[Elecraft] KX3 and charging batteries (when, and why do they get hot?)

Thu Jun 6 21:26:08 EDT 2013

>>...must have halted more often over the course.
>>Is this normal?

Yes, with some concern.  The short answer is I suspect you have a bad cell.

The KXBC3 will attempt to keep the temperature at or below 40deg C, and sill suspend charging as required to allow cool down.  It is a bit troubling that you didn't get more out of the first discharge cycle.  It could be that they just sat on the shelf for a long time, but it could also be caused by a one or more defective cells.  A defective cell may generate heat when charging, causing the KXBC3 to suspend charging.  That can also happen with all good cells, but usually when ambient temperatures are high or during simultaneous transmission and charging.  

I recommend that you measure the cells' voltage individually.  I know, it is a pain to split the case, but you may not have to take them out to check them.  I haven't tried that.  Just be very careful with the probe if you don't.  When measured individually with a volt meter, they should all be within 0.1 volt or so of each other, and all of them should be above about 1v.  Any cells below 0.9v should certainly be considered faulty.  If NiMH cells are run much below 1.0v, they will most likely be damaged.  Any cells that have damage to their plastic case should be discarded as well.

A couple of weeks ago, I charged up a box of 14 used Energizer NiMH cells (2500maH) of unknown origin which had been given to me. I just checked them today, and 10 measured slightly higher than 1.3v each (excellent), one measured 1.28v (fairly good), and 3 measured less than .9v (BAD).  Given this, 10 are good, 1 is a bit questionable but will probably level out with the others when charged a few times, and the 3 low voltages ones will be sent to the battery recycle bin.

>>Were the batteries still way too full when I started charging?
No.  The KXBC3 charges with a constant current of approximately 200ma.  At that current, you will not overcharge 2000aH (or larger) NiMH cells, and no damage will occur as long as the temperature is kept below 40deg C.  So even if they were completely full, you could charge them with the KXBC3 for the full 16 hours without significant damage.  That doesn't mean you should keep them on charge all the time.  That would certainly shorten their life.  I think the KXBC3 documentation mentions something about that.

>>When should such batteries get charged, at what voltage level?
Don't let them go below 1v per cell...about 8 volts total on the KX3 display...8.5V would be safer.  Elecraft recommends setting the BAT MIN menu to 8.5 V (KX3 Owner's Manual Pg. 36) to give you some warning that you need to think about recharging.  This is about 1.06V per cell.  The KX3 will shut down at 8 V which is about 1V per cell, very near the absolute minimum for NiMH chemistry.  There isn't much energy left below 1.06v per cell, so running them below that level really doesn't give you much extra operating time.  Some folks set the BAT MIN to 9V to give them more warning and plenty of time to finish their QSO and shut the station down.

By the way, the NiMH cells do not need to be fully discharged before starting the charge cycle.  They do not have the strong "memory" that the older NiCad chemistry has.  They can be "touched up" as you feel they need it.

My Eneloope and Panisonic NiMH cells are very consistent, especially after several charges.  The voltage of NiMH cells tend to self level (equal voltage of all cells within the battery of 8) when charged with the KXBC3.  I don't have any personal experience with the Rayovac cells, but I think I recall that some folks had some problems with brand new ones right out of the box.  I suggest you check the voltages of the cells individually, and get rid of the ones that measure below 1v and replace them with new cells of the same brand/capacity.  Once you get a good set of cells, you will gain more confidence in them and in the KXBC3, and you will be able to tell where you are in their charge/discharge cycle by the KX3 displayed battery voltage.

Hope this helps.

Mark
KE6BB

________________________________
 From: Arno Dienhart <ad at broadweave.net>
To: Elecraft at mailman.qth.net 
Sent: Thursday, June 6, 2013 4:12 PM
Subject: [Elecraft] KX3 and charging batteries (when, and why do they get hot?)

So I got a new, factory-assembled KX3 yesterday, with charger installed. 

I bought eight Rayovac Platinum NiMH 2000mA cells “ready to go”, inserted
them, and ran the radio on batteries for just a few minutes, when I got the
LO BATT message. So I started charging them, with timer on 12 hrs.

After about two hours, they had gotten over 40° and charging halted until
they had cooled down again. The next morning, charging was in its last 7
minutes, so it must have halted more often over the course.

Is this normal? Or is my ingoing voltage too high, coming from a wall wart
rated at 13-15.8V @ 960mA?

Are the batteries (or one/some of them) defective? I bought them in two
packs of four each.

Were the batteries still way too full when I started charging? I didn’t
check voltage at the LOW BATT warning. When should such batteries get
charged, at what voltage level?

Other than that, I cannot say much because I am still working on an antenna.
But the CW code practice setting works really nice :-). It even goes up to
700 Hz (not 600 as per manual).

Thank you for your expert advice.

73,

Arno

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