Is it OK to mix NiMH batteries?

On Wed, 20 Aug 2008 10:08:39 -0700 (PDT),
wrote:

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?

Yes it will work, but not ideally. Rechargeable batteries lose
capacity from use and time, but unless you have equipment that can
measure the actual capacities you'll just have to guess what those
1,800mAh cells are now capable of providing. They may well have
lost 1/2 of their "new" capacity, and if so, even with a new
Duracell, the entire set of four AA cells will only be as good as a
set of 900mAh batteries. When used, as soon as the first PowerEx
goes flat, the remaining PowerExes will be almost out of juice, and
with those four were used in your camera, it probably wouldn't be
able to operate any longer even though the one Duracell still had
more than 1/2 of its capacity remaining.

You'll be much better off buying either an additional two 2,000
mAh Duracells or even better, a 4-pack of what are called low
self-discharge NiMH batteries. This type (Sanyo Eneloops, RayOVac
Hybrids, etc.) maintain their charge far longer, easily a year or
more, so you won't have to worry about batteries going flat if they
weren't charged recently. Whether they're identified as low
self-discharge types on their packages or not, you can be sure that
you've found them if the package states that they're "pre-charged",
and can be used immediately, without first requiring a charge.

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?

My suggestion would be to use 2 of the PowerEx 1800s and the 2 Duracells
that came with the new charger. At least keep them in matched pairs. The
preferred method would be to buy 2 more matching Duracell 2000s.