The internal resistance of a battery refers to the actual electrical resistance of the battery itself. This value determines how much energy (electricity) is burned up as heat when the battery is under load (current flowing in or out of the battery). It also determines how much of a voltage drop occurs when a load is applied to the battery.
A higher resistance means more energy is wasted and turned into heat. A lower resistance means the battery is more efficient and less energy is wasted.
Typically the internal resistance of a battery will be higher in cold temperatures and lower in warm temperatures.
There are two different types of internal resistance when it comes to batteries:
DC Resistance: This is the amount of voltage drop that occurs in a battery over an extended period of time (typically 10 to 30 seconds). This value can be much higher than the AC resistance mentioned below due to a number of reasons (eg: battery capacitance) and is important for applications with sustained high current draw (eg: all-electric vehicles, power supply units, etc).
AC Resistance: This is the amount of voltage drop that occurs in a battery over a very short period of time (less than a second). This value is important for applications where energy may be put in and taken out of the battery in very quick bursts (eg: hybrid vehicles).
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