View Single Post
  #7   Report Post  
John Rumm
 
Posts: n/a
Default

wrote:

18wh under ideal conditions means with slow discharge, ignoring memory
effect, and batteries as good as new. So IRL lets say we get more like
10wh from our pack.


Now how long can we use such a tool for on one charge? I dont know, but
lets guesstimate 15 minutes of continuous run time. Thats 1/4 of an


In my experience of such tools, that would be pretty unlikely. 5 Mins if
you are lucky (even that would equate to a draw rate of only 8A).

In practial terms that may equate to driving 50 screws on a full charge.

hour, so during that time our 10wh is giving us an average of 40w.
Thats right, 40 watts. And thats for a 15v tool.


or 120W if delivered in 5 mins etc. With appropriate gearing that can
still deliver useful work.

Something like this could be handy in the power tool faq.


Yep, it did occur to me that there is not much in the way of discussion
on the Ah ratings of batteries.

The performance (i.e. the power that can be delivered) is mostly a
function of the quality of the motor and the internal resistance of the
cells. Better quality cells being able to deliver more current on demand
while dissipating less heat.

(Some of the "expense no object" fast electric radio control bods will
draw power from top end sub C cells at 2kW (i.e. 45A drain on 36 cell
packs))

The run time is governed by the capacity of the cells, but is also a
function of the matching of the cells, since better matching will allow
more of the stored energy to be extracted in a useful form.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd -
http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/