[Elecraft] K1 Battery Option Mod - Charger

[alta]

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. 

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.