Re: Just some questions... In lugnet.robotics.handyboard, Mike Schubert writes:
>>> 1) What is a good battery to use?  I don't have 8 nicad batteries.. I
>use
>a
>>> R/C car 9.6V battery pack..  is there some specific need for certian
>mAh
>>> values?
>
>>The recommended battery pack I ordered from DigiKey is rated 9.6V at
>>500mA.
>>So, Don't over kill the board.  And I think the motor chip L293D can go
>>up
>>to 700mA (the TI SN754410NE can go up to 1A)  (I think I am right)...
>>So,
>>watch out...
>
>Oh, you are missunderstands some "electrical" details :)
>You can use a battery pack with 9.6V and 2000 mA or more if you like,
>because the unit is exactly mA/h.
>
>It means you can get this current over a time of one hour. Or the
>half current over 2 hour's.
>
>The current of the motor chip is only limited of the resistor (from
>motor and the cables to the motor). That the reason why you can't use
>a "big" motor with the L293. The Motor get's too much current.
>(but you can limit the current with a resistor)
>
>hope it helps :)
Yes. Exactly the right point. The two different things to woory about, are the
mA/Hr rateing of the battery (which tells you how long a particular current
can be delivered), and the _internal resistance_ of the battery, which limits
the maximum current it can deliver. Now if you take some examples. The small
batteries used in some 'micro' radio controlled cars, have mA/Hr rateings of
just 500mA/Hr, but are run at currents (in some cases) as high as 15Amps,
giving a battery like usually measured in just a couple of minutes. If shorted
through a system only rated at 700mA, these can blow it just as easily as a
much larger battery. In the case of the Lego battery pack for the 9v motors,
few people realise, that this actually contains a 'self resetting' thermal
fuse, just to prevent a short from damageing anything else (it is a tiny 3/8
inch 'disk' hidden behind one of the battery clips). So the answer if you want
to use a larger capacity 'pack', is to use one as large as you want (provided
it is the right coltage of course), but limit the maximum current with a
similar fuse (or a conventional one, though obviously this has the
disadvantage of needing replacement if too much current is drawn). So
something like a 500mA self resetting fuse, and a 10A/Hr battery can be fine,
when much smaller batteries without the current limit of the fuse, could cause
problems

Best Wishes

