Cell/Battery question. Mixing types

In many places on the net you can find warings like the following but with no explanation.

Warning: Do not connect batteries of different chemistries, voltage or capacity in series.

I can see where this is true for parallel batteries but not series.

What happens if the series cells are identical but with a different level of charge ?

3v0

As the series string discharges, ulitimately, the discharge current will reverse polarize the lower capacity cell. May cause it to leak or even explode.

3v0 said:

What happens if the series cells are identical but with a different level of charge ?

3v0

Click to expand...

The weakest or less charged cell will conduct the same current as the most charged cell as they are in series, and will be depleted first to zero voltage.

Further discharge below zero becomes negative contributing nothing to the series string which starts to charge that one in reverse polarity.

That makes sense to me. Thanks to both Mike and Externet.

3v0