed if you fail that.

[foo1bar]

I get the equation bit, but from a input to a solar controller amps perspective i am confused, why would I hook up the rig to only give me half the amps @ 24 volts?

Because you want 24V (or slightly more) because you need at least that voltage for your input.

Or possibly because 24V and 20.8A is a better operating point for your MPPT controller which takes the *power* coming in and transforms it to the needed voltage output (with as much current as it can get for the incoming power)

Or because 24V and 20.8A has less power lost to heating the wires as it goes from the panels to the controller (power lost to resistance of wires is I*I*R - so lower current and higher voltage means more power delivered.


Since you're talking 24V and 12V, I'm guessing you're looking at off-grid.
Gridtie it'd probably be like 15 panels in series to get ~460V for Vmpp and just under 600V for Voc on a cold morning where you live.
(600V max because that's the "magic number" for certain rules - but trying to get close to that so that you can minimize loss in the wires)

[grazza]

Thanks to all for taking the time to reply to what I now realise was a dumb question and kind of knew it at the time, as I said, mental road block moment.

Foo1bar, you were on point, although I did not express this in my question, I was thinking about input voltage/amps into the controller from the solar array so I wanted to bring it back to basics to understand.
I have subsequently just installed my first off grid system on a barge boat for a friend, I used 4 X 250 W solar panels, 2 x strings, 2 panels in series and 2 in parallel to up the voltage to 24 V with a Victron MPPT 100/50 controller, I will be expanding the solar array to 2 strings in series with 4 x panels in parallel. Next I will be looking at expanding the 3 x 110 ah batteries.
Iam still learning and after installing my first system I realise that the mechanics of getting the wires to where they need to be is probably the greatest challenge. I will keep the forum updated as I continue this project which will as the last phase will include a wind turbine.

[littleharbor]

What are the specs. of your panels? From your description it sounds like you are talking about 12 volt panels "2s x 2p to up the voltage to 24v" but 12 volt, 250 watt panels are not common if available at all. Also you mention 3 x 110 ah batteries. Are these 12 volt batteries wired in parallel?

[grazza]

The panels are Black Mono SolarWorld Sun Module Plus SW250, (will post the exact spec later). it is a 12 volt system with the 3 batteries (already installed on the boat) wired in parallel, I connected the output of the controller to the perm hot side behind the main switch panel.

[solarix]

Your problem is that a "24Volt" panel actually produces its max power at about 30 volts (250W / 30V = 8.3A) this allows for a Charge Controller to be able to charge a 24V battery. Two panels in series produce (at most )17Amps at 60Volts. If you just clamp two panels in parallel to a 24V battery, you will still produce about (at most) 17A but at 24V (17A x 24V = 408W)

[littleharbor]

The output from your controller should go directly to your battery bank using the shortest distance possible and appropriate sized wire to carry the 69 amps your 1000 watt array will put into your 12 volt battery bank. It's looking like your controller is too small for that array @ a 12 volt nominal output. Your battery bank is too small and is poorly designed. At least you should be using 4-6 volt deep cycle golf cart type batteries keeping your parallel battery strings to a minimum of 2. Still you are either over paneled or under battery'd.

[grazza]

The current setup was just to get some charge into the current 3 x 110 ah batteries, is is not a static setup, I appreciate I need more batteries.

I hooked the 12 volt panels to give 24 volts to reduce the current in the leads from the PV array to the controller, I did not go to 36 Volts to prevent the PV array open cct voltage exceeding the 100v input to the controller.

I understand that the controller is rated at 700 W, so if I add the additional 4 panels I will lose power at the top end, but I was more concerned about the bottom end.

The distance between the battery banks and the controller is less than the 5m specified in the user manual.

Provided no one can see flaws in the setup I think the choke point will be the controller, I presume I could get another controller and hook it up in parallel, but I would need revisit the cable size as I am stuck with a 12 V invertor?