[Elecraft] [KX3] KX3 Supply Voltage reading low

[Don Wilhelm]

Arlen,

I just measured the terminal voltage of several SLA and AGM batteries.  
After not being charged for over a week, they all measured in excess of 
13 volts.  So my conclusion is that your wall-wart charger is not fully 
charging your battery.
A normal float charge for an SLA battery should be at 13.8 volts - and 
even that is not perfect because the best charging voltage varies with 
temperature.  A good 3 stage charger designed for charging SLA batteries 
is the best to use, and those are a bit more expensive than the 
"wall-wart" chargers, but worthwhile if you consider the life of your 
SLA batteries important.

73,
Don W3FPR

On 8/15/2014 6:03 PM, Arlen Fletcher wrote:
> Hi Mark,
>
> Thanks for the response!
>
> I’m guessing that perhaps you’re considering that the voltage of the SLA is sagging when it’s running the KX3. I wondered that too - so I measured across the terminals of the SLA while it was powering the KX3. It still measured 12.2V, so it wasn’t sagging. The battery (and it’s little charger) report it as being fully charged - though I’m dubious about the accuracy of the little wall-wart charger. :-)
>
> Arlen
> AA7F
>
> On Aug 15, 2014, at 11:42 AM, rv6amark at yahoo.com wrote:
>
>> Re:  …reads 12.2V on my Fluke DMM - the KX3 reports 10.2V while idling.
>>
>> Arlen,
>>
>> My first thoughts are that you need to fully charge the battery.  I don’t have a lot of experience with lead acid batteries, though.
>>
>> The input diode (CMS04 if my memory is correct) should only account for about 0.2v at KX3 receive current levels.  I haven't looked at the schematic for a long time, so I can’t remember what else is in the path from the connector to where the internal voltage reading is measured, but I seem to recall at least one FET (on/off switch) and a current sensor.
>>
>> The 12.2v is getting pretty close to the no-load fully discharged 1.95v per cell, and the 10.2v is pretty close to the 1.75v per cell of a fully discharged SLA battery.  At these voltages, the discharge curves are getting pretty steep, so the addition of the KX3 receive current could cause the extra drop you are seeing.
>>
>> Of course the problem could also lie in the power cable, connectors, or even inside the SLA battery itself.  A check with a fresh, fully charged SLA battery will help isolate the problem.  You will see voltage drop along the cable, even with a freshly charged battery, but 2 volts total sounds a bit high.  I just pulled out an SLA that I haven’t used for a while, but it has been charged recently, and the voltage difference between the battery voltage (no load read by a cheap DMM) and the KX3 display (while in receive) was less than 1 volt.
>>
>> Hope this helps.
>>
>> Mark
>> KE6BB
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