Current drain occurs under two separate circumstances. If you are a radio operator the current drain is a rating on how well, or efficiently, the Field Effect Transistor (FET) circuit works for transmitting a radio signal. Current drain, under the second circumstance, relates to our cars not starting in the morning.
When batteries were first manufactured, the insulation of the cases allowed the current to drain from the chemical container. If earlier type batteries were placed in contact with the earth or concrete, the battery would discharge and eventually be ruined. Modern plastics and battery cell technology has all but eliminated this type of current drain from the multi-celled battery.
Current drain in batteries generally occurs when there is a small load placed on the battery. Especially when the automobile is shut off and the key removed from the ignition. The continuous discharge can be very small, in the mille-amps. A mille-amp is 1/1000 of an ampere or .001. Over a long period of time, a week or more, if the battery is not recharged by running the car, the battery can become too weak to energize the starter.
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Battery current drain can be thought of like a child's small swimming pool in summer. The pool is freshly filled and ready to be used. A small pin leak develops in the bottom of the pool. Over a week of time the pool is now half full, due to the pin leak. After two weeks the pool may be empty if it has not been refilled or recharged by the garden hose.
Batteries can develop a current drain by a number of circumstances. A small wire may have a strand of bare wire touching the car frame. The current drain is very small so the protective fuse or circuit breaker may not activate, shutting the circuit off. A device charger, like a cell phone, may have been left plugged in to the power source. A new stereo device may have been added with an accessory not properly shutting down.
Tracing an automobile current drain can be a frustrating task as the paths for electricity can seem endless. The first rule of thumb is to be sure that the battery is healthy enough by having it properly tested. All batteries have a finite amount of charges to discharges, or cycles, in their useful lifetime.
If you feel your vehicle battery is experiencing a current drain you can try the following to isolate the problem. Before shutting the engine off, turn all stereos, lights and switches to the off position. Check the battery cable connection to ensure it is clean and tight to the battery posts. Check the heavy wiring connections to the circuit box and starter to be sure it is also clean and tight. If you are not comfortable with performing the cable checks take the problem to a mechanic shop that may specialize in electrical problems.