[Elecraft] Internal Impedance of Li-Ion Battery Packs

[Bob]

I wouldn't mess with Li-Ion.  The voltage from the pack is either too low
(3-cell) or too high (4-cell), and they are quite dangerous if physically
abused.

Instead I would go with 4-cell LiFePO4, another form of Lithium battery.
 The iron stabilizes the lithium, and at no part of the discharge or charge
process is there any free metallic Lithium in the cell (that's what makes
traditional Li-ion so dangerous - Lithium burns violently when exposed to
water, even moisture in the air).  The energy density of LiFePO4 is almost
as good as a traditional Li-Ion.

Check out the LiFePO4 batteries offered by BioennoPower.com

I have plenty of information and discharge curves for various LiFePO4
batteries on my website, www.wb4son.com  For example, check out a
discussion of a 30 AH battery here:  http://wb4son.com/wpblog/?p=1250

As for your specific question, you can expect almost any LiFePO4 batter to
remain above 13 volts under load for about 80% of its capacity.  It will
start about 14.3 volts (open circuit), and won't drop below 13.0 volts for
a long time.

You will need to figure out your desired capacity, and also make sure you
select a battery that will support the maximum current you need.

I've run IOTA operations for a weekend (30 AH), and ran our GOTA station
for Field Day off of LiFePO4 batteries (60 AH).  Plus I've been using them
and keeping track of there performance for almost 4 years now.

73, Bob, WB4SON






On Mon, Aug 4, 2014 at 2:01 AM, Jim Brown <jim at audiosystemsgroup.com> wrote:

> I'm in the planning stages for a backpacking operation using a KX3 and
> KXPA amplifier. The application will be 6M JT65 from a rare grid. A key
> element of that is deciding what battery pack to buy. Because weight is a
> primary consideration, Li Ion is the clear choice, but the decision is
> between 3-cell and 4-cell packs.
>
> Real data on LiIon battery packs is damn near impossible to find. I've
> found discharge curves that make sense, but it isn't clear whether the
> voltages shown are open circuit or under load values. The difference, of
> course, is the internal resistance, and that's what I'm trying to get a
> handle on.
>
> I'm considering batteries in the 20-25 Ah range, with the Ah rating based
> on 10A. So my question is, what sort of internal resistance should I expect
> for a battery rated like this? Yes, I know if I had the battery and a
> sufficiently accurate voltmeter I could measure it, but these batteries
> sell in the $250 - $300 range, so I would like to know before I buy.
>
> Here's a pack that I'm considering.
>
> http://www.batteryspace.com/polymerli-ionboxbattery111v252ah27972wh1
> 4arateli-ionwithcarsocketsmartcharger.aspx
>
> A 11.1V (3-cell) battery is clearly the right choice to run the KX3, but
> it's the KXPA100 (100W amp) that I'm concerned about. I suspect that it
> isn't going to be happy much below about 10.5V. That's OK if the published
> discharge curves for Li Ion batteries represent the terminal voltage under
> load -- they show about 90% of capacity with the rated discharge current,
> but if that voltage is reduced by the internal resistance, I've probably
> got at least 10% less.
>
> The link below shows the discharge curves for a single cell, followed by
> my own plots of the same data multiplied by 3X and 4X for 3-cell and 4-cell
> packs. The KX3 and KXPA100 are rated for supply voltage up to 15V, and I've
> been told that 16V is an absolute maximum. That means the output of a
> 4-cell pack is too much, so I think it has to be 3 cells.
>
> k9yc.com/Lithium Ion Discharge.pdf
>
> Can anyone point me to some real data or relate some experience on this?
>
> 73, Jim K9YC
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