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Old 03-17-09, 06:13 PM   #9
Bob McGovern
Lurking Renovator
Join Date: Feb 2009
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Originally Posted by Daox View Post
I'm a little confused. I always figured that all solar charge controllers were mainly just dc to dc converters with some battery charge logic slapped on them. In that case, they'd all convert extra volts into more amperage, unless the batteries were already at full capacity. Now I guess I don't understand how the cheap ones work.
Cheap ones just suppress the voltage by resistive loading or by a form of pulse-width modulation -- turning the panels off and on rapidly to create a 'phantom' voltage lower than the actual. In absorption mode, PWM controllers allow the voltage to climb but suppress amperage. Only MPPT controllers act as what amounts to variable DC transformers and increase current to the batteries as they reduce panel volts. Not entirely sure how they up the amps; may be something like a built-in inverter, transformer circuit, then a rectifier? It's fairly efficient, anyhow -- around 90% for buck MPPT; much lower for boost, which requires capacitor banks.

PWM is often touted as a high-tech charging feature, and certainly it does have the benefit of shaking sulfate of the plates, but really it's a cop-out. Modulating voltage is difficult for solid-state digital electronics; but they know from clocks, right? They can open and close a circuit a certain number of times per minute, doing more OFF and less ON as the batteries approach charge. Interestingly, MPPT has been around for about 60 years -- IIRC, it originally used a couple of magnetos spinning in opposition for a smooth analog transformation.
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