Thursday, June 30, 2011

good day sunshine

I have been researching small solar setups, of the type that feed a
couple 12v deep cycle batteries. I am interested in using a small panel
to charge and maintain a deep cycle battery in my camper shell.

The most interesting piece has been the charge controllers, the piece
that takes the varied level of power from the solar photovoltaic panels
(PV hereafter) and keep the batts optimally charged.

Both the inputs and outputs can be complex. Output is generally
three-stage charging: bulk (80% charge, acceptance (remaining 20%), and
float cycles (maintenance). That's interesting enough.

The input is somewhere between interesting and fascinating. The main
things to understand are:
1. depending on charge phase, the controller needs to put out 14.8vdc
at the most.
2. PV panels don't output 12v; the basic ones put out something like
18V max and many are much higher. This allows enough headroom for poor
solar condtions to still yield nominal 12vdc.
3. wattage ratings are given at highest power output, say 17.5vd x
2.58a = ~50 watts.

So, if the max you can use is 14.8v at the controller that would be
38.13 watts. Leaving a good bit (over 20%) on the table. Normal
controllers do this, effectively necking output from the PV to the
required voltage.

Another, more complex (and $$$) MPPT-type controller sweeps the panel's
output looking for maximum power. It does a DC-DC conversion dow to the
desired 14.8vdc (in our example); during this conversion the excess
voltage is yield as extra amperage. The output is very close to the
PV's rated output, minus about 5% losses incurred in the DC-DC conversion.

Under average conditions the MPPT controller yields about 10% more
power. In certain conditions it can yield 30% more. In rare conditions
when the PV is underperforming and PV output voltage is closer than 5%
to the very close to the required voltage* it can actually be less

The question in my mind is this: is the better controller worth 3x the
price. The difference is about $100. Can I imagine a situation where
having 30% more power would be worth $100. Yeah. So I'm leanintg that
way. And the better controller would be usable with better gear later on.

I may grab a PV and see what kind of real-world vdc I'm getting from it.
If I rarely dip below 15.58vdc I may save up for the better controller.
Really cheap, basic controllers (not the one's we are discussing) are
available for $15, and could be used later for backup.

phase one: get deep cycle battery. build small junction box for
accessories (fan, laptop charger, phone chargers, LED lights). Set up
AC charging regime. Test accessories.

phase two: get PV panel, mount on camper shell. Measure real-world
output in volts. Make decision on controller.

phase three: install conrtoller

A final thought for those unmoved by solar: there are home setups where
you can take as much power from the PV as needed for your battery bank
and put the rest into your wall socke. This is called "grid tie".
The extra voltage is the remainder is inverted to 110v and plugs into
your wall socket like any other plug. The inverter reads wall
voltage/hertz and outputs what the wall wants. Since the inverter
matches what the wall is putting out if the wall power is lost the
inversion stops.

Might be 20w, might be 200w but the power is being put to use instead of
wasted. Think about the positive input on the grid if everyone did that...

* needing 14.8v, getting 15.54

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