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I deep discharged two 4 cell (2S2P) 7.2 V Lithium Thionyl Chloride battery packs each connected to a GPS tracker. I ran the two packs until each GPS tracker stopped communicating. (can't be certain what the cut-off voltage was).

enter image description here

After retrieving the packs I noticed three batteries in each pack had about 15 mV and one connected to the positive cable had the normal 3.6 V (circled in the picture). All fuses were intact.

Are these normal voltages for a deep discharged lithium battery pack? I expected all cells to still be 3.6 as they usually recover after normal discharge conditions with a cut off of about 2.5 V. And why has one cell in each pack recovered to its full voltage?

I'm trying to understand the principles of how a pack like this discharges.

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  • \$\begingroup\$ If 3 out of four only registered 15 mV then because their configuration was 2S2P (i.e. one good cell must have been taken to 15 mV due to it being in parallel with a bad one) then all four should have been discharged to that bad level unless you screwed things up taking them apart? \$\endgroup\$
    – Andy aka
    May 29, 2020 at 9:29
  • \$\begingroup\$ Hi Andy, I added a picture of the pack to give you a better idea there. The exact same thing happened with two packs and I disassembled very carefully, all connections in tact, all batteries were good. According to the coulomb counter I got close to full capacity from each pack. But I'm looking for an explanation of why the voltages are different within the pack, I'm not necessarily saying there's a fault, just the principle of how it discharged for my understanding \$\endgroup\$
    – Karl Peters
    May 29, 2020 at 9:49
  • \$\begingroup\$ If the cells are all OK then what is the 15 mV measurement all about? \$\endgroup\$
    – Andy aka
    May 29, 2020 at 9:51
  • \$\begingroup\$ When I measure the voltage across 3 of the cells in each pack, I get 15mV ... what's it all about... this is my question... \$\endgroup\$
    – Karl Peters
    May 29, 2020 at 9:57
  • \$\begingroup\$ How can a cell register 15 mV? If it did it would be dead and unrecoverable. Think about it. Then you say all the batteries were good. How can this possibly happen if three cells measured 15 mV. That doesn't make any sense. \$\endgroup\$
    – Andy aka
    May 29, 2020 at 10:06

1 Answer 1

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More information and more measurements would help, and the following is only an educated guess, but probably 'has some merit'. The following contains a lot of "just maybe this happened", but just maybe it will provide some clues :-).

Battery data sheet here

There are too many "it may be that's" to be certain, but LiSoCL2's very unusual sudden drop in voltage at endpoint seems a likely factor.

You say the batteries are arranged as 2S2P. This has two possible configurations - with and without the midpoints of the two x 2S strings joined. The photos shows metallic tabs where the mid points are adjacent - circled in red.

enter image description here

But your comments suggest that each 2S string may be joined at top and bottom only.
Knowing which configuration applies would help.

LiSoCL2 cells have a discharge wit time curve quite unlike that of most other battery chemistries. At constant current, battery voltage rises slightly with depth of discharge and then plunges extremely suddenly when the cell is almost depleted.

enter image description here

If a cell has slightly more capacity than one it is in parallel with or than the cells in a string in parallel with the string that it is in, then the sudden collapse of voltage in one string may lead to an imbalance which results in current flow between strings. Once the target load is removed the highest capacity cell may recover - possibly exacerbating the transfer of charge into the other string. If there is a small ongoing load (here, after the GPS stops working) this may clamp both strings to the voltage of the lowest string. While the cells are not meant to be "rechargeable" it is common for many chemistries to exhibit a limited charging capability. In some cases this is negligible and in others (eg Alkaline primary) can be substantial.

SAFT Primary Lithium Batteries Selector Guide - 2009 here
2005 version & 2002

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  • \$\begingroup\$ Hi Russell, this is a good analysis. To answer your question each series string is connected at the top and bottom only. The transfer charge theory is a compelling one if each series string had not been protected with a blocking diode. Also each cell is protected with a bypass diode. That's why I'm still struggling to work it out... \$\endgroup\$
    – Karl Peters
    Jun 2, 2020 at 9:08
  • \$\begingroup\$ @KarlPeters I'd expect (but would not be sure) that if you joined the cells at the two midpoints that you'd get different results. Still a possibility of one cell having higher capacity than its twin and transferring charge. Logging battery voltages along the way would be interesting and relatively easy. An eg Arduino and two resistive dividers for the high side cells would do the job. Storage to non volatile memory "would help" :-). || Questions from curiosity if I'm allowed to ask :-) : || Those are costly batteries - what is the application and how long do they operate the GPS for? \$\endgroup\$
    – Russell McMahon ♦
    Jun 2, 2020 at 10:54
  • \$\begingroup\$ Why use LiSOCl2 batteries - presumably the GPS current drain is high enough that the shelf life of the cells is not a major factor. Maybe it's due to mass or volume constraints. The 1 Ah is quite a significant capacity - you'd need about 16 x 18650 LiIon cells to match these 4 cells. | Or temperature? Or ... . | I don't recall seeing a battery with a NATO stock number before :-) ( = 6135 14 440 1213) \$\endgroup\$
    – Russell McMahon ♦
    Jun 2, 2020 at 10:58
  • \$\begingroup\$ I'm not allowed say what the exact equipment is but this pack should work for about a month - depends on how active it is, it'll go to sleep when it's not moving so could potentially last for months. The reason we use these cells is because of their energy density, fit a lot of power into a small space. Also the self-discharge is very low - you can store them for 10 years no problem. Li-ion cells would work but the pack would be much bigger, and in some use cases the battery can't be retrieved so its rechargeability is useless. \$\endgroup\$
    – Karl Peters
    Jun 4, 2020 at 12:46
  • \$\begingroup\$ how do you think joining at the midpoints would change things? Is there any advantage to doing that? \$\endgroup\$
    – Karl Peters
    Jun 4, 2020 at 12:47

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