I would be concerned about the amount of current required to kill a car battery in a week:
Let's assume that 1 week = 24 * 7 = 168 Hrs
A typical, healthy car battery will deliver anywhere between,
45 to 70 Ampere-hours at small currents, depending on the
size of the car (check your car owners manual)
Assuming a weak battery has a 40 Ampere-hour reserve, it would require an average current of 250 milliamperes to drain
in a week, or about 3 watts of power.
While this does not seem like a lot, it certainly indicates
something is wrong, because the clock and station memory
in a radio should take a lot less than that.
The first step is to make sure that the radio is actually at fault,
not damaged wiring or another one of the dozens of electronic
systems in a modern automobile.
Don't trust your mechanic on this, I have never met a gear-head
that was any good in electronics, but they do come up with
some fairly captivating fish-stories when they do not
understand something. :)
1) The first step is to disconnect the radio and isolate the
2) Remove the radio and hook it up to a 12V DC supply,
or battery through a digital milliamp-meter. Check the
current with the radio shut off.
3) Anything over about 10 mA or so is BAD.
4) If the radio draws any significant current with the switch
shut off, look for a bad switch or a leaky capacitor on the
B+ wire (before the switch, so it should not be hard to find)
Another possibility is the clock/keep alive controller chip,
but this is less likely. The most likely culprit is probably
an old leaky capacitor (electrolytic or solid tantalum),
since these do tend to fail with age, vibration and extreme
temperatures in a car environment.
5) If the radio is at fault, you will need some knowledge and
electronic instruments to find the problem yourself.
A quick way to check an electrolytic or tantalum capacitor
is with an ohm-meter. Connect ohm-meter to the capacitor,
and watch the reading race to infinity as the capacitor charges
up. Then reverse the leads and watch it again. Once charged,
the capacitor should have infinite impedance.
6) If the radio is not at fault, it could be dozens of other
problems, ranging from the body control computer,
immobilizer, remote starter, parking lights, door-locks,
alarm system, bad wiring ... you name it.
7) You should be able to isolate it by measuring the battery
current, while pulling fuses one at a time.
8) Another possible cause is the battery itself, or a bad
(shorted) diode in the alternator. The best way to check
the alternator is with an oscilloscope, while the car is
A bad diode will show up as a missing peak in the otherwise
regular pulse train.
9) Finally, don't over look the relays in the power distribution box.
These can over heat and stick in the ON position with the
ignition turned off.
Good luck, Martin
on Jun 13, 2008