Flash Amps can also be used to provide an estimate of internal resistance. Flash Amps are defined as the
maximum current a battery can deliver for a very short period of time. This test is typically performed by
electrically shorting a battery with a 0.01 ohm resistor for approximately 0.2 seconds and capturing the
closed circuit voltage. The current flow through the resistor can be calculated using Ohms Law and
dividing the closed circuit voltage by 0.01 ohms. The open circuit voltage (OCV) prior to the test is
divided by the Flash Amps to obtain an estimate of internal resistance. Since Flash Amps can be difficult
to accurately measure and OCV is dependent on many factors, this measurement technique should only
be used as a general estimate of internal resistance.
The voltage drop of a battery under load is a function of total effective resistance and current drain rate.
An estimate of initial voltage drop under load can be calculated by multiplying the total effective
resistance by the current drain placed on the battery.
Example: A 1 Amp drain is placed on a battery with a Ri of 0.1 Ohms.
1.0 Amps X 0.1 Ohms = 0.1 Volts
Open circuit voltage = 1.6 Volts
1.6 Volts – 0.1 Volts = 1.5 Volts
(expected closed circuit voltage)
In general, internal resistance will rise during discharge due to the byproducts of active materials that
have been used. However, the rate of change during discharge is not consistent. Battery chemistry, depth
of discharge, drain rate and the age of the battery can all impact internal resistance during discharge.
Cold temperatures cause the electrochemical reactions that take place within the battery to slow down
and will reduce ion mobility in the electrolyte. Subsequently, effective resistance will rise as ambient
temperatures drops. Conversely, as temperatures rise, the electrochemical reactions within the battery
become more efficient and effective resistance can decrease.
The graph (fig. 11) shows the effect of temperature on the total effective resistance of a fresh
Energizer® AA/LR6 alkaline battery.