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Volvo S40 battery goes flat

My car needs a jump after sitting at the airport for a few days. New battery did not fix. Usual mechanic could not find drain (aside from car alarm, which takes several kilotons to set off), said Volvo electrical systems are set up so “full” charge is not really full…(?). Volvo dealer says new battery is “larger than standard” and “alternator may be having a hard time keeping up with the battery,” and that “all working as designed.” …(???)
Are either of these folks correct? Is there anything I can do to avoid repeat embarrassment when I leave town?

What year is this Volvo?



Find a new mechanic!

Your usual mechanic is over his head, and the dealer is a complete idiot!

You have a parasitic current draw as the vehicle sits.

These can be very hard to trace, and it may require leaving the vehicle at shop for a few days to determine the cause.

Here’s an article that explains things that can cause this type of problem, and how to trace it down.


Your shop needs to measure the parasitic current drain. That’s the amount of current the car’s electronics are using from the battery when the key is turned to “off”. You’d think no current would be used in that mode, but there’s various gadgets that remain powered at least somewhat even with the key in “off”, like the radio, and of course the alarm system. On my Corolla the parasitic current drain measures around 25 mA, but on newer cars with much more electronic gadgetry they have these days it can be over 100 mA. Even at 100 mA that should still not drain a good battery in just a few days. In a couple weeks it could drain the battery enough to prevent it from cranking though, esp in cold weather.

Another guess, when an engine won’t crank, it could be due to other problems than a partially drained battery. Could be a bad connection somewhere, or the starter motor could need replacing. A mechanic can do a voltage drop test to search for bad connections in the crank circuitry.

If the battery is just on the verge of not being able to provide enough current to overcome a bad connection or faulty starter motor, one idea as a temporary solution is to make sure the battery is fully charged before leaving for the day. A battery charger can do that.

This could be due to an alternator on the fritz too, or even the alternator is ok, but your typical daily driving routine involves a lot of slow speed stop and go driving, and no high speed freeway driving. That type of driving might not be quite enough to top off the battery and it would show up first in cold weather after leaving the car parked outside in the elements for a few days. Good idea to have the charging system tested.