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Confusion Over Batteries.......

M

Moikel

Jan 1, 1970
0
Hey,

I'm a reletive newbie in the world of practical electronics (although
I've been studying electronics in school and college for years). I'm
pretty solid with the theory of the whole thing but I'm having problems
with the practical side of things.

The confusion is over batteries.

When analysing circuits on paper everything is nice and obeys ohm's law
and the whole lot. But I recently discovered that batteries will only
supply a certain amount of current and thus in certain circuits won't
be able to supply current that would be obtained in theory. Also, their
terminal voltage will drop if asked to supply high currents, I think.

Now I need to know exactly what kind of current I can expect from a
battery at its rated voltage. In other words, how will I know if a
battery at the correct voltage will be able to supply enough current
for a given circuit to work correctly and avoid voltage drops and the
like?

Thanks,

Moikel
 
S

sPoNiX

Jan 1, 1970
0
Hey,

I'm a reletive newbie in the world of practical electronics (although
I've been studying electronics in school and college for years). I'm
pretty solid with the theory of the whole thing but I'm having problems
with the practical side of things.

The confusion is over batteries.

When analysing circuits on paper everything is nice and obeys ohm's law
and the whole lot. But I recently discovered that batteries will only
supply a certain amount of current and thus in certain circuits won't
be able to supply current that would be obtained in theory. Also, their
terminal voltage will drop if asked to supply high currents, I think.

Now I need to know exactly what kind of current I can expect from a
battery at its rated voltage. In other words, how will I know if a
battery at the correct voltage will be able to supply enough current
for a given circuit to work correctly and avoid voltage drops and the
like?

have u taken the internal resistance of the battery into account?
 
P

PeteS

Jan 1, 1970
0
A battery may be modelled the same way as any other voltage source:

A perfect voltage source (that is, Rsource =0) in series with an
internal source resistance. For high current outputs, things get a
little more complex as the effective source resistance can be
proportional (due to internal heating) to output current. In addition,
the source resistance varies with charge level (this is particularly
true of NiCads).

For short pulses, this may not matter.

The typical output resistance (not to be confused with it's output
impedance) is not always stated by the manufacturers, but they do
usually specify the max output rated current.

Because of electrochemical effects, batteries can have a source
resistance that is a step function of output current. If you keep the
current below the max rated current, you shouldn't have to worry about
that, though.

Cheers

PeteS
 
D

Don Bowey

Jan 1, 1970
0
Hey,

I'm a reletive newbie in the world of practical electronics (although
I've been studying electronics in school and college for years). I'm
pretty solid with the theory of the whole thing but I'm having problems
with the practical side of things.

The confusion is over batteries.

When analysing circuits on paper everything is nice and obeys ohm's law
and the whole lot. But I recently discovered that batteries will only
supply a certain amount of current and thus in certain circuits won't
be able to supply current that would be obtained in theory. Also, their
terminal voltage will drop if asked to supply high currents, I think.

Now I need to know exactly what kind of current I can expect from a
battery at its rated voltage. In other words, how will I know if a
battery at the correct voltage will be able to supply enough current
for a given circuit to work correctly and avoid voltage drops and the
like?

Thanks,

Moikel

Batteries are rated for their voltage, and also for their ability to provide
that voltage at a current. The latter is the Amp Hour or milliAmp Hour
rating.

Google for 'battery amp hour rating'.

Don
 
E

ELAL

Jan 1, 1970
0
Batteries are rated for their voltage, and also for their ability to
provide that voltage at a current. The latter is the Amp Hour or
milliAmp Hour rating.

Not quite! The capacity rating (that Ah stuff, for non-EE-folk) has
NOTHING to do with the maximum supply current. Even physical size is a
much more reliable indicator than that!

It only indicates how long it takes to drain the battery at a given
current, so you can guesstimate how long the batteries will last in a
given application.
(That is, at moderate (relative to Ri) currents, at low currents you
need to take the self-discharge into account and at higher currents, the
Peukert coefficient becomes a major factor. But who said that EE was
easy...)
 
J

Jon

Jan 1, 1970
0
Moikel,
Battery manufacturers supply information, often in the form of graphs,
that show charge/discharge data, self-discharge data, etc. This
information is typically available for download from their websites.
Regards,
Jon
 
G

Gandalf

Jan 1, 1970
0
Hey,

I'm a reletive newbie in the world of practical electronics (although
I've been studying electronics in school and college for years). I'm
pretty solid with the theory of the whole thing but I'm having problems
with the practical side of things.

The confusion is over batteries.

When analysing circuits on paper everything is nice and obeys ohm's law
and the whole lot. But I recently discovered that batteries will only
supply a certain amount of current and thus in certain circuits won't
be able to supply current that would be obtained in theory. Also, their
terminal voltage will drop if asked to supply high currents, I think.

Now I need to know exactly what kind of current I can expect from a
battery at its rated voltage. In other words, how will I know if a
battery at the correct voltage will be able to supply enough current
for a given circuit to work correctly and avoid voltage drops and the
like?

Thanks,

Moikel

First off, there's no way to know exactly anything, unless you want to talk
about religion. I would think a Dubliner like you would realize the hazards of
that.

Batteries are chemical in nature and chemical reactions have variable speeds and
results depending on ambient conditions, initial starting conditions, physical
structure, available reactants, mobility of electrolyte, reaction mediators,
catalysts, physical orientation of the structures and intensity of gravity
fields.

To find out about batteries, test them. Model the results. Use the model in your
circuit simulations. There is never any substitute for testing. And even then,
leave a healthy margin for variation in your test results when you design your
circuits.
 
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