[Elecraft] K1 Battery Option Mod - Charger (corrected)

[alta]

NOTE:  The previous post contained an error.  In the leading 
dialogue, it should have said connect the *cathodes* together, rather 
than the *anodes*.

The circuit and concept are simple.  The idea is to use
a 12v lamp as a current-limiter to charge the NiMH batteries at 30-55
ma.  That is why the lamp is connected between the power-in and the
batttery-option on-off switch (slide switch on cover).

As supplied from Elecraft, the isolation diode is at the batteries.
In this mod, it is moved to the main board as described below.  And, 
of course, the removed diode at the batteries is replaced by a solid
connection (so you don't end up with an open circuit).

The resulting protection-diode arrangement (with cathodes connected
together) prevents damage to batteries, radio, or power supply.  And,
it causes the radio to draw power from the highest voltage source,
which is also desirable.  Note that the anodes are on the power 
source side of the arrangement.

The heat produced when charging is somewhat less than the heat
produced by the transmitter in QSO.  I use layers of Suran Wrap to
protect the radio against battery failure, and to reduce frequency
drift due to temperature changes in the box (which happens even
without the internal batteries...).

You can use the battery-option on-off switch to disable battery
activity (charging, running the radio, or discharge through a
non-energized power supply).

If there are any errors in the description or design, I would
appreciate detailed feedback.  I don't want to mislead anyone.

The details:

This simple mod allows the NiMH battery pack to be trickle-charged at
less than 1/20C (30-55 ma) from an external power source of 11.5-14
volts.

- You can elect to have it charge or not by setting the battery
switch off or on.

- It can charge with the K1 off or on.

- If the K1 is on, it will draw power from the highest voltage of
external or internal.

- This is a good match for direct connection of a small solar panel,
say 40ma to 100ma.

- The circuitry is protected against damage from totally discharged
batteries or short at external input.

Here's how.  It's hard to draw a schematic in text, so I'll tell in
words.

1. Move the "internal protection diode" from the battery option
switch to the main K1 circuit board.  Replace the removed diode with 
a direct connection so current will still be able to flow.

2. Connect the removed diode's cathode (bar) to the cathode on
"external protection diode" D1.  Dress with the anode (arrow) wire
pointing to the rear of the rig so that both anodes are about 1/4
inch apart.

3. Connect a small 12v incandescent lamp from the anode of the
"internal protection diode" to the anode of the "external protection
diode."  I used a Radio Shack 12, 60ma bulb with pigtails.  It
provided a charging current of 30ma to 55ma, depending upon input
voltage and battery voltage.  Recall that an incandescent lamp is a
near constant-current device.

4. For two reasons, I placed several layers of saran wrap between
battery compartment and electronics.  (1) The VFO is
temperature-sensitive, and the saran wrap helps stability, even
without the battery option.  (2) Extra protection against battery
leakage.

DISCLAIMER:  Elecraft recommends NOT charging batteries in the box.
For my own usage, I feel that low-level trickle charging is safe and
convenient, and worth slight added risk.  In worst case, it will
introduce less than 1 watt of heat into the box, and typically about
0.5 watt total.  The transmitter introduces somewhat more than that.
However, it does increase the risk of damage to the rig at some
point.  So, please evaluate the risks for yourself, and do what is
right from your perspective.

... Reed K7FLY - K1

-- 
Reed White  -  ALTA RESEARCH  -  www.alta-research.com
Phone: 877-360-2582  -  Email: alta@alta-research.com