a) They "self discharge" even sat on the shelf, rapidly! After a week or so they are often 10 to 20 percent discharged depending on manufacturer and temperatures etc. After a month sat in your camera they may be half dead even if you never turn it on.

Some do this much more than others. But ALL normal Nickel Metal Hydride AA batteries do it to an annoying degree. If its in a heavily used device that is recharged often it doesn't matter and you never really notice. There are newer versions available that can sit on the shelf for up to a year and these are very useful for things that you don't use often. Like a torch, or can be used in clocks, TV remotes etc. Things that usually make batteries last a long time. These are sold as ready to use or pre charged rechargeable batteries.

A "Normal" rechargeable cannot sensibly be used like this as it will go dead before it should. Like a few weeks only before the TV remote fails to work properly... Or the clock stops. These newer batteries sold as "ready to use" don't suffer this problem. A common well known make is the "Enerloop battery". See photo above. Or Duracell Active Charge sold as ready to use.  These types of rechargeable battery are typically 20 percent smaller in storage capacity. But probably still have more actually left in them when you decide to use your gadget!

b) Rechargeable batteries seldom actually contain the "capacity" (or size of fuel tank) written on the side. Its all marketing. Even well known brands lie!  Or at least measure a selected cell under ideal conditions by discharging it very slowly and round it up to the nearest digit! This is the only way to get the "maximum" capacity out. Our gadgets don't do this though! So I test at a rate of 1 amp (about half C rating) as it is typical of real world usage.

A 2700mAh battery for e.g. may not really last as long as a cheaper 2200mAh one. That's why I measure and graph them carefully during a controlled discharge with a special "charger" and a PC designed to accurately measure this and other data.

Uniross 2700 AA  

 Use for high capacity and medium / low current draw and quick use/recharge.

2700

2258

172

 Use for medium / high capacity and high or low current draw and quick use/recharge.

 2300

Enerloop 2000 AA
 (very low self discharge long charged shelf life)
Graph here
(Pack of four tested in series)

Use for medium capacity and high current draw AND long term items.

2000

1822

95
Good for high current devices like cameras and flash guns

 
Vapextech 2200 AA
Graph

 Use for medium capacity and low current draw or short term often recharged items.

1444
Not brilliant!
(several tested)

Extreme 2200 AA
 (very low self discharge long charged shelf life)

 (commonly found on eBay etc) Sold as ready to use and long stand by and slow discharge. I had 2 out of 8 cells faulty however. As long as all of yours are OK they are fine but not quite the claimed capacity. Compare to branded batteries like Enerloop or Duracell Active.

1672

90

Good for high current devices like cameras and flash guns etc.

Duracell (Active Charge) Rechargeable
 (very low self discharge long charged shelf life)

Graph

Tested as a series pack of 4 cells.

Sold as charged & ready to use. Each cell measured 1.328v on opening the pack so very well balanced and closer to fully charged than the rest here (so far!) which were in some cases nearly half discharged as they were purchased.  Best battery tested so far. If only just! The Enerloop's are almost identical!

2000

MUCH MORE TO FOLLOW  Testing takes about a day per battery!