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Close up of a car battery (George Doyle/Getty Images)
Close up of a car battery (George Doyle/Getty Images)

You & Your Car

Fielding a battery of questions Add to ...

How long will a car run without a battery? I know you need a battery to start the engine, but can't it run on the alternator alone? The question stems from a argument with some buddies about whether the alternator can satisfy all the demands of a modern car in winter like heated steering wheel, seats and mirrors, rear defrost, etc., in addition to the headlights, heater motor, wipers, etc. - Georges in Brighton, Ont.

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You are right, you need a battery to start the vehicle.

It is possible to push a vehicle with a manual transmission and clutch long enough to kick-start it with a dead or no battery. But you will essentially be relying on the alternator to be turning enough to supply power. Once the vehicle is running, the alternator has the capacity to supply all the power to operate all the equipment on the vehicle - it has been sized that way by the manufacturer.

If all of the above-mentioned equipment is drawing power from the alternator, it may be running at or near capacity and have very little reserve to charge/recharge the battery, replacing the power used to start the car, unless it is run for a considerable length of time and at high enough speed to provide maximum output.

A common problem is that people think that they can let the car idle for a lengthy period to recharge the battery.

Recharging the battery

I'm having trouble with my Subaru. I love the car and it has been utterly reliable and a welcome companion through six winters. But I have been having alternator problems for the last two years.

I live in a rural area more than 150 km from the nearest Subaru dealer and have it regularly serviced by a pair of local mechanics I have come to trust and rely on. I am a senior and don't drive the car much, probably about 3,000-to-4,000 km per year, mostly for short trips into town. I keep it in my garage at all times. In the last 14 months, it has failed to start several times.

The mechanics have replaced the battery twice and the alternator once. A neighbour said I should let the car idle in the driveway for 10-15 minutes once in a while to keep the battery charged and I have been doing that before every couple of trips. Yesterday, it would not start again and, when they got it to the shop, the guys said it needs another battery. Something else must be wrong, don't you think? - Marge in Berwick, N.S.

Yes, I suspect something else is at the root of your trouble - your driving habits.

You are not driving the car long enough to allow the alternator to replace the power taken from the battery to start the car. The alternator has a tough time keeping up with all the demands placed on the electrical system in cold winter conditions. Unless the engine is run at a reasonable speed - say 1,500 rpm or more - for a lengthy period of time, the alternator output will not be sufficient to recharge the battery as well as operate the defroster, heater, etc.

Short trips and idling in the driveway are not sufficient to keep the battery charged.

I suggest driving at least 15 minutes at or near highway speeds every few weeks. If you can't do that, buy a battery tender, which automatically monitors the charge state and maintains the correct level. This is different from a battery charger or "trickle charge," as they are sometimes known, which constantly apply a charge to the battery, weakening it over time and eventually ruining it.

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