Rob Sigg wrote:OK I got the skinny. My MPPT controller can handle up to 150 volts and it will take any combination to put the max amount of power into the batteries etc. He tells me that its always better to have the higher voltage rather than amps since there is no line loss with voltage. If we went parallel our amps would be much higher, but the net gain would be less due to loss. Now I understand why my volts are so high but my amps arent
Okay that's a new one on me. I don't do large commercial installations and am not familiar with the technicalities involved. Obviously your friend knows more about this. The controller can
handle the high voltages but I don't think that is at the core of the problem.
I have a small set up, which I used for years although since the last time the batteries went bad on me I haven't replaced them. I'm not home much (over the road trucker) so I haven't gotten around to getting my panels useful again.
I'm not qualified to rebut your friend on the advantages of series configuration of panels, other than to remind you that you have a small system, and the issues which may add up to large losses or gains in a large system may not be the same ones with which you must deal.
If your panels are not far distant from your charge controller, and the parallel connections are made near the controller, taking advantage of the extra conductivity in 6 wires versus 2 wires (were the panels paralleled where they are located), then the resistive losses should be comparatively small for your system.
On the brightest day of the year, at high noon, the most you will probably see from a 240 watt panel is 8 amps. I'm taking for granted that the wires supplied by the manufacturer are capable of transmitting 8 amps without significant loss. I believe this to be true because it would not be in their interest to manufacture powerful panels only to have the loss in the output wire so high that the buyer and user would experience poor performance.
If your panels are configured in series, the highest current you can expect is 8 amps. If the panels are in series, the voltages will add but the current will not. One panel produces 8 amps. Three panels in series produce 8 amps. The voltage may have gone to 72 volts because of the series configuration, but the current will still be 8 amps.
Now, if your batteries are in a configuration supplying you with 24 volts, then no matter how many volts you have coming out of your panels, you won't go much higher than 24 volts at the battery end. Actually, fully charged, you'll get about 28 volts, as 12 volt lead/acid batteries are about 14 volts at full charge. The batteries will not climb to the 72 volts of your panels. Again, the batteries will stay at about 28 maximum. To simplify the discussion and for the sake of understanding, let's stick with nominal whole numbers. At noon on a bright day, with your present configuration, you can charge 24 volt batteries at 8 amps, 72 volts or not at the panel end of the circuit. Three solar panels will not provide the current of an automotive alternator, so your 12 volt batteries will probably not climb much beyond 14 volts even while charging. For power, let's say you'll have 8 amps X 28 volts = 224 watts. If the panels were configured in parallel, you'd be charging with 24 amps at 28 volts, and your power would be 24 amps X 28 volts = 672 watts. I know the panels are rated at 240 watts maximum, but the rating is taken from test conditions rather than the actual operating environment, so these numbers from a 240 watt rated panel are actually pretty much okay.
Losses in power lines are a major factor in the transmission of electricity. It is because of this that the high tension wires one sees strapped all over creation are run at such extremely high voltages. In your case, the losses in the relatively short lines from the panels to the charge controller, resulting from the relatively low current of 8 amps, are relatively small, perhaps a few percent. However, the loss of 16 amps of charging current resulting from the series configuration versus parallel configuration is a huge loss. You're losing 2/3 of the capability of your panels.
If you had 6 batteries configured in series to equal 72 volts, then yes by all means the series configuration of your panels would result in less resistive loss. There is no doubt about that and all the calculations would back it up. But again, your batteries are at 24 volts. Lead/acid batteries do not climb to triple their nominal voltage ratings.
You need to get all of the current you can into your batteries. I know this from experience. Solar power is great, but it has its disadvantages. Perhaps the greatest disadvantage is the lack of sunlight. When the sun is shining, you need to have everything right so that your batteries will not sit in an uncharged state for long. If they do, they'll die. Desulfators are not all they're made out to be.
If you tell me you're getting much more than 8 amps with your present configuration then I have a lot of studying to do and I'll have to rethink everything I've stated above.