K2: ext battery suggestions?

[Julius Fazekas n2wn]

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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[Don Brown-4]

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" <[hidden email]>
To: <[hidden email]>; "Elecraft Discussion List"
<[hidden email]>
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
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft
>
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com
>

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[AJSOENKE]

I run the entire hamshack on 4 EV-1 pullouts and 3-55W Solar panels. I  
estimate the batteries will run two rigs and the computers for about 2-3 weeks  if
I include the TiVo.
 
Al WA6VNN



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