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.
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