My question is different. That question is asking about how long to start the car, I’m faking about how long it should stay on after it’s started because it keeps shutting down again.

I put a new battery in my car a few months ago. I recently done ozone treatment and left the boot open for 4 hours each day over 3 days. I didn’t realise at the time However I have just realised slan interior of light stays on when you leave the boot open.

I needed three jump starts today though I drove for 20 minutes. Bearing in mind the battery is newish, would leaving the boot to open hav killed the battery such that I will now need a new one. Or do I just st have to jump start and drive it for longer/how long from r it to recharge.

When will I know the batteryt needs the be changed, if it keeps dying after 3 consecutively jump starts should I change the battery?


1 Answer 1


4 hours of interior lights will not kill a brand new car battery. I am presuming you drove it normally in between.

12 hours (3 sessions with no driving between) might kill a new car battery, but even that seems unlikely -- assuming a rather smallish 48 amp-hour car battery, interior lights would have to be pulling at least 4 amps, which is quite a lot. If they are LED lamps, I'd say "impossible".

And yet, your car has a flat battery. I have a hunch about that.

How long should it take to recharge from flat? Let's do the math. If the auto battery is 100 amp-hours, that is 100 amps for 1 hour (rather unrealistic), 20 amps for 5 hours, 30 amps for 3.3 hours etc.

However, if you are mostly idling / puttering around town at 25 mph / sitting at stop lights, the alternator will not spin fast enough to get much power. So recharging will take longer, except for this:

  • if you are also running high-power accessories, like blower, headlights, wipers, defogger, engaging the A/C clutch, high powered boom-boom radio, heated seats, etc... you may not be making any progress on charging at all. You might even be running the car in a deficit every day, as your battery gets flatter and flatter day by day.

So. Your battery is flat. You needed 3 jump starts in 20 minutes, well, we can figure that one out, eh?

First, 3 jumps in 20 minutes is one every 7 minutes, which means you aren't running the car nearly long enough to charge back up to full. But still, 7 minutes of honest recharge should leave enough for one restart!

That brings me to the "heavy accessory use" theory. Perhaps the "honest recharge" never happens because accessory use is using all the available power.

Now you know what to do about it.


My preferred way is to either a) potter around town for several hours with every accessory turned off and open the windows instead of A/C... Or b) find a pretense for a couple hours of fast driving in open country (no traffic) with modest accessory use. While doing this, don't shut the car off unless you are near services that could jump you.

Should you just idle the car unattended? Heck no! It causes excessive wear/tear (the service manual may say not to do it, even). Just buy a plug-in battery charger and bedone with it. Get one with an amp meter so you can see if it's charging (if not, it's either full or damaged).

Here's my theory. If most of your driving is around-town, and you run accessories a lot ... It's possible this is a chronic (ongoing) condition. Your battery is never really able to recharge fully, and slips a little every day until you get here. It's a theory.

If you want to settle that question, there's a simple chemical tester (called a hygrometer) that can tell you if the battery is undercharged. Or, put it on a battery charger (with an amp meter) and see if the battery gulps down the amps (which means it needs charging).

Cars used to have an ammeter that showed you "charge or discharge". With all this digital dash and CANbus stuff, you'd think they could read it up on one of the displays! Maybe your car has this in some menu.

  • The car is probably a ‘99 civic... so obd may be an option...
    – Solar Mike
    Sep 9, 2019 at 6:45

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