[Elecraft] KX3 and charging batteries (when, and why do they get hot?)

[Rich Heineck]

As the batteries discharge and reach the knee where voltage begins to 
drop off quickly, imbalances in capacity between the cells can lead to 
wider voltage differences between the individual cells.  If discharge 
isn't stopped in time, it's also possible for the weakest cell to be 
subjected to a reverse charge.   For example, 7 cells at 1.2V per cell 
is 8.4V.  If there's an 8th cell in series, running the pack down to 8V 
would mean that the weak cell has -0.4V across it.  This is a bit 
simplified and extreme, but the real goal is to not let an individual 
cell get below 0.9V, which is the typical manufacturer's spec for 
maximum discharge.

Starting with 8 new, good quality batteries is the easiest way to get 
what should be a pretty well matched set, and picking a conservative 
voltage to stop at will help get maximum life from the batteries.  As 
Mark points out, there really isn't much left near the end anyway.

73,
Rich  AC7MA


One thing to be mindful of when setting the low battery warning, but 
particularly when
On 06/06/2013 06:26 PM, Mark Petiford wrote:
>>> When should such batteries get charged, at what voltage level?
> Don't let them go below 1v per cell...about 8 volts total on the KX3 display...8.5V would be safer.  Elecraft recommends setting the BAT MIN menu to 8.5 V (KX3 Owner's Manual Pg. 36) to give you some warning that you need to think about recharging.  This is about 1.06V per cell.  The KX3 will shut down at 8 V which is about 1V per cell, very near the absolute minimum for NiMH chemistry.  There isn't much energy left below 1.06v per cell, so running them below that level really doesn't give you much extra operating time.  Some folks set the BAT MIN to 9V to give them more warning and plenty of time to finish their QSO and shut the station down.
>