battery charging voltage drop

[darconi]

So just recently my battery was constantly discharging and I was getting a dead battery after driving for a while. Mechanic said it was the alternator that was bad and replaced it.

I just got it back and was monitoring the battery voltage using a little voltage monitor that plugs into the cigarette lighter.

I drove it around for a while and then let it idle in the garage for ~20 minutes and after a while, the voltage drops down to the 13.20-13.30 range. It had started around 13.8 when I first started driving it. The battery voltage monitor gives me a yellow warning light at the 13.30 level.

I read that while the car is running and the battery is being charged that the voltage should be in the 13.5-14+ range.

Is the alternator still broken or is everything still fine?

I don't want to end up driving somewhere and being stuck with a dead battery (again).

[mikefocke]

And when you drive it again

it starts right up and the voltage climbs again to 13.8? The idle may put out less juice than the accessories and engine consume. Especially after idling for 20 minutes, the voltage can drop. So do the measurements again...see what the voltage is with the A/C on and the radio going and the headlights on...but you doing 30MPH or 2.5k RPM. Bet it is in the high range again.

[JFP in PA]

So just recently my battery was constantly discharging and I was getting a dead battery after driving for a while. Mechanic said it was the alternator that was bad and replaced it.

I just got it back and was monitoring the battery voltage using a little voltage monitor that plugs into the cigarette lighter.

I drove it around for a while and then let it idle in the garage for ~20 minutes and after a while, the voltage drops down to the 13.20-13.30 range. It had started around 13.8 when I first started driving it. The battery voltage monitor gives me a yellow warning light at the 13.30 level.

I read that while the car is running and the battery is being charged that the voltage should be in the 13.5-14+ range.

Is the alternator still broken or is everything still fine?

I don't want to end up driving somewhere and being stuck with a dead battery (again).

First, get the battery load tested; it is the only sure way to determine its condition.

Once you know the condition of the battery, or replace it, then check your altenator's voltage output.

[70Sixter]

If the battery is good....

The voltage regulator could be the problem by not fully charging battery. If battery goes dead whilst driving and you already replaced the alternator that is a good item to test or replace.

Not sure about Boxsters, but old 911 engines came with Marchal or Motorola alternators IIRC. Whichever you have the voltage reg has to match.

[darconi]

And when you drive it again

it starts right up and the voltage climbs again to 13.8? The idle may put out less juice than the accessories and engine consume. Especially after idling for 20 minutes, the voltage can drop. So do the measurements again...see what the voltage is with the A/C on and the radio going and the headlights on...but you doing 30MPH or 2.5k RPM. Bet it is in the high range again.

If i start up the car again right after, the voltage remains the same as it was before. If i wait a couple of days and then start it back up, it goes back up to 14.1 and again goes back down after a while of driving.

When it drops back down to 13.2-3 when idle, it goes jump up a bit if i go to 2.5k RPM but goes back down immediately afterwards.

Is there a feature on the alternator that it drops the voltage if the battery is getting fully charged? My thought is that the car recognizes the battery has been fully recharged and drops the charging voltage down since its not needed.

[darconi]

If the battery is good....

The voltage regulator could be the problem by not fully charging battery. If battery goes dead whilst driving and you already replaced the alternator that is a good item to test or replace.

Not sure about Boxsters, but old 911 engines came with Marchal or Motorola alternators IIRC. Whichever you have the voltage reg has to match.

The battery is almost brand new and pretty sure its good.

Where is the voltage regulator? Is it part of the alternator? I figure if the mechanic I had replaced the alternator, it would've also replaced the voltage regulator.

[mikefocke]

Alternators have 4 major components:

* Alternator core (the wire winding and the entire Alternator body)

* Bearing (what the shaft and the pulley rotate on)

* Voltage Regulator (rectangular component on the back side)

* Rectifier (Horse shoe shaped component on the back)

The regulator often what goes bad. But yours was replaced. Which doesn't make it good so lets see what we can deduce from the evidence of its behavoir to tell us if it is bad.

Having sat for several days, the voltage in the battery should drop a bit because you use "juice" powering things like the circuitry that powers the alarm and the waiting for a remote key press. And the use (or rev of the engine) seems to recharge the battery with what seems to me a normal over voltage that then drops to nearer nominal when the engine is shut off.

From what you describe the system seems to be working fine.

[darconi]

Alternators have 4 major components:

* Alternator core (the wire winding and the entire Alternator body)

* Bearing (what the shaft and the pulley rotate on)

* Voltage Regulator (rectangular component on the back side)

* Rectifier (Horse shoe shaped component on the back)

The regulator often what goes bad. But yours was replaced. Which doesn't make it good so lets see what we can deduce from the evidence of its behavoir to tell us if it is bad.

Having sat for several days, the voltage in the battery should drop a bit because you use "juice" powering things like the circuitry that powers the alarm and the waiting for a remote key press. And the use (or rev of the engine) seems to recharge the battery with what seems to me a normal over voltage that then drops to nearer nominal when the engine is shut off.

From what you describe the system seems to be working fine.

Thanks for the explanation. I was just surprised that the voltage drops while the engine is on because my toyota ('99) keeps high regardless. Figured it might be just differences between the way they're designed. As long as I don't get stuck in the middle of nowhere with a dead battery I'll be satisfied.

[uabchris]

I am having the same problem! Alternator tested good at Advance and battery is brand new. P0650 code but no check engine or battery light. Sometimes while running the car dies when I pull off the neg terminal...sometimes not. All fuses good. Sounds electrical...?

Chris Hewett

00 Boxster 2.7

[RFM]

NEVER disconnect the GROUND or POWER cables with the engine RUNNING. Some day it will cost you a lot, keep this in mind for the rest of you (car) days. :drive:

[Boxpow]

Lads, I had a discharge problem that would drop the battery down in 3 days so that the alarm would go off. I found it to be a sticking seat switch which cost me a new battery but nothing else. I know its not the same problems but just a little heads up.