[Elecraft] KX3/P3 external power source

[GRANT YOUNGMAN]

I use A123 battery packs.  These are built from 26650 cells, which have a nominal voltage of 3.3v and a nominal capacity of 2500 mAH.  The specs for these batteries are published by A123.  The specified minimum voltage per cell is 2V, but the general consensus is that while that final voltage won’t destroy the battery, discharging to that level can reduce the life of the battery.

A fully charged A123 cell will have a voltage of about 3.5v.  During discharge (depending on temperature and current, but generally consistent with the currents drawn by typical portable ham gear) the battery delivers a nearly flat output voltage until the point that about 90-95% of the available capacity has been used.  When that occurs, the cell voltage is about 3v.  After that, the cell voltage literally goes over the cliff rapidly to 2v (based on manufacturer published discharge curves).

So with my 4S A123 packs I have my low voltage cutoff set to 12v (3 volts per series cell).

I have not found definitive data from A123 on “long term storage”.  My assumption is that “long term storage” means just that.  It doesn’t mean if I’m not going to use the battery for a couple of weeks I put it in storage mode.  If I thought I’d have it stored and unused for several months or more, I might.  I have to admit I don’t really know what “long term” means in this context, but I use my batteries at least once a week, so I don’t generally worry about it.

I use a CellPro Multi4 charger, which has a program for putting A123 batteries in storage mode.  It charges the battery to 50% fuel level, which is defined within the charging program as a cell voltage of 3.351v.  Incidentally, the charger also defines the 0% fuel level as 2.99v, consistent with the 3v per cell (12v for a 4S string) as the logical cutoff point, beyond which you’re not going to extract significant energy anyway and, if you’re not careful, risk damaging the battery by over-discharge.

After a day’s use, even if I’ve kept the battery charge up with solar, I put the battery on the charger to balance the cells.  

The specifics may vary with other LeFePO4 battery technologies, but the principles should be similar.


Grant NQ5T
K3 #2091, KX3 #8342




>> I'm always hearing conflicting information about batteries that leaves me baffled.
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> Contradictions like this are usually resolved by careful study of source documents, not be reading opinions expressed by those who failed to do that, but instead quote the opinions of others who have also failed to do that study. Science is NOT decided by majority vote, nor is it decided by opinion.
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