[Elecraft] FD2004: Solar K2 w/Li-Ion

[Alan WB6ZQZ]

I have installed Lithium Polymer Batteries INSIDE my K2. They work very 
well and provide nominally 11.1 volts at 3.2 amp hours, fit on the standard 
battery tray, and weigh 7.2 ounces plus a bit for tape, wire, perf board 
and fuse holders. If you want a lightweight K2 that has internal power, 
over 3 amp hours of it, this is a great modification. Perhaps Elecraft 
should have a kit for this!

I have not had time to prepare the web page regarding this project, so I 
will give some info here.

I used six batteriesamerica.com cells. Each P611 cell is rated 3.7 volts 
1.6 amp hours at 1.2 ounces and cost $13. Their catalog currently lists an 
improved model number LP1700 that has increased current capacity at 1.7 amp 
hours but is otherwise the same. The size of these cells is 2.35 by 1.3 by 
.33 inches. Two stacks of three each of these cells fit easily in the 
battery area of the stock K2 battery tray on a piece of perf board.

LiPo cells can be charged and discharged in series/parallel if they are in 
the same charge state (voltage) when initially connected.

I started by purchasing the battery kit from Elecraft without the battery. 
Out of this kit I only used the aluminum battery tray and some screws. I 
was planning to use the switch but did not as I don't want any chance 
of  either draining the batteries or accidentally charging them wrong with 
DC power connected to the rig. So I used a coaxial jack on the back to 
bring the DC out and a jumper cable to bring it back in. That guarantees 
that there will never be mistakes in charging and it also prevents 
accidental overdischarging of the LiPo batteries, which can ruin them.

I built my pack on a piece of perf board a little smaller than the size of 
the K2 battery tray. One cell at a time I taped the cells into two stacks, 
carefully soldering the tabs and then covering them with tape, then adding 
the next cell. Remember that this is 'hot' work (power wise), there is no 
way to safely discharge the cells and they can deliver very high current if 
shorted, potentially destroying the cells. I covered the other terminal 
when soldering a terminal so there was no chance of getting a short. This 
is important.

So now I had two 11 volt stacks. I connected an automotive type blade fuse 
holder to each stack (no fuse installed), paralleled the other end of the 
fuse holder wires, and routed the ground and +11v lead to a coaxial 
connector on the back of the lid in one of the transverter holes. I used a 
different size than the radio itself to prevent accidentally plugging in a 
12vdc power supply directly across the batteries. After completing 
installation I installed 4 amp blade fuses into the holders. MAKE SURE YOU 
FUSE THESE BATTERIES.

I then made a jumper cable to go from the new coaxial power out connector 
to the K2's usual power in connector. I disconnect this cable when 
transporting the K2. It is important to not 'accidentally' drain down the 
LiPo batteries as they will be ruined if drained to a very low voltage.

Another way to do this now is to buy two of the 3 cell packs from 
batteriesamerica.com and use those. I RECOMMEND THIS but have not tried it. 
The 3LP 1700 pack is 11.1 volts 1.7 amp hours $48, 2.35 by 1.3 by 1.0", 3.6 
ounces. So it is exactly what I did with tape and solder, but already done. 
Two of these will fit on the K2 battery tray.

http://www.batteriesamerica.com/newpage8.htm is the BatteriesAmerica R/C 
web pages that contains the cells and packs.

Charging LiPo Batteries:

I have read many specifications on charging LiPo batteries. All the 
published charging information I have found indicates a very simple 
charging methodology, but it is quite different from NiCad, NiMH, etc, so 
is not well suited to those chargers.

Use only LiPo or Li-Ion chargers on these batteries!!! The charging 
paradigm is PRECISE FIXED VOLTAGE with a current limit. The number quoted 
by different manufacturers varies slightly, but it is in the neighborhood 
of 4.235 volts per cell. DO NOT OVERCHARGE. Make sure you use a charger 
that is designed to handle 3 cells in series.

The best plan is to buy a MaHa or KoKam charger designed for Lithium Ion 
batteries (LiPo is a particular variety of lithium ion battery).

I am using the MaHa C777 Plus II which has a Li-Ion charging mode. The 
first time I charged them I used clip leads. When disconnecting I touched 
the wrong thing and the 4 amp blade fuses lit up like flashbulbs. These 
batteries can really put out current. USE FUSES. DO NOT CHARGE WITH 
CLIPLEADS THAT CAN SHORT. I made up a charging cable (which was always the 
plan) that has the connection points separated so I can use the MaHa clip 
leads to safely connect to the battery pack for charging.

I don't consistently keep up with the high level of traffic on this list, 
so if you want further info you may need to contact me directly.

Oliver KB6BA suggested the use of separate power connectors for the 
battery. He used larger cells in his K2 but they did not fit on the battery 
tray, so mounting was more interesting, as I understand it.

-- Alan WB6ZQZ


Date: Wed, 17 Mar 2004 12:30:09 -0800 (PST)
From: Daniel Reynolds <aa0ni@yahoo.com>

>I'm looking at preparing for my 2nd Field Day with my K2. This year I'd 
>like to
>look at taking advantage of solar charging and possibly light weight Li-Ion
>batteries. Has anyone had experience acquiring Li-Ion and is it tricky to
>charge them? If anyone has any comments or ideas they'd like to share with 
>me -
>I'd really appreciate it.
>
>...
>
>Date: Wed, 17 Mar 2004 15:25:10 -0600
From: "Don Brown" <DolfinDon@msn.com>

>Li-Ion batteries have special charging requirements if fast charging is =
>required. Normally they are charged at a high rate for about 80% of the =
>capacity then trickled for the remaining 20%. Some of them have small =
>controllers built in to monitor charge/discharge curves. You may get by =
>with slow charging them at a reduced rate, I have never tried it with ...
>these batteries. ...