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Old 07-02-2018, 01:14 PM
Join Date: Jun 2014
Posts: 11,884
Left car light on all night but battery still seems OK

So I absentmindedly left my car internal light on all night long and the battery seemed dead for 1-2 seconds when I turned the key, yet it did eventually spring to life and the car drives seemingly fine. Is there anything bad that could happen to the battery if it didn't fully die but did get recharged in time? It's a SmartCar, for whatever it's worth.
Old 07-02-2018, 01:36 PM
Join Date: May 2001
Location: Scottsdale, more-or-less
Posts: 15,623
Draining a battery is hard on it, but at this point, there's nothing to be done about it.
You probably shortened it's lifespan by a few days or weeks, but - c'est la vie.
Old 07-02-2018, 01:42 PM
Join Date: Mar 2003
Location: Virginia
Posts: 12,312
It's fine. If you want it wouldn't hurt to put it on a battery charger, especially if your daily driving is short. The alternator will slowly charge the battery, but on most cars isn't designed to charge it a lot.
Old 07-02-2018, 01:49 PM
Join Date: Sep 2012
Posts: 1,534
I have left the dome light on all night a couple of times without any ill effects.

I'd suggest getting the battery tested next time you get your oil changed.
Old 07-02-2018, 01:51 PM
Join Date: Apr 2000
Location: Wash DC
Posts: 166
If your car started OK you haven't done any damage to your battery. Normal driving around should recharge the battery to normal levels.

Don't worry, be happy!
Old 07-02-2018, 01:52 PM
Robot Mod in Beta Testing
Join Date: Mar 2001
Location: Pennsylvania
Posts: 21,585
Your typical lead-acid car battery has two plates in it per cell. One plate is made out of lead and the other is made out of lead oxide. In between the plates is an electrolyte composed of water and sulfuric acid. As the battery discharges, the lead and lead oxide plates both turn into lead sulfate, and the sulfuric acid turns into water. When charging, the lead sulfate plates turn back into lead and lead oxide, and some of the water turns into sulfuric acid. From that over-simplified view, the reaction is completely reversible.

In practice though, it's not completely reversible. When you discharge the battery too far, the lead sulfate forms crystals which do not break up and turn back into lead and lead oxide. These sulfate crystals decrease the amount of useful charge that the battery can hold, and too much sulfation will completely ruin the battery.

Ideally, what you should do when you come out and see that the battery has been too deeply discharged is put the battery on a slow charger. Slowly charging it is more likely to break up the crystals as the battery charges since the charge migrates more uniformly through the plates. When you quickly charge the battery, as your car's alternator does, the charge is deposited mostly at the surface and it migrates more slowly to the inside of the battery, which is more prone to forming crystals that won't break up during recharging.

At this point, what's done is done. You've probably reduced the battery's capacity a bit, but most cars have a slightly oversized battery so that they will start reliably in all sorts of bad conditions. The car may be harder to start in the dead of winter, and the battery may die an early death under that type of stress.

With a little luck, you hopefully didn't end up with too much permanent sulfation and the amount of capacity loss will be minimal.

I don't know how "smart" the charging system is in a smart car. Older cars tended to have fairly simple charging systems. Newer ones are much more sophisticated in general and are much more likely to charge the battery in a manner that is more healthy with respect to the battery's longevity.

Sticking a slow charger on the car overnight wouldn't hurt.

ETA: If you have an AGM battery, AGM basically means "Absorbent Glass Mat". The electrolyte is absorbed by a fiberglass mat material, but the battery is otherwise similar to an old-fashioned lead-acid battery. AGM batteries however are less prone to sulfation.

Last edited by engineer_comp_geek; 07-02-2018 at 01:55 PM.

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