There are two main types of rechargeable batteries

Ni-Cad & Ni-MH

Ni-Cad is now considered old technology.

Ni-Cad batteries suffer what is known as "memory" problems, if they are not completely "cycled" IE charged then completely discharged they forget what capacity they are, so if you only used 50% of the charge, and then recharged them all the time, eventual they think they were only 50% of their capacity.

The good news is that a seemingly dead Ni-Cad’s or one that doesn’t hold the charge the way it used to, can be bought back to life.

All you have to do is to run it through a couple of charge/discharge cycles. (i use a torch to discharge them)

Ni-cads are generally cheaper than Ni-Mh batteries.

Ni-Mh are newer technology and don’t suffer from "memory" problems

The capacity of a battery is measured in mAh ( milliamp hours) the bigger the number, the higher the storage capacity, or the longer the battery will go without needing a charge) IE a 2700 mAh battery will last appx 2x longer than a 1300Mah battery between charges.

There are also Li-Ion batteries, but they are mainly used for camcorders and mobiles, not for domestic use.

The Down side

Rechargeable batteries are only good for around 1000 charges, I.e. that’s around 3 years charging them once a day

All Ni-Cad and Ni-Mh batteries are lower voltage than primary batteries IE they are 8v or 1.2v and NOT 1.5v or 9v.

If your appliance states DO NOT USE RECHARGEABLES then DONT it is not designed to work on lower voltages, and may well be damaged by the use of rechargeable

Right now ebay is awash with batteries coming out of China that are making some amazing claims about their capacity, IE 3000 or more mAh, to the layman you wont notice any difference until they don’t last as long as they should.

So you could end up paying more for less!

Stick to known brand names only

There is some good news though, its easy for the layman to check if they have been ripped off, the mah of the battery depends on the density of the electrolyte, so 4x AA 1300mah weigh appx 100g and 4x AA 2700mah weigh appx 125g

NEVER charge Ni-Cad in Ni-Mh chargers or vice versa, although most modern chargers are dual purpose.

Ni-Cad when charged in a Ni-Mh charger can OVERHEAT and possibly explode

And another warring, some Far Eastern factories are dumping massive stocks of Ni-Cad batteries onto ebay, but labelling them Ni-MH in the hope of getting more money for them.

The sellers that buy these batteries in from the Far East may not be aware, and the first you will know about it is when the batteries overheat or leak or damage your appliance.

Best practice Stick to KNOWN brand batteries only