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post #9 of Old 03-07-2008
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Don't mistake an MPPT charge controller for being a "solid state" conversion. I mean, it is solid state in the sense that there is no "buzzer" creating an AC voltage, like the old UPSes used to use to make AC from DC batteries.
But an MPPT controller simply takes DC and puts it into a high frequency switch, converting the DC into a 30KHz AC signal, which is fed into a transformer. (Pulsed DC, but that's still more AC than DC to me.)
Then the other side takes the AC power from the transformer, and sucks it back out in an optimized fashion. The fact that they are running around 30KHz allows them to be pretty efficient, this is essentially the same technology that is in every computer's "switching power supply" these days for $20-100.
The MPPT controller goes one better, in theory, by also having a cheap dedicatred CPU on board that looks at the input and output, and tries to get smart about how to deal with the conversion. (An easy task, like winning at poker all the time every time.[g])

With some panels you an get 48V out, others you can't. If the factory has used three "12"V arrays in one panel, that might get you 48 volts at peaks--if you can rewire the arrays. In order to get better seals and lower costs, more of them are coming hardwired for 12 or 24v operation and you tsake what you've got. The problem is, if you wire up 2x12 or 2x24 to get higher voltage, and EITHER panel gets any shade--you lose output from the whole array. On the other hand, if you have all the strips in each panel set up in parallel (i.e. internal and external, all set for 12V) then any shade on any one part of the array just reduces the output amperage--instead of reducing the voltage of the whole system.
And of course, on a US-market boat, you've still got to get it down to a 12V battery, not a 48V one. Going from 12 to 24 volts give you some extra efficiency, going from 24 to 48 is just gilding the lily. Like calling a four stage charge with an "off" cycle a "five stage" charger. As, ahem, some of the makers are now doing.
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