One thing to keep in mind is that rechargeables do wear out over time. So it's best to always use all four of them "together" and always recharge all four "together". That way when the "set" begins to fail, you'll know to throw them out. If you separate them, a single battery with problems could make the other three (in the flash) appear bad as well.
To that end, I really like these batteries which I recently purchased. For one thing, they are 2700's which is the highest rated batteries I have so far. For another, they are only $7.99 for a set! And the best thing, in my opinion, was that they came with a plastic hold that keeps all four of them together.
In the past, I always bought different batteries so that I could keep the "sets" separate from each other according to the color scheme of the battery. But with the case, I bought several sets of these and don't have to worry about them accidentally getting mixed up.
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