Elecraft KX3 Internal NiMH Power Supply Voltage after Charge

[Steven C Veal]

Hello group:

Last night I installed new NiMH batteries (the Powerex 2700 MaH variety) into my week old KX3 for the first time and set them for a 12 hour charge. After the charge was complete, the KX3 PS display indicates a power supply voltage of only 10.8 volts, which drops to well below 10.0 volts on transmit.

I'm sure that this is lower than it should be. I haven't re-opened the case to check for bad or ill-fitting cells yet as it is a little bit of a tricky open.

So I guess the questions are these.
1) What voltage have other group members experienced with the internal NiMH PS after charge?
2) What charge duration are other folks using?

Thanks in advance for your responses!
Steve, N5SKH

P.S. What a great radio...
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[Greg Troxel]

Steven C Veal <[hidden email]> writes:

> Last night I installed new NiMH batteries (the Powerex 2700 MaH
> variety) into my week old KX3 for the first time and set them for a 12
> hour charge. After the charge was complete, the KX3 PS display
> indicates a power supply voltage of only 10.8 volts, which drops to
> well below 10.0 volts on transmit.

Those are not the low-discharge variety, so are likely totally
discharged on arrival.  2700/200 is 13.5h, and you need to charge more
than rated capacity because charging is not 100% efficient.  So for
2700, 16h is definitely warranted, and I might recommend 18h on first
charge, but you can't set that.  I'd give it another 4h, maybe 6h
(humuan in the loop timing control from 8h :-).  Undercharging is scary
because it can lead to cell imbalance and you really don't want to
reverse bias NiMH batteries.

10.8V/8cells is 1.35 V/cell.  For a NiMH under any load, that's very
much charged.

10.0V is 1.25 V/cell.  At rest, that indicates a discharged cell, but
under any significant load there is no problem.

Set your BAT MIN to lower; probably 8.4V makes sense (1.05 V/cell, which
you may well see under transmit loads), or even 8.2V.

When doing test discharges on NiMH cells, I use 200 mA and discharge to
0.9V, using a MAHA C9000.  That corresponds to 7.2V for 8 cells.  I
recommend getting one of those chargers and measuring your cell's
capacities before putting them into service and then yearly or if they
seem to be acting up.

I have been happy with the powerex/maha batteries, but have found that
the regular (non-Imedion) ones self discharge quite rapidly.

73 de n1dam

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[N0AZZ]

Steve

I have several sets of batteries for my KX3 and I have 2 sets like yours. I
have found from experience the first time they (or any) need to be
conditioned in an outboard charger/conditioner it takes a full 14 hrs. to
cycle the charge discharge.

Then they go into my KX3 full power and the charger takes care of them just
fine on a 12 hr. charge most of the time. After 10 charge/discharge cycles I
remove them from the radio and recondition them again so they can be brought
back up to where they can hold a full charge.


73,
Fred/N0AZZ
K3 Ser #'s 6730/5299--KX3 # 2573--K2/100--KAT100
P3/SVGA--KPA500--KAT500--W2




-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Steven C Veal
Sent: Sunday, April 14, 2013 9:40 AM
To: [hidden email]
Subject: [Elecraft] Elecraft KX3 Internal NiMH Power Supply Voltage after
Charge

Hello group:

Last night I installed new NiMH batteries (the Powerex 2700 MaH variety)
into my week old KX3 for the first time and set them for a 12 hour charge.
After the charge was complete, the KX3 PS display indicates a power supply
voltage of only 10.8 volts, which drops to well below 10.0 volts on
transmit.

I'm sure that this is lower than it should be. I haven't re-opened the case
to check for bad or ill-fitting cells yet as it is a little bit of a tricky
open.

So I guess the questions are these.
1) What voltage have other group members experienced with the internal NiMH
PS after charge?
2) What charge duration are other folks using?

Thanks in advance for your responses!
Steve, N5SKH

P.S. What a great radio...
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[Bill Frantz]

I have always heard that the rule of thumb for charging a
depleted NiCd of NiMh battery is to charge it to 140% of its
rated capacity. If you are charging 2700 mAHr cells at 200 mA,
then you will need to charge them for 18.9 hours.

Cheers - Bill, AE6JV

On 4/14/13 at 7:56 AM, [hidden email] (Greg Troxel) wrote:

>2700/200 is 13.5h, and you need to charge more
>than rated capacity because charging is not 100% efficient.

-----------------------------------------------------------------------
Bill Frantz        |The nice thing about standards| Periwinkle
[phone]      |is there are so many to choose| 16345
Englewood Ave
www.pwpconsult.com |from.   - Andrew Tanenbaum    | Los Gatos,
CA 95032

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[WB4SON]

Acutally NiMH cells have a Coulometric Efficiency of 66%, which means you
need put 1/0.66 = 1.51 (or 151%) of the charge back into the
battery.(assuming it is fully discharged).

If your target cell were 2700 mAH, and it was dead, then you need to put
2700/0.66 = 4090 mAH.  At 200 mA per hour, that would take about 21 hours.

73, Bob, WB4SON






On Sun, Apr 14, 2013 at 3:53 PM, Bill Frantz <[hidden email]> wrote:

> I have always heard that the rule of thumb for charging a depleted NiCd of
> NiMh battery is to charge it to 140% of its rated capacity. If you are
> charging 2700 mAHr cells at 200 mA, then you will need to charge them for
> 18.9 hours.
>
> Cheers - Bill, AE6JV
>
>
> On 4/14/13 at 7:56 AM, [hidden email] (Greg Troxel) wrote:
>
>  2700/200 is 13.5h, and you need to charge more
>> than rated capacity because charging is not 100% efficient.
>>
>
> ------------------------------**------------------------------**
> -----------
> Bill Frantz        |The nice thing about standards| Periwinkle
> [phone]      |is there are so many to choose| 16345 Englewood Ave
> www.pwpconsult.com |from.   - Andrew Tanenbaum    | Los Gatos, CA 95032
>
>
> ______________________________**______________________________**__
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>
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