[Elecraft] Internal Impedance of Li-Ion Battery Packs

[Jim Brown]

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-ionboxbattery111v252ah27972wh14arateli-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