Dave Plowman wrote:

In article ,
Andrew McKay wrote:

You can buy various tools which are operated with rechargeable
batteries. And there is quite a range of battery voltages to choose
between - 7.2, 12, 18, 20, 24, etc.


Question is, is a higher (or lower) voltage better (or worse)? Higher
voltage implies lower current to achieve the same power rating.


They all seem to use the same sized cells - called sub C size. So the only
way to increase capacity is to increase the voltage - you can't parallel
these types of cells.




Actualy you can for discharge, but not for charge Well even then...Oh
well. Its not practicable for power tools.


Otherwise you are totally correct.

Sub 'C's are rated for something like 30-100A peak discharge rate, for
Nicads, and maybe 30A only for NiMh.

So in practice power available is very much a function of the number of
cells assuming the motor is rated to take the correct current draw.

SUB 'C's come in various capacities too - the originals were something
like 1.2A/h but modern NiMh cells are pushing 3A/h.

With a motor efficiency (these are real cheapo motors) in the 50-60%
range a 10 cell unit (12V) at 30A is roughly 360W in and about 200W out.

Or a bit more than 1/4bhp. Of course at this current the pack will last
only 6 minutes flat out