[Elecraft] Thoughts about K2 and solar/battery capacity

[Daniel Reynolds]

--- Jim Brown <jimbrown.enteract at rcn.com> wrote:
> I have no experience with solar panels, but would like to try some. It seems
> logical that more than one would be in order to get enough power to run some
> radios for a contest. My EE background suggests that each ought to be
> connected to the battery through a diode and current limiting resistor, and
> maybe even a regulator to prevent overcharging. Is there standard practice
> for this?
> 
> Jim Brown  K9YC

For a permanent installation (e.g. at home - 100's of Watts of solar power and
several golf cart batteries) a charge regulator would definately be in order.
It would serve the purposes of both diode and current limiting resistor.

For temporary use in a trickle charge mode (current around 1/100 of Ah capacity
of battery), only a diode would be necessary. The 5W panel claims to have a
diode built in to keep from discharging a battery (I have not confirmed this
yet).

For somewhere in between these two scenarios - some intervention is necessary
to ensure that the battery isn't overcharged - whether that involves keeping a
load on the battery (like the K2), checking the battery every so often and
manually disconnecting as needed, or buying/building a charge regulator to
automatically switch to trickle charge to avoid battery damage.

The 5W panel is sort of 'self-regulated' in one regard. It will never exceed
the 1/10 charge rate of the battery since it will at most produce .35 A at
12-14V (.70 A would be the 1/10 charge rate for a 7 Ah battery).

Since I'm slowly putting my solar power setup together for the K2, I'm taking
the path of least expense. I plan on buying the Micro M+ charge controller kit
as time and money allows. In the mean time, I'm going to let the solar panel
get my 7Ah gel cell up to full charge.

Just some quick math ... assuming 100% efficiency (always a bad assumption):
Solar panel produces 350 mA at 14.3 V (5W). 350mA is 1/20 charge rate of 7Ah
battery. Assume I get 8 good hours of ideal sunlight between 8 AM and 4 PM. I
would need about 3 days of good sunshine to fully replenish a 7Ah battery.

Now lets look at it realistically. Add some inefficiency due to clouds
(10-80%), panel orientation (20-40%), misstated panel output (20%), and heat in
the car (10%) -- and it might take as long as 5-7 days to fully replenish the
same battery (assuming it was completely drained - which is a horrible state
for a gel cell to be in). That averages out to about 1-1.5 Ah of charge
replenished each day (2.5 Ah under ideal conditions).

A worst case "manual overcharge" scenario might be to leave the battery
connected to the panel an extra day - which would result in an overcharge of
1-1.5 Ah. This would probably result in some H2/O2 production, and a drying out
of the battery substrate (reducing the long term life of the gel cell). If this
was allowed to continue for a week, it could result in the long term battery
life being reduced from a couple years to a couple months.

Although it is less than ideal, it is a calculated risk I am willing to take at
the moment (so long as I am diligent to check the battery each day).

What would be the ideal battery size and panel size for Field Day?

First I need to figure my load for 24 hours. Assuming QRP operating (1:4 duty
cycle) running 5W CW (2 A) and receiver using .25 A, my total 24 hour power
needs would be roughly 15Ah over a 24 hour period. Assuming no dependence on
the sun, and keeping battery above maximum Depth of Discharge of 20% (i.e. use
only 20% capacity of battery)... ideally, the K2 would be tied to a 12V/75Ah
battery (large marine deep-cycle battery). Maintaing a low Depth of Discharge
(DoD) is critical for long term battery life. In order to replenish the 24 hour
demand of 15Ah, assuming 8 hours of ideal sunlight and adjusting the solar
panel to aim at the sun, a panel on the order of 30W would work best to keep
the system topped off. Battery cost would be around $50-$75. Solar panel  would
cost around $150-$300 (assuming $5/watt - most retail closer to $7-10/watt). 5A
Charge controller would run another $50-$100. Total investment would be around
$300-500.

However, I doubt you will find me lugging around a deep cycle marine battery
and a 30W panel. This is overkill for my individual Field Day effort. Keeping
it light is my plan for Field Day. 

Realistically - I doubt I will last much past midnight. I'll probably operate
12 to 18 hours of Field Day (7-10 Ah). My KBT2 and 7 Ah battery are barely
enough to meet my needs. The solar panel might offset as much as 1-3 Ah (plus
give me 100 extra points for solar power).

To keep from dipping below 50% DoD, I would do well to invest in a 2nd 7Ah or
10Ah gel cell (boosting my total capacity, including KBT2, to 17-20 Ah).

A second 5W solar panel couldn't hurt (but I can only really fit one panel in
the back window of my car during the rest of the year - so the 2nd panel would
only really get used during Field Day).

A charge controller wouldn't be necessary unless the panel and battery were
left alone for days.

5W Solar Panel ... $40 
7Ah SLA Battery ... $25
Operating 1B-Battery portable during Field Day with a K2 ... priceless

- Daniel / AA0NI
K2 #3421

p.s. Now - all I need to do is find someone who will write up a Palm OS Field
Day logging program that will handle dupe checking, keep track of sections
worked, and tally up my score. Then I can forget about dragging out those large
dupe sheets and all of those log sheets and forms into the windy Oklahoma outdoors.