EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: wolfalex on February 25, 2014, 02:17:17 am
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Hello,
I had to swap the Lithium battery of my new 3478A in the last hour. Everything went almost well: The new battery is soldered in and I have not lost the RAM calibration data.
But I'm not 100% happy, because something strange happened and I do not understand it at all :(
I installed the following new Lithium battery:
http://www.digikey.com/product-search/en/battery-products/batteries-non-rechargeable-primary/394467?k=P226-ND (http://www.digikey.com/product-search/en/battery-products/batteries-non-rechargeable-primary/394467?k=P226-ND)
Before soldering the new battery in I measured approx. 3.4V at it's terminals. Also a second new battery of the same type still reads around 3.4V. But after I soldered in the new battery
and measured again its voltage directly at it's terminals it was only 3.025V anymore. And now I'm wondering what has happend to the battery, and if something is wrong?
Maybe during the soldering process the battery terminals reached a too high temperature? Does somebody know what could have happened, and what does it mean?
Thank you very much in advance for every Information I get.
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You measured the battery unloaded.
Now the battery is loaded (minimally).
It's a 3v battery.
You have measured 3v.
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Hello sleemanj,
Thank you very much for your input. I really appreciate it. But I'm not sure if it could be loading in this case. When I changed the battery I used the same type of new lithium battery for backup on the RAM chip with an diode in series. The unit itself was completely swichted off and disconnected from the power grid to get no short with my soldering
iron on the posivite terminal of the battery.
But to test your theory I did reconnect the backup battery to the RAM and did some measurements again:
New soldered in battery terminal voltage without additional backup 3,155 V
Voltage on RAM without additional backup 2,946 V
New soldered in battery terminal voltage with additional battery on RAM 3,156 V
Voltage on RAM with additional backup battery (same type battery) 3,373 V
If you compare the terminal voltage for the scenario with backup vs. without backup battery it seems that the battery is not really loaded. There
is almost no change in voltage, but instead it seems the open circuit voltage did change during the installation in the
unit. What do you think?
But it is interesting that the terminal voltage on the new battery changed from 3.025V to 3.155 V in approx. 90
minutes. So something is still changing.
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Battery has now 3.22V terminal voltage ;) If the increase continues with this pace I'm back at the starting point tomorrow 8)
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Maybe you accidentally shorted it for a few fraction of seconds and now it's recovering ?
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Hello mimmus78,
Thanks to your idea I think I know now what happened and what I did wrong. I feel like such a noob |O Despite being very careful about the soldering iron/ground/short topic I
completely failed on another issue, and I think in this way I have shorted the battery during soldering.
I used the solder tin directly from the spool, and the spool was located on the table outside from the unit. And of course the chassis = GND of battery. And I think what happened was that the solder tin wire touched the chassis during soldering on the plus pin of the battery. The bad thing was I did not see any spark, because otherwise I would have stopped soldering immediately.
I can't believe that I made such a mistake, but I was also very nervous about the battery swap :( Does somebody know what it means for my poor battery, despite losing
precious charge? I know it's hard to tell from the distance, but maybe somebody has experienced the same one time. If the battery did not blow up until know, can I consider
it as safe now?
Thank you very much
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Well I really don't know what is discharge rate of this battery when shorted ...
but if it has recovered in a day I don't think it was so long. I won't take the
risk to redo the swap.
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Recently, I was wondering how about the state of emptiness of the battery in my 3468a.
I took the Datasheet and painted two curves in it.
the RED curve is 0.25mA discharge (12K Ohms)
and the GREEN at 2.5mA discharge. (1.2K Ohms)
This way you can measure the Fullness of the Battery at ca. 20° Celsius.
My curves shows a 50% Fillgrade.
So take a 1.2K Resistor connect with Voltmeter and measure the Voltage at the BR2/3a
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The bad thing was I did not see any spark, because otherwise I would have stopped soldering immediately.
The internal resistance of these batteries are so high that the short-circuit current is limited - looking at the datasheet (http://panasonic.com/industrial/includes/pdf/Panasonic_Lithium_BR1-2AA_BR2-3A.pdf) current-voltage graph, a rough calculation gives ~6 ohms - meaning short-circuited it would only be supplying ~500mA. They are designed for low current, low self-discharge and very different from e.g. an 18650 which will give tens of amps short-circuited.
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Today morning the open circuit voltage at the battery terminals reads 3.27 V. So somehow I think it will never reach the starting point of 3.37 V again. But on the other side I do not wonder that a short does maybe change something internally.
The high resistance is a really good explanation why I did not see any spark, thank you amyk for pointing that out. I guess the resistance is also the reason why the battery did not
rupture in the process.
I think if the discharge current was so limited by the internal resistance then I did not loose very much of the capacity during the process. And maybe with using Wh1sper's Approach
I could verify that.(thanks for the input). But I'm not sure if I want to play around with my battery any more :) Everytime I get near my open 3478A with a cable to do something I'm
afraid of a short circuit and bricking the unit.
So capacity should be ok. The only thing I constantly think of is safety of the battery in future if it stays in the unit. Can this batteries also have a "Thermal-run-away" event? Until now nothing special happened.