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I had used NiMH rechargeable batteries for many years and had become quite used to their ways. I was very happy with them and mostly very impressed by their long life. I had got resigned to the fact that they never stored more than 80% of the maker's claimed capacity! However all that changed one day when I bought a batch of replacement AA batteries and discovered that half of them were defective and the other half had about 20% of their expected capacity.
Thinking that I must have had a bad batch, I bought another batch of a different make. To my horror I found that these too were giving about 20% of the claimed capacity. I started to do a little systematic checking and I think that you might like to see the results I got.
No special equipment, but I own a superb battery charger which can also check the capacity of individual cells. That is what I used.
One of its many nice features is that you can mix both battery sizes and tasks on four separate inbuilt chargers.
Vendor's website
On the left is a well-used Vapextech AAA battery. It still holds 606 mAh That is 76% of the manufacturer's claimed capacity. Not too bad!
On the right are three BTY AA batteries. They are nearly new and have been cycled to develop their initial capacities. They hold 412 mAh to 517 mAh This is 16% to 21% of the manufacturer's claimed capacity. Truly awful!
Think about it! A well-used little AAA battery has beaten new big AA batteries for capacity.
Diameters are 10.01mm and 13.94mm
I have had troubles with NiMH batteries getting stuck in the battery holders. Batteries should not be over-size as a way of increasing capacity.
I have measured the diameters for you using a micrometer. Sorry I didn't check the lengths, but putting a NiMH battery into a metal vernier gauge spanning the terminals is not my idea of fun.
Diameter 13.85mm
These are some well-used AAA cells produced by BTY. They range from 80% to 85% of the manufacturer's claimed capacity. Reasonable!
[Why then did the same maker's AA cells do so very badly in the test above?]
Diameter 9.98mm
to see the table.