# How long time do I need to run the engine to charge the same amount of power that the starter took

I have a Range Rover 4.6l V8, and I'm wondering if anyone have some sort of estimate of how long time I need to keep my engine running, to charge the same amount of power that the starter took to start the engine.

I'm not really looking for exact figures here, but more if it's 5 minutes, or an hour.

(It starts in about 1-2 seconds)

• Nobody knows the answer to this. The answers are purely speculation. This is because the health of your battery, alternator, wiring, and other parts are variable from vehicle to vehicle, even ones of the same make, model and year. Environmental factors, vehicle speed or RPM, etc... so many things make this an unanswerable question.
– cory
May 17 '17 at 15:26

We always used to say 7 miles of "decent" driving ie not idling in traffic, but the newer charging or smart charging systems may need slightly less...

Preferably you drive about 10 miles then you could be sure that your battery is rechared again. Or start up your engine and leave it running for about 5-10 minutes.

• 10 miles at normal city speeds (at least here in the US) is about 20 minutes of driving. Idling produces far less power from the alternator than driving under load, I don't think those two figures are equivalent enough. With that being said, I'm sure that the 10 mile rule of thumb would be good for this case May 17 '17 at 13:24
• @Taegost 50/50. load is put on the alternator regardless of RPM, circuitry keeps the Wattage/horsepower drain constant. Said car battery is charged at the same rate, wether running at 500 RPM or 10,000 RPM. I believe the most common amperage for charging batteries is ~20A at 12.4V or 248 Watts / 1/3Hp drained from engine power. May 17 '17 at 18:06
• @tuskiomi - To clarify, when I mean "driving under load", is driving around and not idling. There is a difference in alternator output whether the vehicle is idling vs. driving around, which means that there's also a difference in how quickly it can charge a battery. The rate of charge is not the same. My point above is only that the charging ability of the alternator when the vehicle is driven for 10 miles vs. just idling for 5-10 minutes is very different, which I think doesn't make for a good comparison in this case (But I also don't think it's a bad one, either) May 17 '17 at 19:09
• @tuskiomi - That's... Not how it works. and the battery does act as a capacitor as well. Just because you can charge a battery at varying power levels, doesn't mean it charges at the same rate for all power levels. This link here gives a decent high-level overview: powerstream.com/car-battery-faq.htm May 17 '17 at 19:25
• @Taegost No, it is not a capacitor. A capacitor can be charged in the megawatt range. The Lead acid cell can only be charged at a fraction of that (50W tops, usually). And aside from that, you're running away from the point you made before, that the input RPM of the alternator doesn't adjust the charge rate. I will concede, that running the alternator at below the required voltage for the circuitry to work will impede the charge rate, but above that threshold, it's all the same rate. May 17 '17 at 19:36

There is a factor not considered in other answers, which is the original charge of the battery.

If you start the car with a healthy, fully charged battery, only a few minutes are needed, in particular if you are driving (higher revs). But, when the battery is depleted, the time could be really long. A few months ago I had to start an oldish friend's car that was left parked for a month. I had to jump start it, I let it run for a few minutes, and then drove it for 20km in a mixture of city and highway. I stopped it for 3 minutes, and when I tried to start it, it wouldn't crank at all. So I left it overnight with a battery charger, and from the next day the car would start and recharge normally.

• This is a good point, and my battery isn't the best one... May 18 '17 at 6:53
• Is this true? Does a battery charge faster when it's almost full? Your anecdote doesn't prove that point. May 18 '17 at 18:12

Given the AC is running, radio is on, and the headlights are on.... 15 minutes of actual driving is recommended to replenish the used current. 10 minutes if you keep some accessories off for the wait. Assuming you're not driving at higher speeds just drive for 6-8 miles and you should be good to go.

Edit: Headlights and AC are the first thing we ask someone to turn on if a suspected alternator needs to be tested. The actual load of both AC and Headlights are about equal, but on average turning both on dips roughly 1.5v off the alternator. And if it's a weaker (dying) or bad (dead) alternator it will no longer charge the battery with these on. Radio is only a major factor in the case of aftermarket setups which could take several tens to hundreds of watts to operate. A stock radio won't do anything as compared to the AC and Headlights.

• How much juice do the lights, radio, and AC or heat use? Obviously the answer is "it depends" but just on a rough order of magnitude basis? Which is the most important to turn off? May 17 '17 at 17:27
• Is it also true that running the windshield defroster also runs the AC? So, one should avoid that, too? May 19 '17 at 15:45
• Depends on the defrost. Some use the internal fans where others have internal warmers (usually back glass) both use up energy but not as much as the AC compressor usually. This is due to the fact it's on the alternator belt and uses that energy up. May 19 '17 at 16:00
• Ballpark figures: Head+tail lights ~15-20A, AC compressor clutch ~10A, defrost ~10-15A. May 20 '17 at 20:08

This would vary from car to car. If you want to be safe I would hazard a guess of 10-15 minutes at a fast idle. With lead acid batteries you never want to go below %50 of their capacity and want to leave them charged when your done. An external trickle charger might fit the bill.

A P38 has a 100A+ alternator and Rover V8's require very little power to start - the older generation had facility for a hand crank handle through the front bumper.

I'd suggest 5 mins of running would be OK, maybe 10-15 if you're running lights/AC/heater as those can be quite a load.

Why are you asking - what issue are you having / worrying about?

• Well, my battery is bad, and the car has been standing for about a month since the expansion tank sprung aleak. And now I want to get it running again, but I don't want to drive off with the risk that something is bad. So I was wondering how long I need to have it running before I restart it if I have to. May 18 '17 at 7:02
• OK so if you're starting it with the main aim of charging the battery, you want to start it and hold it at a high idle (maybe 1500rpm) to get the alternator working, with no other loads (so no a/c, no lights etc.), that should put a good amount of charge back into the battery in 5 mins. Have you fixed the header tank or is that a problem? May 18 '17 at 9:57
• Also it's a bit of a plug but www.lr4x4.com is a very good forum for all things Land Rover, good technical info, factory service manuals etc. etc. May 18 '17 at 9:59

If your stuck in snow storm with no help available. it's 20 minutes on 20 minutes off. Repeat.

Yesterday, I left my Honda Amaze in the parking lot with parking lights ON. When I came back after 6 hours, I saw that scary dying blink and cursed myself because, I knew that it would be hard to start the engine. Battery went almost completely dead !

It was midnight, thought of leaving my car and return back the next day with some solution. Just then, luckily, two guys from the nearby coffee shop which they had just closed, were passing by. They came to my rescue and pushed the car, the engine sprang to life. However I was worried what if the car stopped in the middle of 3 mile journey. I searched online and followed this post in the forum.

I drove in total 6 miles, then parked my car below my apartment with the engine ON, headlights, radio and the AC OFF. Revved it up on neutral at 2000 rpm for 20 mins. And praying that it must start now after I shutdown. And boom.. Thats exactly what happened. I could then start it normally. I tried twice and both the time, battery worked normally.

So to conclude, if you have a new gen car, and if your battery goes dead due to overdraining, then you must try the technique.

Thanks mighty V8 engine for the post.