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Old 12-11-2015, 20:50   #1
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GILow's Avatar

Join Date: Sep 2008
Location: Adelaide, South Australia
Boat: Swanson 42
Posts: 3,585
Dual charging sources, getting the voltages right

Hello Cruisers,

I while ago I tried to find a modern regulator that would work with our Aerogen 6 wind generator because the current regulator is a little bit too dumb to understand what is happening with the MPPT solar charge controller. In short, most of the time the sensed voltage is too high for the Aerogen's regulator, and it switches immediately to dump mode. And, to compound matters, I realised the regulator was actually draining power back out of the battery bank at night when the bank was fully charged. Whether it was doing this by design, by accident or by decrepitude is anyone's guess, but I am glad I discovered it before it did any damage. Also, we seem to have the only Aerogen regulator that does not have the user adjustable pot to set the cutover voltage.

Anyway, after a fruitless search during which I discovered that every "smart" regulator I could find wanted to dump power from the batteries, rather than diverting power away from the batteries straight to the dump load, I figured I would make my own regulator.

The reason I don't want to dump power from the batteries is that I have a very smart German made MPPT controller with lovely programmable charge curves (which I have left in the factory programmed state) and I want the controller to be able to manage the battery charge state without any interference from other devices. Our boat is a dock queen 5 days of the week, so the controller has all week to get things just right before we harass it over the weekend.

I have some left over picaxe chips and associated goodies to run on an I2C bus, including memory modules, LCD displays and clock chips, so I figured why not see what I can build. A quick check shows that I can do all the switching of the reasonably high currents generated (up to around 30 amps) using MOSFETs, so now I am left with deciding on switching voltages.

The simplest circuit logic seems to dictate that I pull the wind generator out of the charging circuit when it detects the end of the bulk phase, and keeps it out until it detects a drop to below the float charge voltage. But this is where I get confused. If float voltage is (for my current AGMs), say 13.8 volts, and the end of bulk is 14.7 volts, then if I bring the wind generator back in at 13.7 volts (for instance) I am missing out on helping with the whole absorption phase, plus a good chunk of the bulk charge phase.

Not the end of the world, but not the most efficient method.

Can anyone suggest a smarter algorithm, remembering that I am trying not to interfere with the overall smarts of the MPPT controller, plus my current AGMs will be swapped for some other battery chemistry when we actually leave the pen for real.

Matt
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