Car Advice

Myth-Busting: Going for a drive will charge your flat battery

We’ve all heard the one about idling your car or driving it around the block to recharge a flat battery…shame that it’s nonsense then.

It used to motoring law that if you came out to your car and turned the key in the ignition and nothing happened that you’d wheel up another car, connect a set of jumper leads and start the car that way. The flat battery would get enough spark, via, the other car to start your car at which time the alternator would take over running the car’s electrics. Most would leave the car idling or go for a drive around the block and think that was that, the battery was recharged.

But almost everyone thought, and plenty still do, think that idling your car after the battery has been jump-started or driving around the block is all you need to do. It’s exactly what my neighbour did just this week. He went out to his car and it wouldn’t start, so, he did what most people would do and called a breakdown service.

The breakdown service arrived and jump-started the car. It fired into life. Great. But then my neighbour left his car to idle for ages. I noticed it because his driveway is right next to my office window and the smell of diesel exhaust fumes wafting through my office window was becoming a little too much to take.

Sure, I could have closed the window and got back to my day, but after having witnessed the flat-battery scenario and the long idle afterwards I felt it my civic duty to intervene. My neighbour explained he was charging his battery by letting it idle. Um, no, he wasn’t. All he was doing was gassing me and wasting fuel.

See, your car battery is there to power the starter motor which kick-starts the engine. Once that’s done, the alternator (think of it as a generator) which is powered via the motor and produces electricity, takes over powering your cars electrics and ‘topping’ up the battery or surface charging and this will ultimately cause more damage to your already sick battery. Your alternator can’t recharge your flat battery, not even if you let the thing idle all day or drove around the block 10 times. Not even if you drove on the highway at 100km/h.

See, if your car’s battery is flat, it’s flat. The only way to charge it is with a suitable multi-stage battery charger which will restore your battery to good health. So, if your battery is sick and you can’t start your engine then a battery charger is the best way to go…they’re cheaper than a new battery.

And if you do replace your battery then remember to use a memory minder to ensure the sensitive electrics in modern cars aren’t damaged by the disconnection. See, even when your car is not running there are bits and bobs that still need power and they draw from the battery.

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1 month ago

Completely wrong, going for a decent drive after a jumpstart will certainly charge the battery, and enough to the point where you cam crank the motor over to startup multiple times.

Drive-by autocorrect
Drive-by autocorrect
23 days ago

I came here while looking for the difference in charging time between idling and driving. This article however is only disinformation: if you just left your lights on overnight, the charge in the battery was depleted quickly, and using a charger or driving or even idling after jump starting it should restore it (almost) to full health, if not too much time passes before the battery is full again. Only if the battery has been (partially) depleted for a long time, especially if you are not using the car for months on end, some of the electrode material changes to a state from which it can’t be recharged anymore, and the remaining lifetime shortens significantly or capacity can decrease to the point that you may have to replace the battery before you can use the car again.

6 days ago

Actually this article seems completely false. Flat Battery? Be more specific on the voltage. Are we talking about a car battery that’s been drained overnight (leaving headlights on) or left the door ajar and cabin lights drained the battery from starting? If so then you are incorrect. In fact if you are able to catch your mistake early enough and say you drained the car battery with headlights left on (Older Cars usually have this problem because they don’t have automatic headlight shut off feature) you can still jump start this battery with another car and then idle the car for at least 30 minutes. This helps charge the car battery to a level where it’s safe enough to drive and go for a local drive around the block for 10 minutes so you are within walking distance of your home in case the car dies or stalls leaving you in a situation where you require another jump start. Usually the 30 minutes idling in the car and you can also step on the accelerator some to rev the RPM not too high while in Park it does increase the current to the Car Battery than just plain idling.

After you’ve driven around your block for 10 minutes and the car hasn’t died or stalled on its own it should be safe enough to go on the highway for a 75 MPH+ drive down a long stretch of highway say 5 minutes away and then get off the ramp and return. Repeat this for at least an hour and you should have juiced the car battery enough so when you finally return home and are parked in your garage or driveway you can shut off the car. Then turn the key and restart the car and if it successfully starts you’ve actually brought the car battery high enough where the battery hasn’t been weakened too badly where it can still be reused. However during this critical phase of a weakened battery state I recommend after you successfully restart the car that you idle it for at least another 30 minutes or go for another hour of highway driving. This should be enough to start the car the following day in the morning. As long as you keep starting the car and driving it for a good 60 minutes each day you will get the Car Battery back to usable status. Check the Car Battery Voltage and see where it is at before you start the car overnight in the morning. The Car Battery Voltage should have settled somewhat and hopefully in the 12.00V+ range. Although I’ve managed to start cars with drained batteries in the 11.8V range and some as low as 11.1V. But that’s with manual headlights off and the radio and A/C off prior to starting. Always turn these gadgets off prior to starting the car.

However regarding the fumes issue. You should have just closed the windows and let the guy idle charge the car. The best thing you could have done for the guy was bring out a voltage meter to see where his battery voltage was when charging. It should be in the 14V-15V range when the alternator is charging it. If it’s in the 12V range or lower when the alternator is charging it most likely it has a dead cell issue but it might be possible to resurrect it if done soon enough. However, I recommend having a spare car key. One just for keeping in the car to start it and idle, and then the other set for locking all the doors on the outside to prevent someone from stealing your car by just opening your door as some folks tend to leave the car outside warming up and a car thief just strolls down and gets into the car and drives away. If this car thief is smart he / she will not pull over and turn off the car or he / she will get stranded unless his / her buddy jump starts the car. 🙂

Battery charges or trickle chargers are only good for maintaining a pretty good car battery usually in the 11V range from draining further. Trying to resurrect a car Battery that hovers in the 10V or less range seems like a dead cell issue and harder to resurrect. I only managed once to resurrect such a battery by adding some distilled water into the car battery to top it off each of the 6 holes and then idle charged it in the garage after jump starting it with another car.

Somehow I gave up on that old car battery and unhooked it for a few years and at one point my garage door was broken so it sat there for at least 2 years. Then I rehooked the old car battery and miraculously it somehow was able to start the car which astounded me as during the time it was not in use it somehow repaired itself slightly and maybe fixed the dead cell. Sadly I ended up swapping it with a brand new battery since I needed reliability and put this old battery aside for 6 months and accidentally used it as a jump start battery and drained it quite a bit. Then I forgot to charge it back after incorrectly using it as a good jump start battery and now that old car battery seems unlikely to want to be revived. Been using a desulfator and trickle charger to see if can get back to normal but somehow after adding the distilled water this time the voltage dropped from the drained 10V down to the 6V range and I’ve been trying to resurrect this old car battery for almost 2 weeks and nearly about to give up on it.

However, if somehow it gradually begins to increase the voltage slightly there might be hope for it but not counting on it. At the moment I’m considering swapping the new car battery out for the old car battery and then jump starting it and doing my idle alternator car charging technique to local driving to highway driving hoping that it somehow kicks enough juice to resurrect it above 6V as the desulfating and trickle charging isn’t showing any hope. If this doesn’t get it to above the 6V range after an hour of driving then I think it truly is a goner.

These days the best way to keep a car battery lasting longer is I employ a quick negative disconnect after I get the car in the garage. This prevents phantom power draining the car battery if you end up not driving the car for a week or a month it can add up. In some cases some cars are quite hungry and after a few days could make the car unstartable without a jump start if it was in a weakened state.

So far I’ve been doing this since the new car battery and it’s helped maintain its lifespan longer than had I not. The car battery hovers around 12.65V when disconnected overnight. You can also trickle charge the battery while it’s in the garage as a way to top it off as even a disconnected car battery slowly drains voltage over time just sitting there unused. I logged this in my records for many months and even a year to see how much voltage a car battery drains sitting there disconnected over time. I found it fascinating.

Don’t be upset at another mate charging their car. The other thing you could do is tell them to park the car facing the other direction furthest away from your window. At least the fumes won’t be as nauseating or deadly.

Anyhow I noticed you are in Australia – .au and the km reference. 😉

Cheers Mate!

Isaac Bober

Isaac Bober