bad alternator = hard starts?

[turbo970]

well i worked on the subie today. cleaned the crank and cam sensors and checked the vacuum lines. any ways in order to get the subie to run i had to let the starter crank for a while before it would run. it idled real low for a min then ran fine. i tool my multi meter to the battery and it read 12.33 volts. does the coil need to reach a certain voltage or amperage in order to run right. basically asking if this bad alt could be a big part of my starting issue. cause with a fresh battery the subie starts right up.

[evolutionmovement]

Yes. The battery should only really start the car. The rest of the time, it's kept charged by the alternator, which also runs the electrical. A weak or repeatedly discharged battery can also kill an alternator prematurely as well as a bad alternator killing a battery by either overcharging it or making it do the work, draining it. A dead battery sitting for as little as 24 hours can be killed and trying to bring it back to life with an alternator will likely not only fail, but could bring about the early demise of the alternator (perhaps even soon after). Batteries should only be charged by chargers. The relationship between the two is the reason that the failure of one is so often followed by the failure of the other. I'm not saying you're doing one of these things, just giving the information as I've found there's a lot of misunderstandings around this stuff (and hopefully not including myself). My guess is the alternator, maybe with an intermittent condition as it slowly dies.

[turbo970]

well i'm hoping a new alt and a charged battery will make the subie derivable finally

[91Beater]

The low voltage can be bad battery(self discharge), drainage, or bad alternator.

Go to Harbor Freight and get a $5 DMM. With the car running, you should see 14 to 14.5v at battery. If not, you could have a bad alternator or poor connection causing the alternator to not get proper feedback.

Check alternator case to battery negative terminal with the car on. Should be close to zero. If not, grounding is not good.
Now connect an alligator clip in between meter's leads then connect the car through the meter in ammeter mode.

Remove the jumper wire. The reason to have a jumper is to prevent surge current fro flowing through the meter. If you don't follow this, you might blow the fuse in the meter. You should only see 10mA or so. If you see dramatically more, you have a parasitic drain. Check after market mods like stereo.

Part throwing is a technique used by unskilled hacks. Don't do it.

[turbo970]

i have a dmm and with the car running the voltage read 13.8 but thats after cranking on it for a lil while. i replace the positive and ground cables and a new jumper from the alt to the battery. seems to start a lil easier but not to the point of driving it

[91Beater]

What's the resting voltage at battery after sitting overnight?
You need to check parasitic drain before committing to parts changing.

[turbo970]

12.5 or so after sitting over night

[91Beater]

12.5 or so after sitting over night

That's a bit low. If you're getting 13.8 to 14.5v with the car on, you either have parasitic drain or battery that doesn't keep charge.

For the 1st, you test it like I explained.
For the latter, you charge the battery fully with a charger... then leave it disconnected for a day or two. If it drops below 12.65 as fast as your overnight, you have a cell that's on its way out.

Car battery likes to be 100% charged always. Accidental discharge or two isn't a death sentence, but dramatically shortens life. Heavy extended drain will pretty much end it.

Has the battery ever been drained until flat and left by leaving something on for 3 days at airport parking lot or the like?

Basic battery is about $100 at WalMart here. Lead is expensive... so don't even bother replacing battery until you do paratistic drain test.

[turbo970]

i run agm batteries. and the battery was at 75% charge when i put it in the car.

[Legacy777]

12.5 or so after sitting over night

Just to clarify....is this with the car running or not?

If it's not with the car running, that sounds like a pretty normal voltage to me.

[turbo970]

12.5 is with car off but seems to settle at 13.8 13.9 at idle and 14.2 with med idle no load on system

[Legacy777]

12.5 v with the car off is normal, and those voltages with the car running seem normal to me as well.

[turbo970]

they went up when i put a jumper from the battery to the alt.

[91Beater]

Where on alternator did you jumper to battery and for what purpose?

I didn't ask the TS if he/she is using AGM. Since it was mentioned though, 12.5v is LOW for AGM. AGM's resting state is higher than flooded battery. 12.9v after one night and 12.7v after a week or two is normal. It is not "about..."
Resting state voltage needs to be measured accurately. 12.3 indicates totally flat. As you can see, needle meter that's only accurate to +/- 5% or so is useless.


The operating voltage reported by TS at battery is within parameters. 12.5v after sitting overnight indicates internally faulty battery or parasitic drain.

The TS needs to test parasitic drain. I'm not sure if he doesn't understand how to do it or he doesn't want to follow my advise.

I have a backup power supply that uses a pair of 12v battery hooked up back to back. They're a pair of AGM 12v 17Ah.
After one month.. one reads 12.4v after resting. Other one reads 12.7v. This most likely means that one of the six cells in that pack is dead.

And those AGMs are trouble. The acid can not come out unless the case is smashed open, but they're prone to failure in my opinion.

"i run agm batteries. and the battery was at 75% charge when i put it in the car.".... if that means the battery was stored for months at 75%... that battery is history.

[turbo970]

no i ran the battery down and put it on the charger. but i put it back in before the charger got the battery up to 100% charge

[91Beater]

Since you're unable to or refusing to test parasitic drain, we can't know why your battery keeps losing charge.

[turbo970]

it doesnt lose charge unless i am trien to start the car. and the jumper i put on goes from the charge post to the pos post on the battery. the old one was all corroded

[91Beater]

it doesnt lose charge unless i am trien to start the car.

I'm afraid you simply don't understand electricity.

You struggle with starting, because your battery is degrading..
or
parasitic current is causing it to lose charge.. that you just refuse to test.

battery doesn't work like a gas tank that can provide full power from 0 to 100% capacity.


So, go ahead and swap parts after parts to your heart's content or take it to a repair shop.

[phxmotor]

For god sake... put a new battery in it. Just get a 60 dollar walmart battery. It will last four years no problem. Bad-old batteries cause sooooooo much trouble. Put a new one in ... your charge voltages seem fine. But try this after putting the new one in; disconnect the neg terminal while running... does it still run? the answer will be yes, then reconnect the terminal... now turn on the heater, a/c, and rear window defrost. This will put a big load on the alternator... now disconnect the neg terminal... does it still run? If so, the alt is fine. And your problem is just an old battery.
Keep it simple already.
Don't keep beating a dead horse...or in this case... a dead battery.

[turbo970]

its a new red top from my previous car its just that i gotta crank on it to get it started it drains the battery

[evolutionmovement]

Did the battery sit without charge or did it get heavily discharged? Lead acid batteries, unless deep cycle and even then, do not tolerate sitting dead. Even a day sitting dead can be enough to kill it. They are used because they are cheap, handle temperature ranges reasonably well, and can punch a lot of current quickly. If the system works right, the alternator keeps the charge up, so the heavy discharge life limitations aren't much of an issue. I've never had a battery last less than 10 years (I also don't have a big stereo or use all the power accessories at once and use the rear defrost sparingly).

[turbo970]

soon as i drained the battery from trien to start the car i put it on the charger. it was just one day that i didnt get the battery fully charged. and i use agm batteries over wet cell batteries.