[Elecraft] KX3 Please evaluate LiFePo4 battery/charger choice, UPDATE

[Fred Jensen]

On 11/21/2013 4:26 PM, Arno Dienhart wrote:

> When I hooked it up to the KX3 in RX (170mA draw), it started showing 13.5V
> and rapidly sunk to 13.1V within five minutes.
>
> Now, after ten minutes, it seems to have settled at 12.9V.
>
>
>
> Is this normal for such a battery, to have such a large difference between
> no load and small load?

>
> And also, is it normal for the Voltage do go down quite rapidly during the
> first few minutes?

Yes and yes.  That's what my A123 pack does.  Right after a full charge, 
voltage is about 14.8/9 no load, and 14.1 or so with K2 receive load 
[~270mA].  It drops fairly quickly [minutes] to 12.8 or so.  It will 
stay there or very close, dropping to perhaps 12.2/1 under full TX load, 
until near full discharge.  At that point, the voltage falls very fast, 
resembling the glide angle of a brick.

I charted mine using the K2 in RX as a load since it is constant, but 
will trip out on low voltage before discharging the pack too far.

Incidentally, the internal resistance of an LiFePO4 pack is extremely 
low and they'll deliver 20-30C or more into a short circuit, so don't do 
that to them. :-))  I recommend fusing the power leads close to the 
battery and using fast-blow fuses.  It will add a little resistance in 
your power leads, but it's a lot better than smoke.  With two fuses, my 
stable RX voltage starts at about 12.8, and on TX [5W, ~1.7A depending 
on band] goes to about 12.1V.  I'm fairly certain that is drop in the 
power cable and fuses.  Radio works super.

And, FWIW:  I've never gotten my pack below about 30%.  Made 30 CW Q's 
on W5N/RO-015 on day 1 and 27 on W5T/DE-003 on day 2 and it was at 30% 
when we got back to Mike's in El Paso that night.  Really remarkable 
batteries!
>
> I'll run it down, verify that the under-discharge protection works, and then
> charge it again for a longer time to make sure balancing sets in and does
> its magic, if needed.

I'm no battery chemistry expert, but I don't think running them down, 
like NiCd's, [except maybe to check the protection as you say] is 
necessary or desirable.  I do know they like to be stored at around 2/3 
charge, my charger has a mode to do that.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2014 Cal QSO Party 4-5 Oct 2014
- www.cqp.org