What is the best way to keep a battery charged on a car that's not going anywhere?

September 6, 2010 4:31 AM

How often, and for how long, should I turn over my car's engine in order to preserve the battery?

I have a 1999 Ford Cougar which will be effectively off the road for up to three or four months this winter. A friend is able to keep an eye on the car, and turn the engine over periodically to maintain the battery charge. However, conscious of the fact that starting the engine itself requires power, I would like to know how long I should have her turn the engine over for, so as to charge the battery more than is taken from it in starting the car. Also, is once a week suitably frequent? The car will be kept outdoors during the UK winter.

posted by Biru to travel & transportation (21 answers total)

Oh, and I don't know if this is relevant, but it's the 2.5L V6 model I have.
posted by Biru at 4:33 AM on September 6, 2010

Why not just disconnect the negative (black) lead?
posted by hariya at 4:42 AM on September 6, 2010

That I did think of, but if the battery is disconnected and cold for several months, will it not still be taking a risk?
posted by Biru at 4:48 AM on September 6, 2010

Once a week should be plenty, unless the weather is very cold. Rev the car up slightly for several minutes at a time. If he has trouble starting the car, as in the starter motor is sluggish, he'll need to do it more often.

Taking the car for a 15 minute spin to warm the engine up will help a lot. Make sure the revs climb up high enough to have some spare charge left to put back into the battery.

You might also consider a solar car battery charger.
posted by Solomon at 4:50 AM on September 6, 2010 [1 favorite]

Yeah, it would be ideal to have her drive it around, but to have her on the insurance would be ridiculously expensive given her age and driving history etc.

As a guide then, once a week, five to ten minutes of running above idle, perhaps around 2,000 rpm?
posted by Biru at 4:59 AM on September 6, 2010

I did some reading about this subject a couple months ago, and it needs to be a decent drive to offset the power needed to actually start the car, greater than 15 minutes or something along those lines. A >15 minute drive once a week would be enough. And I think idling isn't quite enough.

A mains charger (or possibly a solar charger) is actually a better idea, and will do a better (i.e. MORE) charge than the alternator can.
posted by antiquark at 5:01 AM on September 6, 2010 [5 favorites]

IMHO- Once a week is more than necessary. If you have no drain on the battery (no blinking lights or clock), once a month is plenty.

If it's parked outside, I would drive it around once a month to prevent the tires from flatspotting or rotting.

Spend 30 bucks on a A/C to D/C charger for peace of mind.
posted by WhiteWhale at 5:09 AM on September 6, 2010

More often is better than less often, but you'll need to get the revs up high. 2000 probably won't be enough. I'd go for about 3000 or probably even more. That's going to waste fuel and create a lot of exhaust if the car is standing still.
posted by Solomon at 5:11 AM on September 6, 2010

You should not rev a cold engine.

I think you would be better served getting a cheap float charger to put on the battery rather than letting it idle for long periods or paying for insurance to have someone drive it. In the US, Harbor Freight sells one for $5. I don't know what the UK equivalent is, but if you look around I'm sure you can find something.
posted by Rhomboid at 5:19 AM on September 6, 2010

I think the solar charger is probably the best idea for me. I like the fact that it can be attached to the cigarette lighter and left inside the locked car.
posted by Biru at 5:23 AM on September 6, 2010

"IMHO- Once a week is more than necessary. If you have no drain on the battery (no blinking lights or clock), once a month is plenty."

When I lived in the upper midwest, once a week was REQUIRED ... if I let it go two weeks, I had to call for a jump. It was parked outdoors (in a giant parking lot where it took forever to walk to the car ... we called the whole process "walking the car" and my entire week focused around suckering someone to walk with me in the below-zero weather to sit in the freezing car with me for fifteen minutes and then walk back).

Probably my climate was considerably colder than yours. Does the friend have grocery store errands to run or something once a week? That makes turning it over less irritating.
posted by Eyebrows McGee at 6:46 AM on September 6, 2010

It'll be kept outdoors in Greater London, which according to Wikipedia shouldn't really even get below freezing.
posted by Biru at 7:11 AM on September 6, 2010

My dad has a car he keeps in storage at his vacation house. It's there, undriven, for very long stretches (sometimes up to 4 months). The locale sees etremes of hot and cold. He simply disconnects the battery while it's stored and reconnects it when he is there and needs to use it.

He does keep a block charger in the storage unit just in case but I don't think he's ever needed it.
posted by devinemissk at 7:15 AM on September 6, 2010

I tried a solar charger on a truck I used to drve only occasionally and found it pretty worthless, although I didn't park in full sun. I also have a Battery Tender and it works much better.
posted by TedW at 7:46 AM on September 6, 2010

Read this for some useful info on lead acid batteries. Not too technical, and the section of the FAQ on self-discharge is what you want.

Were I confronting your scenario, I'd drain the gas tank and remove the battery to the inside of a garage, and charge it monthly. Not a big deal.

If you don't want to remove a heavy battery, disconnect the negative lead and charge it every month or two.

There really isn't a rule of thumb that I know of which will tell you how long to idle an engine to recharge the battery. It depends on how long you cranked, what the state of charge of the battery is, the temp of the battery, it's age, and the charging current / profile. It's analogous to asking how long it takes to fuel your tank... (size of tank, degree of emptiness, rate of fill, etc.)... easy to calculate if you know the quantities, but a total guess otherwise.
posted by FauxScot at 7:59 AM on September 6, 2010

If you disconnect the battery, there's no need to start the car. When leaving the battery connected, all of the computers in the car will draw a small amount of current, just to "keep alive."
Cranking the engine is the largest draw a car's battery will ever encounter. My own testing has shown me that, generally, instantly after cranking the car, the battery itself constitutes a draw of about 30-40 amps. That is, the alternator has to output 40 extra amps just to recharge the huge expenditure of amperage used to start the car. This inrush lasts about two minutes. After that initial period, the battery current draw starts to drop off pretty quickly. After ten minutes of idling, a healthy battery typically draws less than 10 amps (and usually even less than 5 amps).

That being said, if you're concerned about preserving your battery, the best thing to do is either leave it disconnected, or attach a trickle charger. Running your car for ten minutes a week during the cold winter isn't really ideal for the engine. During cold starts, the engine computer makes sure that the car runs rich (using extra fuel) for a number of reasons. I've seen problems arise in vehicles that are subjected to repeated cold starts followed by short trips. See, that extra fuel doesn't burn completely and has the tendency to saturate the engine oil. At operating temperature, those hydrocarbons evaporate out of the oil. But if the car never warms up, they accumulate, causing sludge and lubrication issues. Engines that are subjected to short trips also tend to build up moisture in the crank case. Just like with the fuel, any moisture in the air inside the empty spaces of the engine is evaporated at operating temperature. Unburned hydrocarbons and excess moisture are what combine to cause sludge build up in engines.

Now, it's easy to dismiss this as something that can't possibly cause any problems in the space of three months, but it's not impossible. Here's a little story:
I was attending a training program that lasted about four months. This training program had a small fleet of (extremely good condition) training vehicles. Every day when we arrived, we parked the cars outside and at the end of the day, we parked them back in the shop. Shortly after I started, a couple new vehicles were delivered and they joined the routine of taking two short trips a day along with all the rest of the vehicles. By the end of my four months there, we started to notice that these two new cars had a noticeable rough idle. So, we hooked up the diagnostic computer and see that the cars are trying to correct for an excessively rich condition. We open up the oil cap and see a pale ring of moist sludgy oil accumulating on the cap. After we changed the oil on these cars, the fuel adjustments went back to normal and the idle smoothed out. So, these ill-effects of repeated cold starting can take hold and cause problems pretty quickly.

If you're going to start the car once a week, make sure it gets driven until it's at normal operating temperature. If you can't ensure that it'll be driven, just unhook the battery or put a charger on it.
posted by Jon-o at 8:03 AM on September 6, 2010

If the battery is the only concern, I'm not sure I'd worry about it at all for 3-4 months. I've ignored my car for longer than that with no apparent ill effects (other than being colonized by mice once, but that's a different story). I live in a much milder climate than Eyebrows McGee, but then so do you.

As Jon-o says there's also lubrication and moisture to consider.

Regarding insurance, does your friend have her own insurance? Many policies cover occasional driving of other peoples' cars.
posted by hattifattener at 8:32 AM on September 6, 2010

Straight up: your premises are flawed. Listen to Jon-o.
posted by TheNewWazoo at 9:17 AM on September 6, 2010

Ps: In addition to the reasons I explained above, I recommend actually driving the car also for the sake of the brakes and tires, both of which suffer when left stationary. Although the corrosion that builds up on brake rotors seems to scrub off pretty quickly, some heavy build up can permanently affect the rotor surface and may require resurfacing in severe cases. At the very least, they will make terrible noises and vibrations if unused for four months. In the winter, they might even seize from corrosion.Tires will also dry rot very quickly if left underinflated and stationary. Ideally, your friend should drive your car for a half hour, twice a week.
posted by Jon-o at 9:39 AM on September 6, 2010

This might be a big "duh", and maybe it's because I have a pretty old car, but I bought one of those solar chargers and it was useless for me because my cigarette lighter doesn't have power unless the key is turned to at least the 'accessories' notch. I could leave my spare key in there and lock all the doors but that just seems like an invitation for someone to break a window and drive off. Be sure your cigarette lighter works even when the key is not in the ignition.
posted by lemniskate at 10:13 AM on September 6, 2010

While you haven't explicitly mentioned this, putting the recommended dose of fuel stabilizer in your tank before you store it will ensure that things operate smoothly when you're ready to use the car again. You'll want to run it for a few minutes after adding to let the stabilizer get through the entire fuel system, which will prevent your lines from gunking up.
posted by kdar at 10:43 AM on September 6, 2010