[Elecraft] KX3 Smarter Charger

[Rich Heineck]

During charge at 200mA, the NiMH batteries can typically reach 1.5V 
each, for 12V total.  Add in the extra voltage for path resistance, 
isolation diode, and charging regulator overhead, you get up over 13.5V, 
which is why the recommended voltage is 13.8V.  Charging at lower supply 
voltages would have required a switching boost regulator which then gets 
into problems of space, RFI, and cost.  It is possible to put some 
charge into a depleted set of batteries from a lower supply voltage but 
they just won't be able to get fully charged.

Because the battery holders will hold any kind of AA cell, we opted for 
safety and charge reliability when it came to selection of how the 
charger operates.  With 8 NiMH cells in series, the influence of things 
like temperature, cell condition, and other factors make automatic 
detection of full charge at moderate charging currents unreliable.  A 
timed charge may not be elegant, but it's the approach recommended by 
the major battery manufacturers when not doing a fast, smart charge.  
And the latter requires much higher current and was just not feasible in 
the KX3 due to safety, power requirements, heat, RFI, and a few other 
things.

Hopefully this helps explain a bit why the charger operates as it does.

73,
Rich  AC7MA




On 01/30/2013 10:33 AM, Alan Davenport wrote:
> Your questions are valid. I asked the same ones myself. I'm also disappointed in the charger. I also found that it is very fussy about input voltage. I run my 12v rigs in my shack off deep cycle marine batteries which I keep topped off with a high end marine smart charger (ironic!) When the smart charger shuts down because the batteries are topped off the charger in the KX3 refuses to charge its batteries while stating "input voltage too low". There is nothing wrong with my shack batteries. This issue (input voltage too low) occurred even when the shack batteries were brand new. (I had replaced them all just prior to hurricane Sandy.)
>   
> I'm sorry Elecraft. All your other gear it a grand slam home run. This charger is just a bloop single.
>   
> 73 de Al, W2GZN
>   
>   
>
> ________________________________
>   From: "kissov at me.com" <kissov at me.com>
> To: elecraft at mailman.qth.net
> Sent: Tuesday, January 29, 2013 5:53 PM
> Subject: [Elecraft] KX3 Smarter Charger
>    
> I don't claim to know about battery charging but why can't the charger stop when the batteries get to "X" volts and start charging when the total battery voltage drops below "Y" volts, would this feedback system add to the cost of the charger.  I bought a "smart charger" at Harbor Freight for $5.99 the red light goes green when the batteries ( 4 NiMH) are fully charged and when you put depleted batteries in it it blinks red. Why can't the charger in the KX3 be totally transparent? load and forget.  I found the whole charging instructions confusing after an initial 16 hour charge do I occasionally give it a 4 hour charge or what ? do I want a lot of hot batteries in there?
>
> K6CG
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