| Parasitic
(or ignition key off) drain is the cumulative load produced by electrical
devices, for example, emissions computers, clocks, security alarms,
radio presets, etc., that operate continuously after the engine
is stopped and the ignition key has been switched off. Normal
parasitic loads are below 75 milliamps (.075 amps). When the
parasitic load is greater than 75 milliamps (.075 amps), batteries
will drain more quickly. Glove box, trunk, and under hood lights
that do not automatically turn off when the |
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door is closed
or shorted diodes in alternators are the most common offenders. Cooling
fans, power seat belt retractors, radios and dome lights left on, alarm
systems, and electric car antennas have also caused batteries to drain
overnight. Leaving your headlights on will generally discharge a fully
charged car battery, with 90 minutes of Reserve Capacity (36 amp hours),
within a couple of hours.
It is highly recommended, especially if you are using a sealed wet "Maintenance
Free" (Ca/Ca) battery, that you allow it to thaw if frozen, fully recharge
it in a well ventilated area with an external battery charger, remove
the surface charge, and load tested both the battery and the charging
system for latent damage from the deep discharge. You could have a damaged
or bad battery. If the alternator is warm and the engine is cold, then
check for a shorted diode in the alternator.
Below are some methods that are used to test the parasitic load with
the engine NOT running, under hood light disconnected, all accessories
switched off, and the vehicle doors closed:
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Connect a 12-volt bulb across the positive and negative battery terminals
to test the bulb and the battery. If it glows brightly, then remove
the negative battery cable and connect the bulb in series between
the negative battery cable terminal clamp and the negative battery
terminal. If the bulb continues to glow brightly, then start removing
fuses or connections to the positive battery post one-at-a-time until
the offending electrical component is identified by the bulb dimming.
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A better approach is to use a DC ammeter, for example a Fluke 175,
inserted in series with the negative battery cable terminal clamp
and the negative battery terminal or a clamp-on DC ammeter, like a
Fluke 336 or i410 around the negative battery cable. Starting with
the highest scale, determine the current load. If the load is above
75 milliamps (.075 amps) after the initial surge, then start removing
fuses or connections to the positive battery post one-at-a-time until
the offending electrical component is identified by the parasitic
load dropping to within 75 milliamps (.075 amps).
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