Chemistry type, voltage, battery capacity and the technology used by the manufacturer all affect the size and weight of cordless power tool batteries and their power tools.
Higher voltages and battery capacities increase the size and weight of a battery while newer technology can decrease the size and weight, allowing smaller batteries to be just as powerful and long-lasting.
Li-ion batteries are by far the smallest and lightest. A Li-ion battery is half the size of a NiCd battery of the same battery capacity and voltage, and up to a fifth of the weight. This is a big advantage but it comes at the cost of being more fragile.
NiMH batteries are also lighter than NiCd batteries. For the same weight as a similar NiCd battery, a NiMH battery can have about an extra 30% in battery capacity.
It all comes down to choosing the right tool for the job. For example, if you will be using your tool every day on workpieces made of medium-weight materials, such as wood, you may wish to consider tools and batteries that have a medium range of voltages and battery capacities (e.g. an 18V 2Ah battery) so you can carry out the work efficiently but without its weight becoming tiring to hold.
An ideal battery would be about the size and weight of a 500g butter tub, although some Li-ion batteries can weigh as little as 180g.
There are now a number of small, lightweight tools on the market that run on very small cordless power tool batteries. They are perfect for light DIY work such as screwing up insulation boards and drilling holes to hang pictures.
The Li-ion batteries that power these tools are the smallest cordless power tool batteries available. At 7.2V and 1Ah, they weigh only 80g.
At the other end of the scale, cordless power tools are available for heavy-duty metalworking jobs. They require batteries with voltages of up to 36V and capacities of 5Ah. These batteries can be the size of a 1kg margarine tub and weigh up to 1.3kg, although newer technology is bringing them down to under a kilogram.
Ultimately, you have quite a lot of choice over the size and weight of your cordless power tool battery. The manufacturers or your supplier will be able to give you more advice on the best tool for your set of jobs.