[Elecraft] Questions on KXBC3 charging mechanism

[Walter Underwood]

I think this is answered in the manual. For 2500 mAh cells, this is on page 4.
The KXBC3 charges at 200 mA throughout the charging time set, suggesting that 2000 mAH cells would be charged in 10 hours (referred in battery literature as a C/10 rate). However, the charging process is not 100% efficient, so additional charging time must be added, typically 20%. So a fully depleted 2000 mAH battery would require about 12 hours to reach full capacity.

The charging time must be adjusted for batteries of different capacity as follows: Charging Time (hours) = 1.2 X (cell mAH rating) / 200

For example, to charge fully depleted 2500 mAH cells: 1.2 X 2500 / 200 = 15 hours (Set charger for 16 hours) 

https://ftp.elecraft.com/KX3/Manuals%20Downloads/E740167%20Elecraft%20KXBC3%20Installation%20and%20Operation%20Rev%20B1.pdf

wunder
K6WRU
Walter Underwood
CM87wj
http://observer.wunderwood.org/ (my blog)

> On Mar 27, 2021, at 3:46 PM, Geert Jan de Groot <pe1hzg at xs4all.nl> wrote:
> 
> 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
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