Hi Eric
I think you'll find that MPPT gets most of its gains from decreasing the voltage and increasing the amps during the middle of the day, rather than in those parts of the day when the voltage is low.
Because of the problem you mention of voltages being low for part of the day, the solar panel folks had partly solved this by increasing the voltage of the panels, so you'll see that 12v panels operate at around 17 or 18 volts in good conditions. By having the voltage under good conditions at this higher level it means that under poor conditions the panels would still produce enough voltage to charge a battery.
But in good conditions we're wasting power cos the panel is putting out say 18 volts and battery needs say only 12 volts (lets say the battery is in a low state of charge). Say the panel is producing 10 amps, that means that its producing 10 x 18 = 180 watts but a lot of this power is wasted because the power thats going into the battery is 10 x 12 = 120 watts.
And its this difference that the MPPT controllers seek to use to harvest some extra power. By dropping the voltage down and increasing the amps, the controller can eke some extra power out of this gap between the solar panel and the battery. Of course, as the battery charge level increases, the controller has to follow that up, as well as tracking the variations in solar panel voltage as the sun goes behind clouds and so on.
It would be nice to see a unit that has this MPPT capability as well as load diversion, so we ultilise both the areas of wastage.
One of the problems with the PWM load diversion is that the controller adapts the load that is dumped by adjusting its voltage. This is fine for loads such as water heaters but may not work so well for loads such as dehumidifier or a fan.
In this respect the agricultural approach adopted by the controller referred to in the earlier link may have an advantage because it dumps its load at full voltage and it could be possible to drive a device such as a dehumidifier. Perhaps it could be combined with a timer so that the "dump" always ran for at least a fixed minimum time, so the load device isn't doing so much start/stop operation.
It sure looks like there is useful controller just waiting to be invented.
Cheers
Chris
On 27/06/2012, at 8:20 AM, Eric wrote:
Hi Chris,
Yep, this conversation has mixed two different charging technologies. MPPT is a variation on a buck/boost charger that converts low voltage input to a higher set voltage output, i.e. taking 3A at 12.0V and turning it into 2.4A at 13.8V (with various losses related to the conversion). Since solar panels spend part of the day at voltages that are too low to charge batteries, the MPPT controller makes that low voltage output useful, providing the battery with more total charging current over the course of the day.
Diversion load controllers take excess current (MPPT adjusted or not) that the battery can't use because it's fully charged, and route it somewhere else. That somewhere else can be a water heater, a de-humidifier, a ventilation fan or some other load that would be useful while the sun is shining. I guess that one could even run some sort of motorized bird deterrent. If the boat is using a wind generator, the load could be diverted to lights since the power could be available while it's dark. I don't know if any of the existing diversion load controllers use sophisticated multi-stage charging profiles, certainly the simple version that you described does not.
There's no technical reason that both devices can't be built into the same unit. I would think that the buck side of an MPPT controller is already a different circuit that the charging profile controller, they're just in the same box or on the same IC (kind of like the GPS circuit in your smart phone).
Obviously, the super-cool device would use MPPT to convert every last watt into usable charging current, a sophisticated charge controller to extend the overall life of your batteries and a diversion controller that would take any excess current that the charge controller decided it didn't need and send it to another useful load. When I say it like that, it sounds so simple. It makes me wonder why it hasn't been done already. I'm sure that a device like this would be useful beyond the marine environment, it sounds perfect for any off-the-grid application.
Fair winds,
Eric
Marina del Rey, CA
PS. Now that I think about it, I'm guessing that the Mars Rovers had charging controls even better than that.
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