[Elecraft] internal battery voltages

[Francis Belliveau]

All,

I frequently see numbers on this subject that conflict with one another and
sometimes it is because they use dispay numbers.  Using display numbers adds
an additional inaccracy caused by blocking diodes that are in circuit and
may not be well matched from rig to rig.

The information that I have on SLA battery technology indicates the
following for Open Circuit readings:

    Full Charge = 13.2 volts (that is a resting voltage)
    Full Discharge = 10.8 volts (not good to go below)

The right point to stop discharging can get more complex because you do not
want to have one cell to get significantly discharged beyond the others.
Therefore, as the battery gets older you should stop a bit higher.  I use 11
volts as my absoluet cut-off point nd generally try not go push things much
beyond 11.5 volts.

One test that I have seen says to charge the battery, let it rest for 4
hours and if the open circuit voltage has dropped below 12.9 volts the
battery may have a bad cell.  It certainly was not fully charged at any
rate.  I generally hope for 13.2 volts after the 4 hour rest period to
indicate that I have fully charged the battery.

Keep in mind that the best use of this test it to determine that all cells
in the battery have received a reasonably complete charge.  The thing that
most often kills an SLA battery is having one cell that got a little extra
low (dropping below 10.8 volts can easily cause this) to never receiver a
full charge because the other cells take enough overcharge to fool the
charger into believing that the battery is fully charged when it wasn't.
One remedy is to leave the battery on a float charge for an excessively long
period of time.  This folat charge capability is one reason that SLA
technology is so well suited for the K2.  You use it on the weekend and then
flost charge it for weeks until it is needed for another weekend.

I hope that this information does more to clarify things than confuse the
issues.  The numbers above are average numbers for batteries near room
temperatures.  Keep in mind that temperature can easily complicate charging
and usage schemes for SLA technology.

73 to all,
Fran