[Elecraft] Internal Battery Charging Question

[Steve Holton]

The internal voltmeter shouldn't be used for this. It may be off by several 
tenths of a volt from the actual voltage presented to it - over and 
above  0.2-0.6V below the power jack due to the drop across D10.
I looked into this because I was setting up a RS 3 Amp supply (vintage 
several years ago) to supply K2 #2846 and charge the KBT2. The Power supply 
indicated 13.8 volts and the internal meter said 13.3. Looking into it I 
found 13.8V at the power input J3 and 13.62 volts on the cathode of the new 
low drop D10 and 13.62 volts at the voltage divider for the voltmeter. The 
low reading of about 0.3V ( about 2.2% ) comes from two sources. The R9 in 
the voltage divider is 3 ohms off and that'll make an error of about 1/4% 
low. Second the A/D in the micro-controller doesn't measure absolute 
voltages, but relative a reference which is the uC "5V" Vdd which was 
measured on this machine to be 5.11 volts contributing about 2% too low a 
reading. The voltage is best measured at the J3 or the cathode of D10.

The RS 3 Amp supply was adjustable once it was opened up - there was a pot 
that could adjust the voltage which I did.

73
Steve N1NB

Original Message:
---snip
I have a question regarding my K2's internal battery, the KBT2 option.
Tom:
You should adjust the PS to read 13.7 as read by the K2 internal =
voltmeter
----snip