[Elecraft] Two battery questions - K2 portable operations

[N2EY at aol.com]

In a message dated 10/16/04 2:15:31 AM Eastern Daylight Time, 
doug at dougforman.com writes:


> It seems to me that with [ 2 x 6v (3.0Ah) = 12v (3.0Ah) ] while [ 2 x 
> 12v (3.0Ah) = 12v (6.0Ah) ].  


That's correct. You'll have twice the capacity. But for the same size and 
weight, it should be possible to find two 6V 6AH batteries to put in series.

> 
> 
> Also, I'm trying to calculate (roughly) the useful battery capacity 
> based on considering the minimum current draw from a rig, and the 
> maximum current draw from the same rig.  For example, given the battery 
> configurations above, and considering the K2 approximate minimum current 
> draw of 150mA and approximate maximum current draw of 2A (at 10w), what 
> is the potential operating time ranges available from the batteries?

That depends entirely on how much time (percentagewise) is spent at each 
condition.

Here's how to make the calculation:

First off, I'll assume CW. Analysis of typical plain-language Morse Code has 
shown that the key-down time in a typical transmission is 44%. (derived from 
the standard word "PARIS") In the case of a QSK transmitter like the K2, it 
means that *while you are transmitting*, the rig draws 2A 44% of the time and 150 
mA 56% of the time. The "key down" time is only 44% of the transmit time.

But how much of the time are you transmitting? 

Unless you're sending bulletins, a lot of messages, or calling CQ a lot, you 
probably spend at least 60% of the time listening and 40% transmitting. So the 
actual keydown time is 40% times 44% (.4 x .44 = .176) = 17.6%. To allow for 
tuneup and such, let's say 20%.

So the K2, when operated in CW under the conditions assumed, draws 150 mA 80% 
of the time and 2 A  20% of the time. So in a typical hour of operation, the 
K2 will use up:

.15 x .8 = .12 AH receiving
2.0 x .2 = .4   AH transmitting

.4 + .12 = .52 AH total

Using a 6 AH battery, you could get up to about 12 hours operating time as 
described above.

However, such calculations are very approximate. For one thing, they assume 
that your battery is fully charged at the start and can deliver name-plate 
capacity. Also assumes that your keydown time is as calculated. Less keydown time 
produces a lot more operating time. For example, if you listen more and send 
less, the operating time increases dramatically. Look at what happens if the 
operating is such that the rig then draws 150 mA 90% of the time and 2 A  10% of 
the time. So in a typical hour of operation, the K2 will use up:

.15 x .9 = .135 AH receiving
2.0 x .1 = .2   AH transmitting

.2 + .135 = .335 AH total

Using a 6 AH battery, you could get up to about 18 hours operating time at 
the lower duty cycle.

Note also that battery capacity isn't linear. Lead-acid battery capacity is 
specified at a certain current demand level. At lower levels it increases, and 
at higer levels it decreases. If a given lead-acid battery can deliver 2 A for 
8 hours, (16 AH at the "8 hour rating"), do not assume the same battery can 
deliver 8 A for 2 hours - you'll be disappointed, because it will run out of 
juice before then. OTOH, at 0.2 A it will probably last longer than 80 hours, 
indicating a capacity greater than 16 AH.

Last time I used my K2 on FD (all CW), a single "7 AH" battery lasted more 
than 12 hours and was still going strong. 

73 de Jim, N2EY