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Old 01-27-2014, 11:01 AM
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Does driving my car charge the battery?

Somewhere I got the idea that driving your car for 30 minutes at highway speeds helps to charge up the battery. My car battery has never died (it's pretty new, less than a month old) but I want to make sure my car starts tomorrow morning which is forecast to be -25 F. We've had a few pretty cold mornings this month and I'm concerned that it's lost a lot of it's "juice."

I am driving to and from work today, which is 30 minutes each way, but my route is entirely city streets, so my speed never exceeds 30mph. Is it advisable that I take a 30 minute highway drive tonight, or will it not make a difference?

If it matters, I have a 2008 Honda Fit. Thanks in advance.
Old 01-27-2014, 11:08 AM
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Short answer, yes. You should be just fine without any special effort. A good battery and a small engine will start fine in very cold weather.

Longer answer, for better starts in very cold weather, an engine block heater helps a lot. For a one time or rare need, remove the battery and keep it in a warm place until you need to go.

Last edited by Amateur Barbarian; 01-27-2014 at 11:09 AM.
Old 01-27-2014, 11:11 AM
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Anyone with a short commute should try not to give their battery a hard time. Modern cars have lot's of electrical stuff, and some of it puts a load on the battery; notably - heated windows, heated seats and the heater fan (note the theme here).

A normal 30 minute drive will probably put the charge you used to start the car back in, but I think it's marginal. Once the car is moving at 20mph+ the battery is being charged, so long as you don't have all that stuff working. Maybe take the long way home a couple of times a week.
Old 01-27-2014, 11:18 AM
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Cold inhibits the ability of the battery to convert its chemical energy into electricity. All the power is still in there, it just has a harder time getting out when cold. So, keeping the battery inside like Amateur Barbarian suggests will work, but if you leave the battery out in temperatures so cold that it will not function, it will still work once it comes back up to temperature.

To make it easier to start the car, make sure other electrical systems like the fan, headlights, and radio are off.

Your battery is always recharging itself, but it does so faster at highways speeds without a lot of external draws (a/c lights, etc). Unless there has been a problem with your battery, the cold is your issue, not how you drive, and the highway will not help you start the next day.
Old 01-27-2014, 11:21 AM
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The battery should charge as long as the engine is running. It will charge a bit more at higher engine revs (at least on most cars) but even sitting at idle it should charge.

When the car sits unused, there are some things that are still running (like the memory backup for the radio and the engine computer) which will slowly drain the battery. How long this takes depends on the design of your specific car, but typically a car can sit at least a couple of weeks before the battery drains enough for it to become an issue (many cars can go more than a month, and some can go several months).

If the car has been sitting for a week or two, then you might want to run it for a bit to get it all nice and charged up. If you just drove it yesterday, then it is already charged and won't see any benefit from any additional driving.

If the car is going to sit for any length of time and you are worried about the battery, go down to ye ol local auto parts shoppe and buy yourself a battery tender. This basically plugs into an AC socket in your garage and attaches to your battery to keep it nice and freshly charged for extended periods of time.
Old 01-27-2014, 11:30 AM
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Alternator output depends on engine speed, not vehicle speed. When the car is moving at any decent rate at all, engine speed is usually high enough to ensure decent charging, because the transmission will be in a gear that puts the engine speed in about the same range unless the speed is down at stop-and-creep.

The issue with city driving is not the lower speeds, it is all the time spent Idling at stop lights. Even at at idle, most modern alternators have enough output to provide modest charging, even with the lights on. If this is not the case, then you will see your headlights brighten as you accelerate...this was not uncommon with old cars and is still the case with lots of motorcycles.
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Old 01-27-2014, 12:16 PM
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yeah you should be good with just the commute.
Old 01-27-2014, 01:10 PM
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Thanks folks
Old 01-27-2014, 01:14 PM
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I'll add that the 30-minutes should be enough. However if you are driving with the heater fan on high, headlights on, and rear defroster also on, then the battery might not get charged very well.

You have some options:

1. Don't worry about it and see if it starts. If not, see #5 below.
2. Remove the battery and take it somewhere warm. A warm battery works better.
3. Speaking of which, run a drop light with an incandescent bulb outside and under the hood near the battery. The heat from it will help.
4. Get a little trickle/float charger and put it on the battery. It will fully top off the battery overnight.
5. Buy one of those portable jump starters. If the car battery cranks slowly and won't start the engine, put on the jump starter. Do this well before draining the car battery.
Old 01-27-2014, 01:29 PM
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With the extreme cold we've been having this Winter, I've been going over my Mom's house, starting up her 2003 Hyundai and letting it idle for 20 minutes or so every day. Since she only drives it once or twice a week, I've been worried about the battery dying. It sounds like I've been doing the right thing, yes?
Old 01-27-2014, 01:58 PM
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Quote:
Originally Posted by Honey View Post
With the extreme cold we've been having this Winter, I've been going over my Mom's house, starting up her 2003 Hyundai and letting it idle for 20 minutes or so every day. Since she only drives it once or twice a week, I've been worried about the battery dying. It sounds like I've been doing the right thing, yes?
Just idling is very rough on a gasoline engine. You'd be better off driving your Mom's car for 10 or more miles once a week. Another option would be hooking up a trickle charger to the battery. Is the car garaged where there is electricity available? Heck if it's garaged, is it a heated garage?
Old 01-27-2014, 02:41 PM
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Quote:
Originally Posted by JerrySTL View Post
Just idling is very rough on a gasoline engine. You'd be better off driving your Mom's car for 10 or more miles once a week. Another option would be hooking up a trickle charger to the battery. Is the car garaged where there is electricity available? Heck if it's garaged, is it a heated garage?
No garage unfortunately. I can take it for a quick spin, although driving it with no heat (it takes a looong time to warm up) in -10 weather is going to suck.

Anything for Mom though.
Old 01-27-2014, 02:59 PM
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There have been several threads about this, and the universal answer for cars that are driven infrequently (less than once a week for at least 10 miles or so) or have batteries older than a year or two or need to start in the freakishly cold temps we've been having is...

...a battery tender. These are cheap and need only an extension cord long enough to reach a plug. They will float-charge your battery, incidentally creating just a tad of warming heat, for weeks and weeks if necessary. You can even, as suggested in another thread, basically build the tender into the car so that all you have to do is connect the extension cord to a slightly accessible plug.

If you live where it's sunny, or at least bright-cloudy, get a solar batter tender and put it on the rear package shelf or dashboard, and wire it straight into any always-hot lead. (More or less directly to the battery is best, but any accessory lead that doesn't switch off will do.) It will gently trickle-charge your battery without even the need to plug it in. These are $50 or less.

Last edited by Amateur Barbarian; 01-27-2014 at 03:01 PM.
Old 01-27-2014, 03:30 PM
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For this situation trickle charging won't do anything, it is all about temperature. Does anyone know at what temperature you need to worry about a relatively new well-charged battery not being able to start an average car?
Old 01-27-2014, 03:30 PM
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Quote:
Originally Posted by Honey View Post
No garage unfortunately. I can take it for a quick spin, although driving it with no heat (it takes a looong time to warm up) in -10 weather is going to suck.

Anything for Mom though.
If you have to drive a ways to work or somewhere, swap cars with Mom. That might work especially on the days she isn't going to be driving.
Old 01-27-2014, 05:03 PM
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Quote:
Originally Posted by JerrySTL View Post
Just idling is very rough on a gasoline engine. You'd be better off driving your Mom's car for 10 or more miles once a week. Another option would be hooking up a trickle charger to the battery. Is the car garaged where there is electricity available? Heck if it's garaged, is it a heated garage?
Yes. The engine and charging system will work much better under load, while driving. Just idling the car for 20 minutes will not provide as good a charge as actually driving the car for 20 minutes.

It will take about 20 minutes of driving to charge your battery. That is why short trips wear batteries out, they never get back up to full charge and the drain from starting the car doesn't get replaced. There are other factors to consider such as how may lights, wipers, and accessories are being used during short trips.
Old 01-27-2014, 05:48 PM
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Quote:
Originally Posted by flight View Post
For this situation trickle charging won't do anything, it is all about temperature. Does anyone know at what temperature you need to worry about a relatively new well-charged battery not being able to start an average car?
Cold cranking amps are rated at 0 degrees F. A few degrees below that is not likely to shift the curve much. However, batteries do decline in CCA performance pretty fast - in a warm climate, you might get 5-6 years from a battery, whereas the same battery would be "failed" - unable to start the car reliably - its second 0-degree winter.

Trickle charging is part of the solution in that keeping the battery full to the very brim with magical fire will help ensure it can crank the engine; charging also creates a little bit of heat, raising the battery's core temperature and thus increasing the cold amperage. But an older battery at very low temps is going to struggle, especially with a large or high-compression engine.

In those cases, a block heater, a battery heater or both is the only solution other than a new battery every year. But trickle charging - battery tending - is a pretty miraculous thing for vehicles driven infrequently or needing every bit of juice to start.

(I just had to start a very temperamental vehicle with a very large engine - 7 liters, 10.5:1 compression, carbureted, no choke - after almost 3 months, in ~15 degree temps. With an aging battery. I almost got there. Had to throw my small charger on it in boost mode to get it over the hump and stay running. I hadn't planned to start it until things warmed up, but a trickle charger would have helped immensely in this particular case.)
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