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First time poster, long time follower here.

I am looking into the safety aspect of using primary lithium cells in parallel. The reason behind using parallel cells is to increase overall capacity. In an ideal world the two cells would be the exact same voltage and capacity, in this case there would not be a problem:

schematic

simulate this circuit – Schematic created using CircuitLab

The problem comes when partially or fully discharged batteries are mixed with new batteries, thus creating a situation where the discharged cell could be reverse charged by the new cell. This is a big "no no" for primary lithium cells and could result in explosion. BAT1 in this case is the discharged cell:

schematic

simulate this circuit

I need to put some form of protection for the cells. I cannot use series diodes like in the circuit below due to and unacceptable voltage drop (the cut-off for my circuit is 2.2v and it needs to last a couple of years!):

schematic

simulate this circuit

I looked at using a PMOS configuration as posted at Protect lithium battery in parallel although this seems to only protect for reverse polarity i.e connecting the cell the wrong way round, not reverse charge between cells.

Are there any other ways of protecting the cells from being reversed charged ?

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  • \$\begingroup\$ What is your load current? \$\endgroup\$ – John Birckhead Dec 1 '20 at 21:52
  • \$\begingroup\$ If it needs to last a few years, why not simply balance those batteries beforehand to make a single larger battery to begin with? And use similar batteries, instead of one old and one new, or one charged and one discharged. \$\endgroup\$ – Justme Dec 1 '20 at 22:56
  • \$\begingroup\$ Consider wiring the cells in series instead, and using a buck converter to provide the lowest voltage your circuit will work with reliably. Even though the buck converter will cost you some efficiency, you will gain the ability to use those batteries deeper into their discharge curves. Also, the batteries will be running at lower average currents which prolongs their life. \$\endgroup\$ – Mark Leavitt Dec 1 '20 at 23:14
  • \$\begingroup\$ @John Birckhead John, my load current is usually around 15uA with short peaks of around 50mA. \$\endgroup\$ – zombie365 Dec 2 '20 at 0:02
  • \$\begingroup\$ @Justme unfortunately I cannot stop the end users from mixing old and new cells. not everyone in the world reads the "do not mix batteries" warnings. \$\endgroup\$ – zombie365 Dec 2 '20 at 0:06

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