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you will see that the these curves show that the maximum power voltage doesn't change much as the sunshine level drops. The wikipedia article shows curves with a more dramatic alteration, but all the curves I have seen for market panels look like this. (above) So, what you do is you figure you're going to lose a volt IR in transmission, and that if your remaining (max power point) voltage at max sun is around 6-8% too high for the battery, you will not lose much power because that is within the range where you will only lose around 3% of your power. You'll lose that much power by forcing it through a transistor. So, then your panels output voltage can drop as much as 12-14% and you will still be within range to only lose 3% of your power. The stats of panels show that the output voltage won't drop more than that. There are things helping you by the way. When it gets cold and your panels output more voltage, your batteries require more because their internal resistance rises. When it gets hot and your panels output less, your battery requires less. When your panels output current drops and the voltage drops a little as well, guess what, the battery doesn't ask as much voltage because it's internal resistance is causing a smaller IR drop because of the smaller current. .. So we are going to do tests to verify this stuff, so stick around if you are waiting for the test results.
So if you pick just the right voltage of panels for your condition, you can happily charge with relays and get as much as any other method. Unfortunately, only maybe 20% of panels lie in the proper voltage range. The rest are going to require fussing. There are multiple things that drag you out of this ideal zone. If you are covering more distance, you may consider using relay charging with 18/35/70 volt panels to offset the IR loss of the cable, or, you can go high voltage/mppt. Relay charging with 19/37/72 volt panels would only make sense if you were charging a long ways away from your panels, and for panels in the low voltage range like 14/28/56 volts... well, if you want them to reliably approximate maximum power, you are going to have to series/overvolt and mppt. Consider the 14 volt panel you lose a volt in IR losses, now you're at 13, and you can only trickle charge. If you put a blocking diode in there, you are dead. In my opinion, forget the blocking diodes, just have your controller turn the panels off at night. Relays suck power for their coils, but only 1-2 watts for 60-120 amps of current capability. That is a lot less than losses in 60 amps of blocking diode. (like 20-30 watts)
Also, I don't think mppt controllers are generally designed to boost ... assume they can only buck and supply them with extra voltage.
I'm going to try and use relay charging. We'll place the panels close to the charging station and use 2 guage cable. I have an mppt controller, so I'm going to make a comparison and cough up real data.