# Reverse charge protection for parallel primary lithium cells (CR2450)

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:

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:

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!):

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 ?

• What is your load current? Commented Dec 1, 2020 at 21:52
• 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. Commented Dec 1, 2020 at 22:56
• 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. Commented Dec 1, 2020 at 23:14
• @John Birckhead John, my load current is usually around 15uA with short peaks of around 50mA. Commented Dec 2, 2020 at 0:02
• @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. Commented Dec 2, 2020 at 0:06

Suggestion 1: Place a PTC self-resetting fuse between the two cell holders. If the user installs cells at differing states of charge, the PTC fuse will trip and limit the current

Suggestion 2 (I am sure you already though of it): Use a single, bigger cell

Suggestion 3: Use only one cell and have the user change the cell twice as often. If internal resistance becomes an issue, add a supercapacitor to supply the impulses of current when the load is fully on.

Suggestion 4: Don't use cell holders - solder cells with tabs directly to the PCB

Suggestion 5: Implement an ideal diode for each cell with a MOSFET and an ideal diode IC.

Do not connect cells in series. It avoids the inrush of current between the cells, but it introduces a new problem: the voltage of the cell with the lowest charge will reverse during use, which is also dangerous.

Here is a solution for you.

Schematics with a comparators. Use it.