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My bike light uses two CR2032 batteries, which I replaced with CR2025 and some padding. It stopped giving light, so I removed the batteries and measured voltage. One battery gave 2.8 V, and another -1 V. I checked with new batteries that the polarity was right and confirm that the battery had negative voltage. I replaced with a new CR2025 and the previous positive battery and the light worked again.

The light was working before, so the "negative" battery had to be around 1.5V, and I did not notice it being backward inside the light when I removed it.

How can a battery reverse its voltage like this?

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  • \$\begingroup\$ How did you measure that voltage, exactly? \$\endgroup\$ – Marcus Müller Oct 23 at 16:47
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    \$\begingroup\$ Did you replace both CR2032s with CR2025s at the same time and were both CR2025s new and identical when you first installed them? \$\endgroup\$ – pericynthion Oct 23 at 17:01
  • \$\begingroup\$ @MarcusMüller I measured voltage with a multimeter and the setting continuous voltage, 20. \$\endgroup\$ – miguelmorin Oct 23 at 20:09
  • \$\begingroup\$ @pericynthion I replaced both CR2032s with CR2025s at the same time. I believe the CR2025s were new, as I have no other use for these batteries, but I can try again with new batteries and the same light if needed. \$\endgroup\$ – miguelmorin Oct 23 at 20:10
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Since the cells are in series, they both have the same discharge current. When the cells are perfectly matched they both have an identical discharge curve. If the cells are not identical, one will discharge completely before the other cell. Since the current through the cells is identical one cell can end-up reverse charging the depleted cell.

Consider the following basic simulation, where two capacitors with dissimilar capacitance are discharged in series.

enter image description here

enter image description here

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If the two cells are in series but one has significantly less capacity that the other, it can discharge first. The other cell will then discharge the expired cell even below zero as you have seen. If cells are in series you should ensure they are in the same condition, do not pair a new cell with a partly used one.

This same effect can occur with rechargeable cells and in that case the cell that reverses polarity can be damaged permanently. You should always be careful about over discharging a series battery.

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How can a battery reverse its voltage like this?

This is unusual (assuming that the batteries were measured in the same way), and outside of normal battery operation.

The polarity of the batteries is not different, and they have the same voltage, if the battery was at a negative voltage, it was probably damaged.

The difference between batteries is only size and capacity, but not voltage or polarity:

The 2025 battery is 2.5mm deep and holds a capacity of 160-165 mAh The 2032 battery is 3.2mm deep and holds a capacity of 225 mAh

enter image description here
Source: https://www.abcdiamond.com/what-is-the-difference-between-cr2025-and-cr2032-batteries/

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