[Elecraft] KX3/P3 external power source

[Jim Brown]

Yes. There are two VERY different limitations on minimum battery 
voltage.  One, which is set by the radio, is the minimum voltage at 
which the radio function is acceptable.

The other limit is the minimum voltage to which the battery can be 
discharged without damage to the battery. Li batteries of all variations 
are damaged if discharged below a certain voltage. That voltage depends 
on the chemistry used -- LiPO, LiFePO3, etc. For LiFePO3 chemistry, the 
individual cells that make up the battery should not be discharged below 
about 3V per cell. Once the cell has fallen to that voltage, it is 
nearly discharged, and continuing to draw current from it will cause the 
voltage to fall very quickly, and damage can occur.

73, Jim K9YC

On Thu,6/8/2017 10:28 AM, Fred Jensen wrote:
> Not sure I understand your question Matthew. Lithiumbatteries [all 
> chemistries] have exceedingly low internal resistanceand consequently, 
> will deliver a huge current if shorted.  Power leads should be fused.  
> The flat, automotive blade-type fuses seem to work best... large, low 
> resistance contact area.
>
> The voltage vs time function during discharge is probably the primary 
> concern for ham QRP use because it determines the total energy you can 
> extract from the battery before your radio quits. Good quality 
> LiFePO4's have a discharge curve that is very flat after an initial 
> drop from full-charge.  Mine [an A123 4S1Ppack] maintains a terminal 
> voltage above 11.5 V until it is nearly discharged.
>
> Re low-voltage cutoff: Some packs have a cutoff internal.  My K2 shuts 
> down [draws negligible current] at about 10.5Vhowever I've only had 
> that happen once. Most of the time, other factors [weather, tired 
> butt, bugs] will terminate an outdoor session well before the battery 
> depletes.