|Previously we have been considering the power supply for a circuit to be a pure emf. Real batteries are better modeled as an emf in series with an internal resistance. The internal resistance r is included to account for power dissipation which is not recoverable within the battery when current is flowing.|
Since the internal resistance of the battery is in series with the load the
equivalent resistance of the circuit is Req = R + r.
The current is thus reduced owing to the internal resistance, i = emf / (r+R),
from what it would be in its absense.
The potential difference across the load, equivalent to that across the battery,
is less than the full emf of the battery
because of the voltage drop across the internal resistance.
Vload = iR = emf - ir
|The so-called terminal voltage of a battery is lower than the emf when it is discharging because of the voltage drop across the internal resistance. If, on the other hand, the battery is being charged by an external source such as a recharger, the current will be forced through the battery in the opposite direction; the terminal voltage will then be higher than the emf by the amount of the voltage difference across its internal resistance.|
|Thus if no current flows through a battery, its terminal voltage in that case will be identical to its emf. Good voltmeters have extremely high resistance; the input resistance of modern multimeters typically exceeds 10 megaohms (10 x 106 ohms). Thus the current flow in the circuit shown at the left will be much less than a microamp. Since the internal resistance of a fresh battery is typically one ohm or less, the voltage drop across the internal resistance will be insignificant when the terminal voltage is measured with a good voltmeter.|
|To properly test the effects of internal resistance in a battery, a load must be added so that a current will flow through the battery. If no currents flow, no information about the internal resistance can be gotten. Ideally the load added will represent the conditions under which the battery will actually be used. After the load is added to the battery under test, the terminal voltage of the battery is measured with a voltmeter as shown at the left. Thus a battery tester is a voltmeter in parallel with a load resistor, both connected across the battery to be tested.|
The table below shows measurements of the terminal voltage of two AA alkaline batteries, one new and one used, made with a voltmeter and with a battery tester with load resistance of 15.6 ohm.
15.6 ohm load
|new||1.53 V||1.49 V||96 mA||0.04 V||0.4 ohm|
|old||1.25 V||1.00 V||64 mA||0.25 V||3.9 ohm|
The Anatomy of the On-Battery Tester
Life with and without Batteries
Battery Know How, an ACDelco Training Program presented by the Automotive Parts & Accessories Association
Last updated Aug. 4, 1998.
Copyright George Watson, Univ. of Delaware, 1997.