[Elecraft] RE: Solar Panels

[John Clifford]

I've found that a 10w solar panel will keep up with a K2 with the internal
battery OR an external gel cell IF you run the radio during the day with the
panel attached.  A 10w solar panel will also completely recharge the K2's
internal battery and replace what you pull out of it if you run all night
and recharge the next day.

A 20w panel will have no problem keeping the K2 charged indefinitely, and if
you want to run 24 hours a day it will still keep the K2 and external
battery charged indefinitely as long as you get enough sunshine.

My K2 with pretty much all of the options draws about 280 mA during receive
(OPT PERF).  Transmit at 5w is about 2.6 A.  Given that I listen for
interesting CQs mostly and reply to them, I spend about 50% of the time in a
QSO and about 50% of that time transmitting, and CW is basically a 50% duty
cycle mode.  So... that means my average current usage is about 570 mA while
the radio is on.  A quick back-of-the-envelope check says that my 2.9 AH
internal battery should last about 5.5 hours with this kind of current draw,
and this is verified by my experience (actually, a little less time).

You can determine how much juice you need to get from a solar panel by
computing your average current usage times the hours you will be operating,
and then determining how much current you can get from a solar panel times
the hours the sun shines (6  hours is a good standard -- even though the sun
shines more than that, unless you are physically moving the panel so that it
is perpendicular with the sun's rays the middle six hours of the day is when
your charging is going to happen).

Most nominal 12v panels actually produce about 17v of current under load.  A
5w panel gives you about 290 mA, which will keep up with your K2 in receive
mode, or if you use your K2 at night for a couple of hours this will top it
off the next day.  A 10w panel gives you 580 mA, which will keep your K2's
battery fully charged if you run it using this panel during the day, or it
will completely recharge a drained internal battery if you don't operate.

The 20w panel is the way to go if you want to use your radio every day and
not have to worry about the battery running down.  You can use a charge
controller (I use the Micro M+ from SES Logic,
http://www.seslogic.com/microm%2B.html) hooked up to your solar panel and
tied in parallel to your batteries, and you can charge up a 7 AH external
gel cell and the internal battery while operating during the day... so you
have a battery that will last you all night.

I've built a 15 AH battery pack by putting two 7 AH gel cells inside a .30
caliber ammo can with retaining brackets, and a flat panel with a dual
Powerpole jack.  Each battery is wired to a separate jack, and I have a
2-to-1 combiner so I can run both batteries in parallel if I want, or I can
just plug into each one separately.  (I've toyed with the idea of using one
of these ammo cans as the enclosure for the K2 also... with the lid on
they're waterproof and I'd set it up with all of the jacks accessible from
the front.)  This battery pack, and a 20w panel or 1A power supply along
with the charge controller, will power my K2 pretty much indefinitely.

Re solar panels, I like the BP Solarex MSX series, because they are fairly
compact for the power you get, and they are extremely rugged (the cells are
mounted on an aluminum plate and epoxy covered).  SES Logic has good prices
for these as well, and the owner Mike Bryce is a fellow ham.  I have no
connection with this company besides being a satisfied customer.

Hope this helps,

 - jgc

John Clifford KD7KGX

Heathkit HW-9 WARC/HFT-9/HM-9
Elecraft K2 #1678 /KSB2/KIO2/KBT2/KAT2/KNB2/KAF2/KPA100
Ten-Tec Omni VI/Opt1

email: kd7kgx@arrl.net