Lexus: How to test Battery/alternating testing

I thought I'd layout the procedure to check your battery and charging
system. First, the battery has to be fully charged. Then take a voltage
reading, which should be about 12.5 volts. Next start the engine and the
voltage reading should go to 14.5 or so. This tells you the alternator is
putting out. The last question is whether it is putting out sufficiently.
So, turn on the headlights and see if the voltage stays about 14.5. If so,
the charging circuit is OK.

Next you need to verify whether there is drain back from the battery when
the engine is off. To do this, disconnect one of the battery terminals and
place an ammeter in series with the battery terminal and the removed cable.
There may be an initial surge of current, but should quickly stabilize to
less than 50 milliamps. Make sure you close the car door and have all
lights off. On some cars this reading can be as high as 80 milliamps, but
any higher than this will cause a dead battery after a few days. If higher
you need to remove fuses one at a time or check for leakback through the
alternator. Sometimes something as simple as a trunk light switch can cause
the problem.

Lastly, you need to check the battery to see if it can hold a charge and
whether it can provide enough power to start the engine. To do this, again,
the battery must be fully charged. Take a voltage reading (should be about
12.5). Disable the engine so it will not start. This is best done by
removing the fuse for the fuel pump. Crank the engine for 5 seconds (use a
watch). Wait 15 seconds and do again. Repeat this several times. You
should be able to do this 10 or more times before the battery goes dead. If
it fails sooner, the battery is defective (assuming you don't have excessive
current draw problem with your starter).

Lastly again starting with a fully charged battery, disconnect either
battery terminal. Take a voltage reading. Wait 12 hours and read again.
Repeat this for two days. The voltage should no drop more than.1 volt.
Reconnect the battery and crank the engine. If it cranks, battery is OK.


John Ackerman
net