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K2 - KBT2 and solar power?

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Peter Wollan

K2 - KBT2 and solar power?

After using my K2 for its first Field Day, I now see a reason to install
the KBT2 built-in battery. But, I particularly liked using solar power
-- a 10-watt panel running through Don Brown's solar charge controller
to a gel-cell battery in parallel with the radio while operating. Worked
really well. Can something like that be done with the KBT2?

The charge controller cuts off the solar panel when the battery gets to
about 14 volts, but it needs to be near the battery to sense the voltage
correctly. The solar panel can generate 20 volts at 1/2 amp -- what
happens if this particular charge controller is plugged in between the
K2+KBT2 and the solar panel? My guess is that the controller would keep
the voltage down to 14 volts, but would never see the battery as fully
charged. Do the diode and resistor in the KBT2 prevent overcharging?
It sounds less controlled than the KBT2 is designed for. Anyone have
experience with this?

Peter N8MHD

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Don Brown-4

Re: K2 - KBT2 and solar power?

Hi

The resistor and diode in the KBT2 limit the charge current to the internal
battery but really do not prevent overcharging. If the input voltage to the
K2 id about .2-.3 volts greater than the float charge value of the battery
(13.8V) then all is well but if the voltage is higher you may overcharge the
battery. The switch off point on my solar controller is set at 14.1 volts.
With the duty cycle the average voltage to the battery will be about 13.8
volts or equal to the float charge rating of the battery. If the charge
controller is connected to the K2 power input the diode drop in the battery
charging circuit would lower the voltage switching point to the battery and
the battery will not come up to a full charge. There are two ways to solve
this problem. The best way is to install a fused connector on the back of
the K2 that connects directly to the battery terminals. The solar controller
would connect to this connector when using a solar panel to charge the
battery. A power supply will still operate the K2 and charge the battery
through the normal power input when not using the solar panel. The other way
to solve the problem is to increase the controller switching point to about
14.5 volts by installing a 220K resistor at R9. I have not tried this but
should work. The controller would plug directly into the K2 power connector
to charge the battery. If you use this method then the controller should not
be connected directly to the battery.

Thanks

Don Brown
KD5NDB

----- Original Message -----
From: "Peter Wollan" <[hidden email]>
To: <[hidden email]>
Sent: Saturday, July 07, 2007 7:33 AM
Subject: [Elecraft] K2 - KBT2 and solar power?

> After using my K2 for its first Field Day, I now see a reason to install
> the KBT2 built-in battery. But, I particularly liked using solar power
> -- a 10-watt panel running through Don Brown's solar charge controller
> to a gel-cell battery in parallel with the radio while operating. Worked
> really well. Can something like that be done with the KBT2?
>
> The charge controller cuts off the solar panel when the battery gets to
> about 14 volts, but it needs to be near the battery to sense the voltage
> correctly. The solar panel can generate 20 volts at 1/2 amp -- what
> happens if this particular charge controller is plugged in between the
> K2+KBT2 and the solar panel? My guess is that the controller would keep
> the voltage down to 14 volts, but would never see the battery as fully
> charged. Do the diode and resistor in the KBT2 prevent overcharging?
> It sounds less controlled than the KBT2 is designed for. Anyone have
> experience with this?
>
> Peter N8MHD
>
> _______________________________________________
> 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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Don Wilhelm-3

Re: K2 - KBT2 and solar power?

In reply to this post by Peter Wollan

Peter,

Yes, the solar controller must connect directly to the battery.

Take a look at the "Using a SmartCharger with the K2 Internal Battery"
article on my website http://w3fpr.qrpradio.com to see how I made a
direct connection to the battery (I highly recommend that the connection
be fused for safety).

Substitute your solar controller for the SmartCharger and you have it
complete.

73,
Don W3FPR

Peter Wollan wrote:

Don Wilhelm-3

Re: K2 - KBT2 and solar power?

In reply to this post by Don Brown-4

While Don Brown has offered an alternative to increase the voltage of
the controller, I would not recommend doing it that way - what you end
up with is a solar controller that is specifically modified to use with
the K2 only.

I would want the solar controller to work with any 12 volt Lead Acid
battery and the charger modified for this K2 use would overcharge a
battery connected in the normal way.

If I did modify a controller to have a higher output voltage I would
mark it in BIG BOLD letters to serve as a caution against connecting it
directly to a battery - I know such mistakes do happen.

Don B. did say that the direct battery connection inside the K2 is the
best method, and I fully support that.

73,
Don W3FPR

[hidden email] wrote:
SNIP...

The other way

> to solve the problem is to increase the controller switching point to about
> 14.5 volts by installing a 220K resistor at R9. I have not tried this but
> should work. The controller would plug directly into the K2 power connector
> to charge the battery. If you use this method then the controller should not
> be connected directly to the battery.
>
> Thanks
>
> Don Brown
> KD5NDB

Leigh L. Klotz Jr WA5ZNU

Re: K2 - KBT2 and solar power?

Administrator

In reply to this post by Peter Wollan

Don and Don are more qualifued than I am to answer this, and they
already have, but I wanted to point out another option. With the KBT2,
the 10W panel may be too big, and you can use a 5W solar panel and the
KBT2 without a charge controller *if* the K2 is on and the current draw
is about the same as the panel output. For example, the Northern Tool
flexible folding panel for $40 puts out 340ma at noon, and my K2 with
all options (DSP) on, pulls 320ma. I monitor the battery voltage and
plug in the panel directly to the extra battery connection Don
mentioned. When the K2 is off or if I want to charge the battery and
turn off options, I use Don's controller inline.

Result: after my FD operation 1B2B/WNY (we were driving to Canada from
Syracuse) I found my KBT2 fully charged.

73,
Leigh/WA5ZNU
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