This is a pretty short question but it is one that I have had a hard time finding an answer to.

My understanding is resistance and voltage is consistent across a series circuit and I should use Ohm's law to work out what resistor to use to bring a component within its current (mA) specification. However wouldn't adding the resulting resistance just add to existing internal resistance in the battery? Why is the internal resistance not subtracted?

 

Because most of the time the voltage source can be considered to have 0 resistance. There are exceptions. Sometimes the voltage source resistance needs be to measured.....and used, for the proper operation of circuit.

 

Because it <should> be a vanishingly small and ignore-able portion of the circuit resistance.

A lot of what goes on in circuits is 'order of magnitude' stuff, getting the "exact" value is not necessary or even reasonable.

The trick is knowing what is important, and when.

 

hi Mc,
Welcome to AAC.
This is one of the reasons why voltage regulators are used in electronic projects.
Most times the internal resistance of the power source is not known by the designer of the project.
The potential voltage of a battery when off load for example could be say 12v.
When current is drawn from the battery the voltage output fall due to a voltage drop within the battery.
ie: Vdrop = Idrawn * Internal battery resistance
As more current is drawn from the battery the greater the Vdrop would be.

For many applications this drop in voltage would not be a problem, provided it stayed with the equipments operational specification.
More critical applications that require a stable supply voltage, would have a voltage regulator between the battery and the equipment.

The same requirements apply to mains powered equipment, the local mains supply voltage can vary.

E

 

Internal resistance of battery changes (increases) in the course of a lifetime, for example from 0.1 Ohm to 10 Ohm.
Good designer takes that into account and makes his circuit functionally operative in range of internal resistance changes.

 

Plus, it is unlikely that a cell or battery manufacturer will guarantee to a lower limit on internal resisance.

 

Thank you everybody for the informative information!