New Yorker

A few months ago my battery ran down (car sat for a week with the trunk light on; trunk not fully closed). So I got a boost from another car, started mine up, and drove it for an hour. Everything was back to normal, and I've had no battery issues since. Car fires right up in even the coldest weather. But… charging my battery from nothing, could the alternator have been damaged? (I heard somewhere that alternators are not designed to recharge batteries from zero.)

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Brettus

no problem - can be a good idea to get a long drive in after a full battery discharge but not essential .

EDZRIDE

It's not hard on the alternator but it is hard on the battery (unless it's a deep cycle battery)!

Razz1

Go out and get yourself a Schauer battery charger for 35 bucks and re-charge the battery at 2 amp

Jedi54

should be okay for the alternator, some batteries just don't take well to being drained down so far.
If it's holding a charge, you should be fine.

StealthTL

Alernators don't like it - the battery damage is already done, don't make it worse. You want to re-charge slowly, SLOWLY.
When the alternator sees such a low voltage it tries to pump it up all at once - like 100 amps until it comes up. Alternators have carbon brushes and the don't last long at full output, they burn away.
Battery charger FTW.....

S

nycgps

I would say unless its a deep cycle battery, you should replace it after it has been "deeply" discharged by your trunk light.

Cuz one day it will just stop working, and Lets hope that day will not be the coldest day and you're in the middle of nowhere ...

but if you dont want to replace it for whatever reason, use a battery charger. Its not that expensive. If you dont want to pay for it, will replace the battery soon but in the mean time u want to make sure its fully charged, take it to Autozone, they have free charging service, takes 30 minutes.

Socket7

I wouldn't make a habit of doing it, it's not good for the battery or alternator, but I also don't think you've caused any damage to your alternator or your battery (you have cut down the lifespan of the battery though, but if it still puts out 12v, you haven't damaged a cell).

If you're the kind of person who does this regularly, invest in a battery charger. If you do it once every few years, forget about it. I've done this a couple times on my old accord and never had an issue. For all i know it's still running the same battery today.

New Yorker

Thanks for the insights, everyone.

Brettus

hmmm all these years of recharging a battery from zero with the alternator and now i find out it's not a good idea .
Never had to replace an alternator in 25 yrs of driving and always get good life out of my batteries
I did hear once that it can be a good thing to occasionally totally discharge your battery and recharge it - something to do with "memory" .

Huey52

Nope. Only NiCad batteries used in older laptops & cellphones had a 'battery memory' issue and should be discharged completely before recharge. Your standard automotive battery is lead-acid and should not be deeply discharged on a regular basis.

"Deep cycle" automotive/marine/home power battery backup batteries are designed to withstand deep, repetitive, discharge. If you plan on leaving your trunk ajar for extended periods often, consider a deep cycle battery like the Optima Yellow Top.

Don't feel bad New Yorker, I left my trunk ajar a couple years ago. I did use a Sears Craftsman charger at lowest setting to slowly recharge my stock Interstate, but your one time alternator recharge - not to worry. In the future do use a charger.

Nubo

Are you sure? I thought it was Generators that had brushes, and we switched to Alternators to get away from that.

Brettus

you didn't have to use it in your sig.

skrubol

It's not good for the alternator or the battery, just like you aren't supposed to jump start someone from a running car. Trying to pull a dead battery up to 14v or so takes an incredible amount of current, beyond what the alternator should supply or the battery is meant to take.
Most of the damage is done in the first minutes of running, as after that the battery is getting up to a decent charge.
There's really no avoiding this situation when getting jump started though, so I wouldn't worry about it too much.

Huey52

Huh? Does not compute. I said Nope, not Dope.

Brettus

/\ oops sorry man - i'm having a bad day

StealthTL

Alternators have brushes too. (Anyway, Mazda manuals always refer to the rx-8 'charger thingy' as a Generator.)

The alternator is made to keep the battery topped up - if you turn on the lights, 100 watts, the alternator puts out the 100 watts, battery stays constant.

If the battery suddenly needs 100 amps to get back to normal (1200 watts.....) the system will try to supply it as quickly as possible, damaging the alternator and the battery.

S

Huey52

Note: US automobiles haven't had [DC] generators since the early 50's, same period as 6VDC systems were coming to an end. We've had [AC to 12VDC] Alternators since then. Alternators have onboard AC to DC rectifiers.

Generators (DC) and Alternators (AC) both have "brushes" to electromechanically interact with their windings.

btw: remember that a generator is the opposite of an electric motor, which is why electric automobiles can generate a small charge to their batteries via motor-braking action.

skrubol

Alternators don't have to have brushes, but I believe car alternators do to avoid having large (heavy) permanent magnets.

Huey52

Yep (as opposed to nope).

'Brush', by definition for automotive alternators: A block of conducting material, such as carbon, held against an armature commutator or rotor slip ring to form a continuous electrical path.

GaMEChld

Or a Battery Tender, and charge it at 1.25 Amps! Now THAT'S gentle! XD

skrubol

The brushes in alternators don't carry the full current though, only a few amps for field current.

StealthTL

OK, one more time -
our 'generator' (Mazdas words, not mine)
has 'brushes' (carbon, ferro-ceramic, guacamole, whatever)
that will be 'damaged'
by dead battery resuscitation.

S

Jedi54

^^^ so what you're saying is that this car has NO alternator?



haha.

ken-x8

Bada-bing! Generators have commutators, alternators have slip rings. Remember the old days when part of the drill of fixing a generator included sanding the commutator? (Or, in a machine shop, turning it and re-cutting the isulators.)

Bada-bing! again.

Big difference in brush wear between a generator with output current coming off a commutator, and an alternator with just field current going through slip rings.

I wonder if a drained battery can look like a short long enough to damage an alternator. I've jumped fully-drained batteries and then driven off more than enough times without any harm to the alternator. (Or generator.) Not good for the battery to charge at a high rate, of course. But they do charge at 20 or 30 amps for a brief time after a normal start.

Ken