[Elecraft] Lithium Polymer batteries in the K2

[Alan WB6ZQZ]

Status Report on the Lithium Polymer batteries in Elecraft K2 project:

The six LiPo cells arrived (3.7v 1600 mah each) from 
www.BatteriesAmerica.com a couple of days ago. The voltages match within 
20mv so I did not worry about matching the stacks. I weighed them, and with 
(extra) packaging they came to 7.5 ounces (for 11-12v 3200mah capacity). 
Not bad.

Note that BatteriesAmerica calls these Platinum Polymer rather than Lithium 
Polymer. This is apparently an advertising ploy. They list them in the RC 
section.

I assembled two stacks of three cells each on a piece of perfboard that is 
slightly smaller than the K2's battery tray. They are arranged three in 
series in each stack, then the stacks are paralleled via fuses. Per the 
directions on the package I was very careful not to short the cells. I 
secured each cell to the board, covered one terminal and worked on the 
other terminal. Each layer was flipped over so the terminal polarities that 
needed to be connected were adjacent, and the ribbons were soldered 
together and then covered over with tape. I used 3300 volt fiberglass tape, 
but electrical tape should work as well. I want to be very sure these 
connections never short.

I finished it off with a pair of flat-blade fuse holders, one for each 
stack, and then a black/red zipcord to a slighly larger coaxial power plug 
on the rear panel. I used an existing unused hole, but a new one could be 
drilled if necessary. Note that fuses are not installed until last to 
reduce the danger of shorting the packs. I covered the positive connection 
on the coaxial jack with heatshrink as well since this terminal is always 
hot. It would be a good idea to remove the fuses from the battery pack when 
working on the rig, though there are no exposed connections.

This batteries on a board assembly was double-sticky taped into the K2 
battery tray. It fits easily, with room on all sides and the top - it does 
not touch the K2 case lid. I installed the battery tray per the regular 
procedure, but did not use the regular K2 battery wiring of course.

I made up a jumper cable to bring power from the new battery connector to 
the K2 power in jack.

I weighed the K2 complete at 4 pounds 9 ounces. This includes ATU, LiPo 
battery, DSP, and the SSB kit plus a palm paddle mounted on the side.

The K2 meter reads 11.1 volts on receive, slightly lower on transmit. The 
wire in my adapter is not very heavy, that may improve with a better 
jumper. I used fairly heavy wire in the battery pack itself.

The radio puts out 10 watts into the dummy load, same as it does on a 17 
amp hour SLA.

So, it works!!

Later on I will charge the pack and report on that process.

-- Alan WB6ZQZ