" writes:
I had a set of 4 PowerEx 1800 mAH batteries from several years ago. I
threw one of them away because my charger wouldn't charge it anymore,
so now there are 3 of them left. I just bought a new charger, which
came with 2 Duracell 2000 mAH batteries. Is it OK if I take one of
the Duracell batteries and combine it with the 3 PowerEx batteries to
form a set which I can use in my camera that takes 4 batteries?

In general, no. In the best case, the capacity of the set of 4 is only
equal to the weakest of the 4 cells. But in the worst case, you will
reverse-charge the weakest cell and destroy it.

When you use 4 cells in series (which is almost certainly what a camera
that takes 4 cells does), you want all 4 to have the same capacity plus
or minus a few percent. When you use the camera, all cells deliver the
same amount of current to the load (because they're connected in
series) and all approach "empty" at about the same time. When the
weakest cell finally drops below 1 V, all the other cells are also near
1 V, the total voltage drops below 4 V, and the camera ought to shut
down *while the weak cell still has some amount of charge*.

If you have one cell whose capacity is less than the rest, it will reach
"empty" while the other cells are still at 1.2 V or above. The total
pack voltage might not drop enough for the load to detect "low battery",
and continue to draw current - which the remaining cells are happy to
supply. Since all the cells are connected in series, this current flows
through the "empty" cell too, and actually starts charging it in reverse
polarity, with the "positive" terminal more negative than the case.
This will eventually destroy the weak cell. This is most likely to
happen when there are more cells in the pack (8 cells is worse than 4),
and when there is just one cell weaker than all the rest.

In the particular case you're asking about, you propose to add one
higher-capacity cell to 3 lower-capacity ones. That's less likely to
be a problem than with one lower-capacity, so you'll probably get away
with it. But given the low cost of 2 more new 2000 mAh to make up a
set of 4 all-new cells, why not do that?

I keep NiMH cells in "sets" of 4 when they are in good shape. But if
one fails, I use the remaining three in applications that need 1 or 2
cells, and use a new set of 4 for the devices that need 4.

Dave