[Elecraft] Solar charging regulators:

[Ferguson, Kevin]

Using a series pass regulator (e.g. 7815) is a poor idea in PV applications.

These regulators will definatly prevent overcharging...in fact they may well
prevent
charging at all.

Most of the panels don't have much voltage headroom, especially in the
summer, and
the 78XX series of regulators typically need to drop 2-3V or so, "wasting"
4-7 of the array's
PV cells. Many low cost arrays only have enough cells to produce 14V under
load, so
a series regulator will seriously lengthen the charge time. The 17-18V input
volts that
the regulator wants to drop to 15V at the output can only be supplied by
typical panels at near
zero current.

Even if a Low-dropout regulator were used, there is a basic mismatch with
the application.
A series regulator wastes the most power when the battery is flat, and
wastes no power at all
after the battery is fully charged.  So when power is needed the most, the
least is available.

Since the array behaves more as a current source than a voltage source,
shunt regulators 
are a much better solution. This will allow 100% of the panel power to flow
to battery untill the voltage
reaches the setpoint.  In other words, the regulator starts out wasting no
power, and wastes all the 
array power after the battery is fully charged. Sadly, there are essentially
no off-the-shelf shunt regulators 
that are sutiable.

They are pretty simple to design, however.  Basic idea is that  the circuit
should behave as a perfect
zener diode, with high dissipation rating, and temperature stable (actually,
-26mV/C would better
match lead acid chemistry) clamp voltage. National's LM10 combines a
precision, buffered voltage reference 
and an op-amp. This chip combined with a power N-channel MOSFET or NPN power
darlington 
would make a very low parts count shunt regulator. I think there are
schematics for such in the 
application note for this part. 

Alternativly, most of the active circuits published for linear amplifier
screen-grid bias 
would be suitabe if adjusted for 15V output.

There are designs for switching regulators that will milk the last possible
watt out of a PV array, though
in clear violation of the KISS principal. These are known as tracking charge
controllers.

The cost of PV panels is so high, that a little extra care, complexity and
cost in the charge controller is easy
to justify...especially if it allows a lower cost array to meet the needs of
the application.