3478a Lithium battery swap question - Page 1
Forum section: electronics / testgear

author: wolfalex
author_rank: Regular Contributor
timestamp: February 25, 2014, 02:17:17 am
content: 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 allI installed the following new Lithium battery:http://www.digikey.com/product-search/en/battery-products/batteries-non-rechargeable-primary/394467?k=P226-NDBefore 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 batteryand 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.

author: sleemanj
author_rank: Super Contributor
timestamp: February 25, 2014, 03:44:27 am
content: You measured the battery unloaded.Now the battery is loaded (minimally).It's a 3v battery.You have measured 3v.

author: wolfalex
author_rank: Regular Contributor
timestamp: February 25, 2014, 04:23:28 am
content: 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 solderingiron 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 VVoltage on RAM without additional backup 2,946 VNew soldered in battery terminal voltage with additional battery on RAM 3,156 VVoltage on RAM with additional backup battery (same type battery) 3,373 VIf you compare the terminal voltage for the scenario with backup vs. without backup battery it seems that the battery is not really loaded. Thereis almost no change in voltage, but instead it seems the open circuit voltage did change during the installation in theunit. 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. 90minutes. So something is still changing.

author: wolfalex
author_rank: Regular Contributor
timestamp: February 25, 2014, 03:24:59 pm
content: Battery has now 3.22V terminal voltageIf the increase continues with this pace I'm back at the starting point tomorrow

author: mimmus78
author_rank: Unknown
timestamp: February 25, 2014, 03:50:14 pm
content: Maybe you accidentally shorted it for a few fraction of seconds and now it's recovering ?

author: wolfalex
author_rank: Regular Contributor
timestamp: February 25, 2014, 04:47:17 pm
content: Hello mimmus78,Thanks to your idea I think I know now what happened and what I did wrong. I feel like such a noobDespite being very careful about the soldering iron/ground/short topic Icompletely 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 swapDoes somebody know what it means for my poor battery, despite losingprecious 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 considerit as safe now?Thank you very much

author: mimmus78
author_rank: Unknown
timestamp: February 26, 2014, 09:13:45 am
content: 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 therisk to redo the swap.

author: Wh1sper
author_rank: Regular Contributor
timestamp: February 26, 2014, 09:21:48 am
content: 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

author: amyk
author_rank: Super Contributor
timestamp: February 26, 2014, 12:54:57 pm
content: Quote from: wolfalex on February 25, 2014, 04:47:17 pmThe internal resistance of these batteries are so high that the short-circuit current is limited - looking atthe datasheetcurrent-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.

author: wolfalex
author_rank: Regular Contributor
timestamp: February 26, 2014, 02:57:25 pm
content: 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 notrupture 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 ApproachI could verify that.(thanks for the input). But I'm not sure if I want to play around with my battery any moreEverytime I get near my open 3478A with a cable to do something I'mafraid 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.