[Elecraft] KX3 Smarter Charger

Wed Jan 30 14:55:12 EST 2013

Smart chargers for Nimh rely on the cell to show a certain percentage of
voltage drop when full charge is attained - around 10 mV per cell. This
occurs most reliably at fairly high charge currents. With 8 cells in
series, that voltage drop can easily be masked by what is happening in
other cells, such that while some cells are still charging, others may be
getting seriously overcharged to their detriment.

This type of charging also requires a constant current charge to allow
detection of voltage depression. Constant current supplies use switching
regulators - so more noise in the shack.

Additionally, as Nimh cells age, that voltage depression becomes less
pronounced. I've had even good quality smart chargers miss end of charge
even when only a single cell is involved.

Then you have to keep in mind that these cells are very close to some
expensive high density electronics. I think rupture or outgassing of Nimh
cells is rare, but I wouldn't want to chance it. It seems Elecraft's more
conservative approach is wiser.

Chip
AE5KA

On Wed, Jan 30, 2013 at 1:30 PM, Walter Underwood <wunder at wunderwood.org>wrote:

> I can think of one reason to avoid charging every time external power is
> applied -- external batteries. If you are running from 14V LiFePo batteries
> on the trail, you don't want to charge the internal NiMH batteries at the
> same time.
>
> wunder, K6WRU
>
> On Jan 30, 2013, at 11:22 AM, Rich Heineck wrote:
>
> > 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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> --
> Walter Underwood
> wunder at wunderwood.org
>
>
>
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