[Elecraft] Questions on KXBC3 charging mechanism

[Geert Jan de Groot]

Hi,

Searching the web and searching the mail archive didn't find the result 
I was looking for, but I found a number of broken links pointing to 
now-defunct Yahoo groups on this matter..

I'm confused on the charging method used on the KXBC3 charger.
The standard low-current charge method for NiMH is "14 hours, 0.1C", so 
in the case of my 1900mAh eneloops, 14 hours of 190 mA. The KX3 manual 
doesn't say much, but the KXBC3 manual specifies a 200mA charge current.
That would suggest that 14-16 hours should give a full charge without 
too much overcurrent.

However, while charging I noticed that the KX3 draws close to 500mA, so 
I am not sure. Given that normal receiver operation draws 200mA, I fail 
to see how this would work with 200mA charge current - numbers don't add up.

I looked at the schematics. There is a low-drop voltage regulator (U2, 
LT1965) that seems to be used as current source. The regulator can be 
controlled via pin6 (shutdown pin). The voltage regulator supplies 2 
resistors R8 and R9 (12E each). The 3.3V output voltage over the 
resulting 6E resistance suggests a current source of roughly 500mA, 
which is closer to my measurements. There is an additional 20E resistor 
R10 which is switched using Q1 which would drive the current up to 710mA.

However, even being "low voltage drop" the regulator would need 3.3V 
input voltage and likely a little more, say 4V. Batteries on their way 
charging have a voltage of 10.5V, which means that this circuit alone 
needs 14.5V input voltage to function, but the charger is specified . So 
the current source can't give full current because the input voltage is 
too low to function.

Also, the charge time isn't 14 hours, but selectable 4, 8, 12, 16 hours. 
I don't see how that would work - people doing 2850mAh cells, as 
reporting on the reflector, won't be able to get a full charge even 
after 16 hours, not with the 200mA specified in the manual. And what are 
the 4 and 8 hour charge settings for?

I guess the figures really don't add up and I don't understand how this 
is supposed to work. Can someone enlighten me?

73,

Geert Jan