Cells returning unexpected voltage readings - Page 1
Forum section: electronics / beginners

author: JacobP
author_rank: Contributor
timestamp: March 16, 2017, 02:32:47 am
content: Good evening everyone,My wife purchased a secondhand toy for our son and when we got it home, it wasn't working correctly, so I did some investigation. First thing I suspected was that the batteries were dead (obvious, I know). Since it was handy, I grabbed my DMM and got some values that I didn't expect.It is powered by 4 C cells and the previous owner had put in two that were national brand and two that were a store brand. I tested one of the store brand batteries, putting the red lead on the positive end of the battery and it read 0.8280v. The national brand battery, measured the same way with the read lead on the positive end, read -0.3125v.I assume the difference between the cells is a result of the previous owner mixing older and newer batteries. However, that still leaves me two a question: How can the voltage on one cell be positive and the other be negative? it doesn't seem like that should be able to happen.

author: IanB
author_rank: Super Contributor
timestamp: March 16, 2017, 02:39:15 am
content: If you put a mixture of different cells in a product (don't do this!) then maybe some cells will be much stronger than others. If this happens, then when the weaker cells are fully discharged the stronger cells can push current through them from the negative end to the positive end and reverse their polarity. This is bad, because pushing current through discharged cells like this can damage rechargeable cells and can make alkaline cells leak and spill their guts all over the battery compartment.This is what I suspect happened to the cells you measured. You measured a discharged cell that had had its polarity reversed by stronger cells pushing current through it.

author: JacobP
author_rank: Contributor
timestamp: March 16, 2017, 02:53:27 am
content: Thanks for the explanation! This is one of the most interesting things I've learned all week.Something else interesting that I observed: if I put the cells together so they are in series, the overall voltage is ~.5v, or the sum of the voltage from the individual cells added together. What causes that? Is one battery basically acting as a resistor?

author: IanB
author_rank: Super Contributor
timestamp: March 16, 2017, 03:05:10 am
content: When you put cells in series the overall voltage is always the sum of the individual voltages added together. This is what putting cells in series means, all the voltages get added up end to end. Therefore (0.8 V) + (-0.3 V) = 0.5 V.

author: helius
author_rank: Super Contributor
timestamp: March 16, 2017, 03:20:39 am
content: Being in series with stronger cells isn't the only reason that a cell may develop a "reverse" voltage. It happens just from sitting in storage too long. As the cell deteriorates, it loses ionic conductivity, and so the internal resistance becomes higher, andthatis what causes the voltage to drop. Beyond a certain point, the resistance is so high that the main electrochemical cell is not any stronger than the unintentional cell caused by the case corroding: so the voltage appears to be reversed (it can't supply much current at all this way though)