Confusion Over Batteries.......

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

Reply to
Moikel
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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

Reply to
PeteS

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

Reply to
Don Bowey

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

Reply to
Jon

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

Reply to
sPoNiX

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...)

Reply to
ELAL

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.

Reply to
Gandalf

Yes, that's because real batteries have internal resistance.

Ask the manufacturer. Doing tests is always useful, applying various loads - but use extreme care - a "D" sized NiCd, for example, can weld metal, for a very short period of time. (and then, while still shorted, will happily immolate itself.)

Good Luck! Rich

Reply to
Rich Grise

The "reserve" or "stamina" watchword for a cell or a battery is Capacity, quoted in mAh for popular consumer sizes, or in Ah for big rechargeables. Jim

Reply to
Jim Gregory

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