Most likely you were talking to a milk expert. In order for one battery to
discharge into another there must be a voltage
difference between two - one battery charged and another drained.
Discharge/charge current will be limited by voltage difference (open ckt.)
divided by sum of internal (source) impedances of both batteries. After
initial surge current will exponentially go down to zero as result of
equalizing of battery voltages. Since battery internal impedances are very
low and there were no other power dissipating loads in a ckt., there is no
signifficant energy loss.
In other words both batteries may be connected together. As soon as they
both charged -there no current from one to another. Charging current will be
shared by both differently every time due to difference in charge and
capacity..
Unlike bad milk there are no bad electrons.

Good luck.