[Elecraft] KX2 battery options.

[Detrick Merz]

I've done both, for my KX3, using cells salvaged from old laptop batteries.

4 cells: at full charge, pack voltage is too high for the KX3. On the flip
side, nominal pack voltage will remain high enough for the radio to operate
at higher power until the pack is nearly dead. I've dealt with this two
ways:

1) only charge the pack up to ~15.5v. This keeps the peak voltage within
the acceptable range, but sacrifices some capacity. It may also increase
the longevity of your cells (i.e. allow more charge cycles). It may also
prevent any cell balancing from ever happening.

2) Charge the pack fully and use a buck  converter until pack voltage falls
off a bit. This will allow you to maximize the pack capacity but keep the
input voltage where the radio wants it. The downside is the possibility of
birdies (buck converters are switchers) and having to carry yet-another
thing. Some power is "wasted" by the buck converter, but it can be removed
from the equation pretty quickly (it doesn't take terribly long for the
pack voltage to drop to where the radio is happy).

3 cells: peak pack voltage is perfectly acceptable for the radio, but may
not allow operating at full power. Nominal pack voltage (11.1v) also won't
allow full power out. When the pack is near dead, output power may need to
be significantly reduced. I've dealt with the 3 cell pack by using a boost
converter. This allows for higher output power but sacrifices efficiency
(some power "wasted" by the boost converter), and may result in birdies
from the converter.

Since I already have both 3- and 4-cell packs, I use both of them. I ended
up picking up a boost-buck converter. It's pretty small (deck of cards),
and hasn't created any RFI that I've noticed yet -- even operating out in
the field where the noise floor is very low. It lets me use either pack on
the input side, and set the output voltage where I like. I can run the KX3
at 15W until the packs go dead. I've also used it with my Bioenno LiFePO4
pack if I needed a little more power to complete a contract.

-detrick
K4IZ


On Mon, Nov 19, 2018, 08:07 Mike Short <ai4ns.mike at gmail.com wrote:

> I have a bunch of 18650 batteries. Should I build a 3 or 4 cell case for
> it?
>
> Mike
> AI4NS
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