"Meindert Sprang" wrote in message
...
snip

The charge of a battery is the product of current x time.
True, but irrelevant

Both batteries are in series and one load is connected to the set,
operating
at 24V. Another load is connected across only one battery, operating at
12V.
So it is evident that one battery is discharged more than the other.
Yes, but you don't need the load - batteries will always have different
charge levels in individual cells, whether in the same case or not. It's
easy to measure the differences.

I start to charge the set in series, so the current through both batteries
is exactly the same.
No. You are failing to understand that the charging process is a chemical
reaction that converts energy into a differnt form.. The current flowing at
the positive and negative terminal of the whole string of cells is the same,
obviously, but the current that flows 'through' any cell is a complex result
of the conversion processes occurring inside the cells of which it is a
string. Each cell can be considered an energy sink. The amount of energy
imparted to each cell will differ, either within a single battery or within
a series of batteries. This is not a problem for the charging process. The
current flowing across any cell, or any group of cells, is most definately
not equal, and there is no reason it should be.

Since one battery is discharged more than the other and
the current throug both is the same, one battery must be charged longer
that
the other.
No. All cells are charged for as long as the charge is applied. It makes
no sense to say that one is charged longer than the other. Perhaps you
meant to say that some will continue to have the full potential applied when
they are not accepting additional energy - this is normal in any charging
process and causes no problem for the battery. It is, in fact, exactly what
happens when any battery is fully charged and a maintaining charge is being
applied. It is no different for the battery as a whole as it is for the
individual cells.

Exactly how am I going to achive that with the same current
through both batteries?
You do not achieve that, and you don't need to.

One battery will reach the full state before the other but is still being
charged with full current because the other battery hasn't reached the
voltage that corresponds with full charge.
One battery wil reach the full state before the other, like one cell will
reach the full state before some others. It's not a problem. It is not being
charged with full current because the 'load' that the system presents to the
charging process is reduced by the reduced charging requirements of the cell
(or cells) that have reached or are approaching full charge. The current
that can be absorbed by a bank of cells depends on the charge state of the
cells in the string, not the charge state of the most discharged cell.
That's why I said before that the system 'monitors' the state of the string
of cells, not individual cells. If the charge rate was controlled by the
most discharged cell then modern lead acid batteries, with typical
manufacturing differences, wouldn't last any time at all.