10. WHAT CAUSES MY BATTERY TO DRAIN OVERNIGHT?
Last Updated on July 11, 2004
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 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, to fully recharge it, remove the surface charge,
and load tested both the battery and the charging system
for latent damage from the deep discharge(s). You could
have a 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:
- Connect a 12-volt 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.
- 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 (.75 amps).
- Additional troubleshooting techniques
can be found in a guide from Exide at http://www.exide.com/products/trans/na/battery_care/electrical_parasitic_load.pdf.
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