Re: Solar power battery system for WRT54GS

SMS <scharf.steven@xxxxxxxxxxx> hath wroth:

Jeff Liebermann wrote:

http://802.11junk.com/jeffl/rf-calc/solar-repeater-206.xls
Unfortunately, to recharge a 30 A-hr gel cell in one day or less
requires a 400watt solar array (non-tracking). The WRT54G will burn
94 watt-hrs/day. The solar array would need to equal that in order to
prevent the battery from running down. At 25 watt solar array would
do the job.

Yes, if it was actually generating 25 watts during the day. The 25W
ratings is based on the panel being in direct sunlight. You really want
a 50W panel to be safe.

Yep, y're correct. 25 watts is what's needed to replace the power
consumed by the WRT54G during 24 hours (88 watt-hrs) with about 3.0
hrs/day winter insolation. The 3.0 hours is the "equivalent" direct
sunlight that a non-tracking flat plate collector would receive during
the winter[1].
http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/
You're correct that 25 watts is insufficient, but for a different
reason. If the solar system only supplied enough energy to replace
what was consumed by the WRT54G, then the system would take forever to
replace any additional losses. The battery simply would never catch
up and take forever to get to full charge.

That's where I found a screwup in my spreadsheet[2], in the
calculation for days to recharge under full load. The calculation was
inverted. Revised version at:
http://802.11junk.com/jeffl/rf-calc/solar-repeater-207.xls
I'm was a bit optimistic at 4.5hrs/day for mid winter. Changed to
3hrs/day.

Revised (and fixed) version specific to WRT54G at:
http://802.11junk.com/jeffl/rf-calc/solar-wrt54g-v101.xls
Note that even with a 40 A-Hr battery and a 50 watt solar array, it
will still take about 3 days to fully charge the batteries under load
in mid winter.

[1] It's really Kw-hr per square meter per day, based on 1000
Kw-hr/m^2/day. The number "3" would be the same as 3000 Kw-hr/m^2/day
or 3 hrs of equivalent full sunlight per day.
http://www.windsun.com/Solar_Basics/Solar_maps.htm

[2] Can someone check the spreadsheet for me? Every time I look at
it, I find mistakes. I can't seem to check my own work.

--
Jeff Liebermann jeffl@xxxxxxxxxxxxxxxxxxxxxx
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
.

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