Car battery vs. cold weather

I’m wondering about the minimum amount you need to run your car to maintain the battery. I live North of Boston where the normal winter temps are in the 20 degree range but we get really cold weather too. I own a car out of necessity but I hate to drive so I can go two weeks without driving (I use a lot of mass transit).

This isn’t Minnesota cold so that’s a whole different topic.

I have AAA so I can get a jump start if needed but I like to count on the car when I need it.

Also, is there a problem with just starting the car and letting it run for a while vs. actually driving it around?

Batteries do not lose charge in cold weather. What happens is their ability to deliver current is greatly reduced, and if the battery is already marginal (small, old, slightly discharged) it may not be able to deliver enough juice to turn the engine. That’s why the spec is “CCA” or “Cold Cranking Amps” - how many amps the battery can deliver at a specified low temperature. (0 degrees F by spec, often adjusted to more accessible temperatures for field testing.)

It’s hard to say where the break-even point is between use of starting juice and replenishing that juice from idling or driving. I’d guess, very roughly, 15 minutes of normal driving.

You’re best off not starting the engine except when you need it. If you need reliable cold starts, invest in a block heater, a battery heater, or both.

It depends on the car. Most cars can tolerate a couple of weeks without use with no ill effects on the battery. Some cars can go several months without draining the battery too much, but that’s kinda rare. In my experience most cars are in the range of a couple of weeks to maybe a month before you run into trouble.

You can always hook up a battery tender (available at ye ol local auto parts shoppe) if you are concerned about how long the car will be sitting.

If you just start the car and let it idle for a bit it may not get up to its full operating temperature. When the engine gets nice and hot, then any water that condensed into the oil gets flashed into steam and is expelled from the engine. Water produced during combustion also gets flashed into steam and expelled out through the exhaust. If the engine and exhaust parts don’t get hot enough, then you’ll get water mixed in with your oil and water which sits in your exhaust system and causes it to rust out fairly quickly.

If the tires sit for too long they will develop flat spots and your brakes will also tend to rust in place and get a bit sticky. Your brake rotors will also start to develop a lot of surface rust. It’s basically better for all of the moving parts of your drive train (axles, CV joints, etc) to actually move so that they remain properly lubricated and protected from rust and corrosion.

Get a solar-panel one and wire it so it can live on the rear package shelf or equivalent. Even in mostly cloudy weather it will generate enough of a trickle to keep the battery topped up.

As you say, there are many issues with letting cars sit a long time between drives. But the OP is concerned (here) about reliable starts. With a newer battery and a functioning alternator/regulator, I wouldn’t worry much about two-week intervals. A month… I’d see about a battery tender of one sort or another.

If you only use your car rarely you can simply remove the battery from the car and keep it in the house until you need it–you just have to loosen a couple bolts.

You might use a trickle charger to keep the battery fully charged:

This is something you might do in extremely cold weather.

My wife has a very short commute (~2 miles each way). Her car and battery are less than three years old, but she’s been finding that it’s starting to crank slowly on very cold mornings. I finally realized that her short commute wasn’t enough to fully recharge the battery after a start, so once a week we take it out on longer errands to give it a good charge. Yes, I know 2 miles isn’t enough to really dry out the oil and the exhaust system, but I’m not going to move farther away from work just for this.

As for how often one needs to drive to maintain the battery…depends on the car, but if the battery is healthy (no internal short at all) and the car is healthy (no power uses except for the dashboard clock and security system), then you can probably go for a couple of weeks without incident.

Depending on what electrical accessories you have running, idle RPM may not provide full charging voltage for the battery. Moreover, idling doesn’t warm up a car as quickly or completely as powering down the road. Heat rejection to the engine block/head is roughly proportional to power output; IMHO the ideal commute for fully warming/charging the engine and battery would be a gentle 1-mile drive on surface streets followed by a 5+ mile drive at highway speeds.

You are better off not starting the car and just letting it idle for a short while to recharge the battery. It’s better to take it out for a long drive about once every week or so.

Or you can just let it sit until needed. You could buy a portable jump start battery kit which can be recharged in your home. Connect it to the car if it’s been sitting for a while before starting on cold days.

Here’s an example:

If you have a garage with power I would just buy a battery tender, as engineer_comp_geek mentioned above.

You can even mount it under the hood in the engine compartment and leave it hooked up to the battery at all times.

You just put the car in the garage, plug in the tender, and unplug it when you leave, just like a block heater used in very cold areas. No hooking it to the battery, plugging in, unhooking and storing, etc. it is just another accessory in the engine compartment. They probably aren’t the solution if your car is parked out in the elements and you have to run a power cord.

I agree that just idling the car for 20 minutes or so is a bad idea. It is a waste of time, gas, creates possibly dangerous exhaust fumes, won’t get the car as hot as it needs to get, and really won’t fully charge the battery.

With modern cars removing the battery tends to bring problems associated with the anti-theft systems they use these days. You can find central unlocking doesn’t work, or immobilisers have locked up. Some radios will need a code to reactivate them once they have been depowered, and you can only input the code X number of times before the unit has to go back to the vehicle makers and be reprogrammed.

I agree with what others have said about battery tenders for vehicles that sit unused for long periods of time. I have a several trucks, tractors and a boat that I use only occasionally, and I keep battery tenders on them all. In my experience, in addition to assuring a charge for starting, it also lengthens the life of batteries, which don’t like to sit for long periods of time only partially charged.

If you don’t have a plug in, as someone else noted, small solar chargers work. Harbor Freight has them. They may be unregulated, but I’ve never noticed any harm (water level going down) from them.