[Elecraft] K2: ext battery suggestions?

Sun Jun 3 10:16:51 EDT 2007

Hi

The following taken from the instructions for my Solar Controller Kit may be 
helpful. I have also used a 35 amp hour electric wheelchair gel cell battery 
and 2 35 watt solar panels with my controller on field day to run a K2/100 
at about 75 watts out. This allowed operation for the full duration of the 
field day (day and night) without using any external power.

Don Brown
KD5NDB

    Sizing Your Solar Charging System

The solar panel and battery needs to be matched to your system requirements. 
Although there is a wide range of specifications that will work, there are 
certain limits that must not be exceeded or damage to your battery may occur 
or at least may shorten its life.  The first requirement is the amount of 
power in amp/hours you will need to operate your equipment. This is simply 
the average ampere draw times the number of hours you want to operate the 
equipment. This number will be the size of battery you will need. You should 
add 15 to 25% to this number to have some reserve capacity. Next you need to 
determine the size of your solar panel. Solar panels are rated in watts or 
sometimes current.  If it is rated in current multiply by 14 volts (the 
fully charged battery terminal voltage) to get watts.

 Unless your panel can track the sun as the earth rotates you will need to 
figure on only about 4 hours of charging time near the rated output of the 
panel with several hours at reduced output. To make the math easy let's use 
four hours of sun per day. Divide the battery capacity by four to get the 
charging current. To convert to watts multiply by 14 volts. This is the size 
panel you will need. You can use a larger panel but you must not exceed the 
maximum charging current of the battery. This is about 1/3 of the batteries 
amp hour capacity. Just make sure the charge current is not higher than this 
number.

OK let's design a system for the Elecraft K2.  Suppose you want to operate 
for 4 hours per day. The K2 draws 250 ma on receive and about 2.5 amps on 
transmit. Let's assume 75% of the time is receiving and 25% is transmitting. 
So .25 amps times 4 hours times .75 is 0.75 amp/hours for receive. When 
transmitting assume about a 50% duty cycle so 2.5 amps times 4 hours times 
25 times .50 is 1.25 amp/hours.  Add the receive requirement to the 
transmit requirement and we get 2.0 amp/hours. Add 25% for a reserve and we 
get 2.5 amp/hours. The internal battery option for the K2 has a 2.9 amp/hour 
battery so lets design for it, as it is a little larger than we need in the 
above example.  2.9 amp/hours divided by four hours is 0.73 amps for our 
charging current. Multiply by 14 to get watts and we get 10.2 watts. The 
maximum charge current for this battery is 2.9 amps divided by 3 or 0.97 
amps times 14 volts is 13.5 watts. A 10 watt panel would be perfect for this 
application. You could probably use a 15 watt panel safely because most of 
the panels will not reach their rated output unless they are exactly 
perpendicular to the sun at high noon at a mid latitude.  The panels should 
be mounted facing south at an angle equal to your latitude so they get 
direct exposure to the sun for the most hours.

Solar Panels are constant current devices. Constant current devices seem a 
little strange because most of us are familiar with constant voltage devices 
like batteries and power supplies. A constant current device like a solar 
panel will supply the same current into any load up to its open circuit 
voltage as long as the load current is equal to or greater than the current 
supplied by the device. For instance if a solar panel is capable of 
supplying 1 amp of current it will supply that same one amp into a short 
circuit, a 6 volt battery or a 12 volt battery. The current supplied is 
determined by the size of the cells in the panel and the amount of light 
falling on the panel. Panels will supply 18 to 30 volts open circuit 
determined by the number of cells in series. This is the reason for a 
controller. Without one the panel would keep supplying the charging current 
as long as the panel is in the light possibly overcharging the battery 
shorting its life.

----- Original Message ----- 
From: "J F" <phriendly1 at yahoo.com>
To: <dougzzz at gmail.com>; "Elecraft Discussion List" 
<elecraft at mailman.qth.net>
Sent: Sunday, June 03, 2007 6:20 AM
Subject: [Elecraft] K2: ext battery suggestions?

> I use sealed GelCells Doug...
>
> These are the found in UPS units, usually in pairs.
> The price is right I can get them for free when the
> cells "fail". It seems that one battery will go well
> before another.
>
> I use them in tandem with the internal battery for
> QRP. Picked up a Solar panel to keep a charge on them,
> but haven't deployed it yet.
>
> You could use motorcycle or mower batteries as well.
> My preference is to stay away from anything that can
> spill.
>
> Cheers,
> Julius
> n2wn
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