Battery Re-Charging Question, Please

Hello:
Have a 1993 Corolla. Hardly ever used.
For a 3 year old battery that has "really" run down, likely all the way,
how much running of the car engine does it take to re-charge it ?
Also, I've heard that if a battery is "run all the way down," it is essentially junk now, and that one shouldn't even consider trying to re-charge it again; that it won't hold the new charge.
Is this true ? Why ?
Thanks, B.
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Try using a good battery charger rather than beating your alternator to death.
--
Ron P

If we are what we eat then: I'm fast,
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Robert11 wrote:

The cheapest fix may be charging with a low amp charger and then try using the battery. It might work fine for you.
Then, it may not hold a charge or have very little capacity when charged.
It would take much, much longer running the car to charge it than using a charging unit. Alternators are meant to replenish the capacity, not charge to 100% from zero charge while the battery is in use.
If you ran it down without recharging to 100% over and over and then left it to discharge completely, it may be sulfated beyond repair.
If you have a charger with a desulfication function you may have some luck. But it takes time.
Batteries in cold climates can freeze if in a very low state of charge, and if it has frozen, the case may be cracked someplace. Check all cells for liquid.
Lastly of course, you may have to purchase a new battery.
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wrote:

The only way to tell if the battery is toast is to recharge it and see. You can jump the car to start it and depend on the cars electrical system to recharge the battery but that might not be the best way to handle the recharge.
Today's modern car batteries are not intended to be discharged completely. They give the best service life when they are used for short busts of power, like starting the car, and then immediately recharged.
A battery that is completely discharged and left to sit for an expended period of time will most likely be junk. They can sometimes be saved if handled properly. The plates will sulfate and you have to recombine that sulfate back into solution. You can do this, sometimes, by an extended slow charge. Use a trickle charger for a month or more and then test the battery.
Jack
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Robert11 wrote:

It depends on whether it's maintenance-free or low-maintenance, but I don't know why. The latter are more common and always have removable caps, and I've had good luck restoring them with a 10A charger run for 5-8 hrs. OTOH a maintenance-free battery (may or may no have removable caps) will usually die after just a single deep discharge, as I learned, again and again.
Recharging a completely dead battery with just the alternator may overload it, so I would rather use a charger, at least for the first few hours. I once left the headlights on overnight and jump started the dead battery. The alternator output measured almost 14V after the jumper cables were unhooked and even 30 minuteslater, but next morning the alternator wouldn't read higher than 12.5V (not a cold morning). But the battery lasted another 1-2 years.
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