[Elecraft] Internal Battery Charging Question

[Bob Lewis (AA4PB)]

The typical "12-volt" supply is actually regulated at 13.8 volts. This
matches the alternator output on a vehicle with a "12-volt" system.
SLA batteries like that in the K2 "float charge" at 13.8 volts. That
is, at 13.8 volts the battery will take just whatever current is
needed to make up for the losses and keep it properly charged.

Because the K2 has a reverse polarity protection diode in line, there
is an extra 0.4 volts of drop between the external power supply and
the battery. That means that your supply needs to be regulated at 13.8
+ 0.4 or 14.2 volts in order to keep the battery properly charged.
Several users have been able to find the internal adjustment pot on
the Radio Shack switching supply and adjust it up (from the original
13.8 volts) to 14.2 volts.

Your Radio Shack supply doesn't appear to have a very good quality
regulator as it varies from 14.7 volts at no load to 14.0 volts with
the load of the K2 receiver. Both are incorrect for the K2. At 14.7
volts (receiver off), the battery is likely to be overcharged. At 14.0
volts with the receiver on, the battery will be under charged. It may
be time to look for a better regulated supply.  One other thought -
are you measuring the voltage at the supply or at the K2? It is
possible that you could have a few tenth of a volt drop in your
connecting cable.