"Larry" wrote in message
...
Ian Malcolm wrote in
:

Assuming the panels can be modelled as an ideal voltage source

Geez. If boat technology gets any better, we'll be able to run a light
bulb off the panel, shining on the panel, and the panel will have so much
power boost there'll be a surplus to charge the batteries!



Larry

Here is something I posted to the "Cruiser's Forum", about MPPT charge
controllers. They are real, and in many cases will increase the charging
current into your batteries. No black magic is needed, but I suppose that
some designs will be better than others. What follows is mostly
theoretical:
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MPPT stands for "Maximum Power-Point Transfer", and it is equivalent to two
switching regulators in series. It operates the solar panel at the load
where the panel delivers it's maximum output power, then converts whatever
Voltage this may be to a Voltage appropriate for the battery (depending on
the battery's charge-state).

This is in contrast to the series-pass, Pulse-Width-Modulation (PWM), or
shunt regulators. In these, the panel output is essentially connected
directly to the battery during the charge-acceptance portion of the charge
cycle, and the panel output is reduced once the battery approaches
full-charge.

An example from my boat: I have three BP 110W panels, wired in parallel.
Each panel has an open-circuit (no-load) output Voltage of 21.7V (ratings at
full noonday sun conditions, with a cell temperature of 25deg C). Each panel
has a short-circuit current of 6.9A. The panel has a maximum output power of
110W, at 17.5V and 6.3A. At any other output Voltage the panel will deliver
less than max power. When I am charging a low battery at (say) 12V, using a
non-MPPT controller (or directly hooking the panel to the battery), the
panel is not operating at maximum power. Looking at the output curve for the
panel, at 12V the panel will be delivering about 6.8A, which is 81.6 W. (I
am reading these Volt/Amp numbers from a chart in the panel specifications.)

With a MPPT controller, the panel would be operated at 17.5V, and be putting
out 110W. Switching the 17.5V down to 12V (assuming 95% efficiency, a number
I pulled out of the air) would give me 8.7 A into my 12V battery. This is a
28% increase in charging current.

Of course, the battery won't stay at 12V for long. As the battery Voltage
rises, the current-boost will be less. At 13.5V, a directly-connected panel
would deliver 6.75A (91W). With an MPPT controller the charge current (at
95% efficiency) would be 7.74A -- still one amp better.

Once the battery reaches full charge and the controller goes into
trickle-mode, an MPPT controller will have no advantage.

These numbers will be different in practice, as the panels won't see
constant full sun, and the panel temperature will usually be hotter. Still,
the MPPT controller ought to give some charging improvement. MPPT
controllers don't use fixed settings, but are constantly dithering the panel
load, monitoring the panel Voltage, and looking for the point of maximum
power.

These days, many panels are wired for 24V (instead of 12V) output, and the
MPPT controller can efficiently convert the output of these panels for use
in charging a 12V battery.

I still have the old-style controller for my panels, but I will probably be
installing MPPT before too long. I may end up putting a seperate controller
on each panel (instead of one controller driven by the three panels in
parallel), because my panels are often partially shaded, and I think I will
get more output if each panel can be independently optimized. I need to do
some testing or modelling to be sure of this. I can't easily add more
panels, so I need to make maximum use of the ones I have.
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Hope this helps.
-Paul