[Elecraft] (Solar) charging the K2 battery

[Don Brown]

Hi

If you are using my solar controller you must connect it directly to the =
battery terminals with reasonably short wires between the controller and =
the battery. The wire length is not critical between the controller and t=
he panel. The reason for this is the controller must sense the battery vo=
ltage to know when to disconnect the panel. If the K2 blocking diode is i=
n the circuit the controller will not work properly. The controller has a=
 blocking diode on board but the voltage sense is before this diode so th=
e controller can accurately read the battery voltage. =20

If you are building the controller into the K2 then connect the battery t=
erminals directly to the controller and install a connector on the back p=
anel to plug in the panel (use one of the transverter connector holes if =
you don't want to drill any new holes). =20

If you are using the controller externally then install a fused power jac=
k connected directly to the battery terminals on the back of the K2. Conn=
ect the controller to this jack with less than a couple of feet of wire t=
o keep the voltage drop down. =20

With this arrangement you can use the normal power input jack to power th=
e radio and/or charge the battery from an external power supply or use th=
e solar power jack to plug in the solar setup and power the radio and/or =
charge the battery.

Because a solar panel is a constant current source the cable length doesn=
't matter from the controller to the panel. You can use a 50 or 100 foot =
extension cord if needed to get the panel out in the sun. If you are goin=
g to use an extension cord you may want to use different plugs than the s=
tandard AC plugs so someone doesn't plug your solar setup into 120 volts =
by mistake :-(  =20

Do not remove or modify any of the K2 internal battery circuitry or you w=
ill defeat the K2's protection features. Do not try to plug the solar pan=
el directly into the K2 power jack. A solar panel can output over 20 volt=
s and depending on the load and the size of the panel you may apply too m=
uch voltage to the K2.

If you have a very small solar panel less than 2 or 3 watts you can conne=
ct it directly to the battery terminals through a separate power jack and=
 fuse without a controller. The risk of overcharging the battery is low w=
ith such a small panel and you can check the battery voltage with the K2 =
display function from time to time to make sure. Only 2 or 3 watts going =
in would limit your operation time but may work in some applications if y=
ou only wanted to work a few hours a day. A small setup like this will st=
ill benefit from the controller by making it all automatic. Any panel lar=
ger than this will require a controller or you will overcharge and ruin y=
our battery.

Please fuse any direct connections to the battery that have external conn=
ectors. Even though the battery is small it can supply a very large curre=
nt melting wires, destroying parts or causing burns if a short occurs. Be=
 careful when working around SLA batteries.

Don Brown
KD5NDB


----- Original Message -----
From: Mike McCoy
Sent: Monday, April 29, 2002 8:36 AM
To: elecraft@mailman.qth.net
Subject: [Elecraft] (Solar) charging the K2 battery

I came across a deal on a solar panel I couldn't resist. It's the Uni-Pac=
 15
and according to the tech sheet it puts out 17.6v @ 15.8w (max). Pretty c=
ool
panel. See it here:

http://www.unlimited-power.co.uk/Uni-solar_Portable_Solar_Electric_Panels=
.ht
ml

I plan on using Don Browns Solar panel controller to charge the 'stock' K=
BT2
battery using this panel. I've read the KBT2 manual & the tech article 'c=
are
and feeding of a K2 battery' and I have a couple question for those with
experience using the K2 battery & solar chargers:

According to the manual, the KBT2 provides current limiting ("typically
250ma eliminating the need for a current limited PS for charging"). Thus
when using a controller I don't need to worry this panel can put out clos=
e
to an amp in full sunlight and overcharge/overheat the battery?

According to 'Care and feeding of the K2 battery' under no circumstances
should I connect the panel (and controller) directly to the normal K2 pow=
er
input. If this panel/controller is regulated to provide 14-14.2v, why
couldn't/shouldn't I use the panel to charge the K2 from the normal power
input?

<snip>

Thanks & 73
Mike - AD5IU


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