[Elecraft] Portable Power

[Fred Jensen]

I'm sure that LiFePO4 is by far the best choice for ham gear and 
weight.  I ran a 2.4 Ah 4S1P on my K2 on receive with a recording 
voltmeter knowing that the K2 would shut down before the battery was 
over-discharged.  Fully charged, it started at 14.2 V, which dropped to 
13.8 V very quickly ... ~20 min.  Thereafter, the voltage remained 
stable, declining very slowly and linearly until it reached about 12.1 
V, at which point it took on the glide angle of a brick and the K2 shut 
down.  This sequence took about 10 hours.  K2 receive drain was just 
over 200 mA ... I don't have many add-ons in it.

For my second test, I repeated the above watching for when the voltage 
reached 12.4 V.  At this point, I began to transmit [CW] into a dummy 
load at 5 watts output.  The battery maintained a useable voltage until 
it reached the sharp "knee" and the K2 died. IOW, because of the flat 
discharge curve I could still get usable energy from it [to transmit] 
even though it was nearly depleted.

I have a 5 Ah SLAB, and I conducted the same two tests.  It took longer 
on the RX-only test of course, but nowhere near twice as long as the 
LiFePO4.  The TX test, with the voltage at 11.9, shut down the K2 when I 
tapped the paddle once.

I conclude that I could get nearly as much total usable energy at usable 
voltages from the 2.4 Ah 4S1P as from the 5 Ah SLAB.  The SLAB may have 
twice the "rated" energy capacity as the LiFePO4, but its internal 
resistance is significantly higher.  When the voltage drop through that 
resistance reduces the voltage to an unusable value, the remaining 
energy might as well be stored on the moon.

There is a safety caveat with extremely low resistance battery chemistry 
such as LiFePO4 and Li-Poly:  With such microscopic internal resistance, 
they will deliver their entire energy capacity about as fast as the 
external circuit can accept it.  If the external circuit is a dead 
short, which may be close to the internal resistance, that can be 
upwards of 20C - 30C, where C is the capacity of the battery.  This can 
be good for a variety of electrical displays, none of which are 
particularly useful.

My 4S1P is TSA-certified for carry-on luggage.  Make sure there is 
absolutely no way the battery can be shorted and I think you're OK.

Fred K6DGW
--Northern California Contest Club
--CU in the Cal QSO Party
--3-4 Oct 2015

2015-08-10 13:17 GMT+02:00 Rick M0LEP <m0lep at hewett.org>:
>> On Mon 10 Aug David Bondy G4NRT wrote:
>>> Should I be looking at LiFePO4 batteries or Sealed Lead-Acid
>>> batteries?
>> You'll find quite a bit of discussion of batteries on the SOTA
>> reflector, but apart from the cost, I can't see any obvious advantages
>> of using SLABs any more.
>>
>> Choose an appropriate LiFePo and you'll have a battery delivering more
>> power at a more useful voltage with rather less weight in your pack...
>>
>> Things get a bit more involved if you intend flying to foreign parts.
>> Again, lots of advice on the SOTA reflector.
>>
>> --
>> 73, Rick, M0LEP   (KX3 #3281)
>>
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