Geoff Schultz wrote:
Jeff wrote in
:

Geoff Schultz wrote:
Get a controller that diverts voltage above the maximum charging
voltage of the battery to a resistive load. In my case this is the
water heater with 12V and 120V heater elements. 100% of the power of
the solar panels will go to charging the panels up to that point.
You can't produce more power than the panels are generating.

I personally use a Morning Star TriStar controller to control both my
wind generator and solar panels and it works great.

-- Geoff
So, are you saying that if the panels are putting out 17 volts and the
battery is only taking 14, then 3 volts are applied to the heater? I
don't think that's the way it works.

That's exactly what I'm saying. It's called Diversion Mode and on the
controller you set the maximum voltage which is allowed to be applied to
the batteries. Anything above that is diverted to the load.

The only time that this occurs is when the batteries are fully charged.
The vast majority of the time the charging load of the batteries drops the
output of the solar cells to a voltage less than the maximum allowable
voltage and thus nothing is diverted.

For details please see the manual:
http://www.mrsolar.com/pdf/morningstar/TS_Manual.pdf

Yes, this is how the tristar works. And for some people, it works
fine. However, for those of us whose power usage far exceeds the
charging ability, the diversion mode would rarely be turned on. In my
case, it would only happen if I had just run the engine, and at that
time the water tank is fully heated.

The diversion mode is suited to boats with excess generating
capability, such as trade wind passagemakers with wind or hydro
generation.

For systems where the solar panels will fall behind the load, the new
MPPT (Maximum Power Point Tracking) systems will generate more power
at the battery. Many panels will put out up to 17 Volts, and their
Wattage will be rated at this level. A normal regulator will reduce
this to an appropriate charging Voltage, perhaps 13.5 Volts if the
battery is discharged. However, the current is not increased so the
net Wattage will be reduced.

A MPPT controller is a DC-DC converter that will drop the Voltage down
without reducing the Wattage. An 85 Watt panel that can put out 5
Amps at 17V will put out 6.3 Amps at 13.5 and stay up at 85 Watts. A
normal controller would only allow 67 Watts.

Thus, the controller is not "creating more power than the panel
outputs," its adjusting the Voltage so that 30% of the power isn't
thrown away. As I've said, I've watched the current go from 8 to 10
Amps when the MPPT is turned on.