Corroded battery readings

[Causailty]

Another question, as you all answered my last one so efficiently.

Using my multimeter to measure if non rechargeable(alkaline) batteries are 'good', which has been successful.

The 'bad' ones were reading mV.
But one 'bad' one was reading negative mV?
And a corroded 'bad' one seems to be reading inconsistently unlike the others, i.e. not one fixed number.

In the hope of learning, I am wondering if there is any technical reason for this and that?

[peter.mitchell]

But one 'bad' one was reading negative mV?

A common reason is reverse discharge;

If the battery is in a device where the batteries are in series, the current will flow through all of them, discharging them. Some will reach 0v before others, because not every battery has the same capacity or internal resistance. When the battery becomes flat, but the batteries around it are not, the battery will have a positive charge on its negative terminal and a negative charge on its positive terminal from the other batteries, with the device completing the circuit, this causes the empty battery to begin charging in reverse.

Another reason may be that the chemicals in the battery behave in unexpected ways when the cells loses charge.

And a corroded 'bad' one seems to be reading inconsistently unlike the others, i.e. not one fixed number.

This is likely due to a poor electrical connection between your multimeter probes and the battery or the case of the battery and the internal materials, though it depends on how much it is fluctuating. The corroded terminals may still also have corrosive chemical on them, reacting with your probes creating a slight charge.

[Causailty]

Awesome thanks