[Elecraft] [KX3] KX3 Supply Voltage reading low

[Don Wilhelm]

Arlen,

As far as I know, there is no selection for observing the battery 
voltage vs. the external voltage.  The displayed voltage should be the 
higher of the internal batteries or the external batteries. There is a 
diode drop, so expect a reduction of 0.3 volts from the measurement at 
the battery.

I suspect that the battery charger is dragging down the voltage from 
your SLA battery (inside the KX3).
As another guess, the cable from the battery to the KX3 may be suffering 
from poor connections that are causing a significant voltage drop by the 
time the voltage reaches the KX3.  It would be nice (and informative) to 
measure the voltage at the K3 end of the power cable with your DMM (with 
the KX3 turned on).

Is this an older KX3 or a relatively new one?  The reason I ask is 
because the early KX3s did not glue the power connector to the PC board 
and could cause a separation of the connector voltage to the KX3 board.  
That can also result in a high resistance where the connector is 
soldered to the board (and produce a significant voltage drop.

I would suggest that you remove one or more of the batteries and see 
what the voltage reading from the external battery may be.  That will 
eliminate any concern that the voltage of the internal batteries is 
'dragging down' the voltage from the external supply.

I would also recommend that you use an external 13.8 volt supply rated 
at 5 or 6 amps if you can to see what the voltage reading results might 
be.  A fully charged SLA battery showing 12.2 volts open circuit is not 
fully charged, and the internal resistance of the partially discharged 
battery may be giving you confusing measurements.

73,
Don W3FPR

On 8/15/2014 6:12 PM, Arlen Fletcher wrote:
> David,
>
> I wasn’t aware of the ability to choose which supply (internal Vs. external) was being reported! Thanks for bringing that to my attention. I’ll bet you’re exactly right - that the internal battery voltage rises a bit when external power is supplied. I’ll check this out as soon as I get home this evening.
>
>