[Elecraft] Two battery questions - K2 portable operations

[Ed Worst]

Hi Doug,

You're right: 2 x 12 is better than 2 x 6 (because you get twice the current
capacity from the two 12 packs).  But a caution... you can't just wire the
battery packs in parallel.  If one battery has even a slightly different
voltage than the other, the higher voltage battery will discharge into the
lower voltage battery which could be a disaster.  I've seen circuits for
putting batteries in parallel (they're called 'battery isolators'); there
might even be one in the handbook or maybe someone on the reflector can
point you to one.  Otherwise a Google search would certainly find it for
you.  It could be as simple as a pair of Schottky Barrier Diodes (they have
a lower voltage drop than normal silicon diodes).  I seem to remember seeing
a circuit in QST lately (meaning 'in the past 5 years or so').  An article
search on arrl.net might find something.

As far as calculating battery life, it seems straightforward, but you have
to make a lot of assumptions to do it.  For example:  if you assume you
listen (0.15A) 90% of the time, and transmit (2A) 10% of the time, your
average current drain is (0.15 + 2 x 0.1)A = 0.35A per hour.  At 0.35A/hr, a
3AH battery would last about 8.5hr.  A pair of batteries would last about
17hr.  The big assumption in that example is the 10% transmit time.  If
you're using CW, your average current draw is much less than the "key-down"
current because you're only drawing 2A during the key-down times, and 0.15A
the rest of the time.  If you're on SSB, you have a similar situation, only
with voice peaks instead of key-down.  So that 17hr is a really conservative
estimate, and could be off by a factor of 2.  In addition to the
calculations, you should do a "real-world test".  I went through the same
calculation scenario with my little K1, concerned that a 2.5A gel cel
wouldn't be enough.  The actual usage however showed that I came home from
my camping trip with a lot of capacity to spare, plus an extra gel cel that
never got used.

Hope this helps.

73,
ed - k9ew


----- Original Message -----
From: "Doug Forman" <doug at dougforman.com>
To: <elecraft at mailman.qth.net>
Sent: Saturday, October 16, 2004 1:14 AM
Subject: [Elecraft] Two battery questions - K2 portable operations


> I'm assembling a portable kit in which I can fit two small sealed
> lead-acid batteries.
>
> For my specific size requirements, I have the option of putting (2) 12v
> (3.0Ah) batteries in parallel, or (2) 6v (3.0Ah) batteries in series to
> obtain the 12vdc I need.  (the specific batteries I am considering are
> the PowerSonic PS-1230 - Mouser # 547-PS-1230 or the PowerSonic PS-630 -
> Mouser # 547-PS-630)
>
> It seems to me that with [ 2 x 6v (3.0Ah) = 12v (3.0Ah) ] while [ 2 x
> 12v (3.0Ah) = 12v (6.0Ah) ].  Maybe the simplicity of this makes me
> question this assumption, or maybe it's been too long since I did these
> kinds of calcs  ;-)
>
> Could someone please confirm or deny my thoughts here?
>
> 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?
>
> Thanks in advance & 73,
>
> Doug  N7BNT
>
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