On Sat, Oct 15, 2011 at 07:45:21AM -0000, Steve wrote:
>
> At the very least your panels should provide at least 3% to 4% in amps
> of your 20 hour AH rating. This is the recommended charge rate for the
> absorption phase of lead acid battery charging.
The commonly-cited recommended rate that I'm familiar with - which I
believe originated with Nigel Calder - is 14% of total capacity. On the
other hand, there's G.W. Vinal's "Amp-Hour Law" cited by Rick Young in
his "Charging System Design", which says that a charging current equal
to the value of the number of amp-hours "missing" from the battery will
not excessively gas or heat the battery (that works out to pretty
serious current, in most cases.) In other words, you're safe (going by
the typical conservative estimate) if you stay below 31.5A for that
225AH battery.
As far as recharging, in the worst case cited - i.e., 3 days from 20%
SOC for 4 6V batteries at 225AH each - you'd need to make up 720AH @ 6V
(225*4*0.8), or 180AH @ 24V; a lot of juice before we even consider any
of the losses. Since you mentioned sailing around Boston, I'm going to
use the insolation average for that area, which is 4.2 hours per day
(note that the winter average will be 25-50% less than that, but we'll
go with the summer averages for now.)
With all of that, you'll need to generate about 14.5A @ 24V (180 / 3 /
4.2) to recharge, which is about 350W. Now, add in the production-side
losses - e.g., a solar panel rated at 100W (i.e., ~35V volts @ ~2.85A
for a panel designed for a 24V system) is actually producing ~70W of
usable energy in the very best real-world conditions (since you're only
using ~24V of that 35V.) In _actual_ conditions, though, it's more like
50W - which can be expressed as a 50% loss. There's a little more lost
in the system itself, but going with what we've got (before this gets
too depressing :), we're looking at 700W worth of solar panels - call it
4 175W panels. Sharp makes some nice ones (the NT-175UC1), which are
32.5" x 62" in size (call it 3x5'). This gives us a rough starting
estimate of 60ft^2, about 150 lbs, and about $2k not counting mounts,
wiring, etc.
In other words, a rather serious project - and not very likely to fit on
a moored 25' Cape Dory. Sorry to disappoint. :(
--
Ben Okopnik
-=-=-=-=-=-
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