correct voltage from alternator
 

Dear All
I've been having some battery problems of late and without a battery tester, im wondeirng whether my alternator isnt kicking out enough

With the engine running it produces 13.89v but what confuses me is that it does not rise even a tiny bit when the engine is revved to 2000 rpm

Is this normal? Does the voltage regulator really stabilize it that much?

If so, gues that points to a poor battery


thanks


MJ

MJ,

13.9 is about normal when the car is running and it should hold steady when the accessories such as the lights and A/C are turned on.

What is your battery voltage when the car is off and having set for a few hours? If low have it charged and tested.

http://www.audiocircle.com/image.php?id=128272

I'd pull the battery and have it tested at a local auto parts store.

the voltage is...REGULATED :)

you have to remember one thing...the alternator only exists to "top off" the battery which is actually powering everything.
if the battery voltage goes down the alternator output goes up to compensate and keep it "level"...
revving the engine to 2000 doesn't really change the electrical power demand of the car over 1500rpm or 3000rpm.

the only time you might expect to see a change in output is below the "minimum rpm range" of the alternator which is probably about engine idle speed.

It's actually both alternator and battery that's powering the car because they are connected in parallel. The higher-voltage device will contribute more power depending upon the balance. There's multiple conditions:

1. When starting, the battery's providing 100% of power to spin the engine. This drops its fully-charged 13-13.5v down to 12v or less depending upon condition of battery.

2. Right after starting, the battery is partially drained and the alternator's providing 100% of power used to run the car AND recharge the battery. Since the alternator's native output is 14-14.5v, current ONLY flows from alternator to the lower 11-12v battery.

3. After driving around for 10-15 minutes, the alternator charges the battery to 13.5-13.8v and current flow decreases. However, at no time does the alternator's output fall below battery voltage (unless regulator fails). At this point, the most that occurs is zero flow into or out of battery.

An amp-meter will confirm these states for you. Shows tonne of current flowing out when starting (-), then 10-20a flowing back into battery from alternator (+). This rate slows as battery is charge and reaches zero. It MAY pulse to use the battery (-) if you have sudden surges such as turning on lights & stereos, but quickly goes back to zero as the alternator catches up.

OP, what kind of battery problems are you having?

The battery is slowly draining. I can't work out whether it can't hold a charge or whether there is a voltage leak.

I tested for a leak with a lamp connected between the earth of the car and the neg terminal and found a tiny leak from the car radio, which is in fact always on as it has a meory and a clock. But an electrician here say's its normal for a tiny drain.

So i reached the conclusion that it must be either the charging system or the battery itelf. What you have written is interesting as it seems that using a voltmeter is useless unless you switch to amps.

It is the simplest conclusion i can come to without using a battery tester. If the battery is poor and not holding a charge well, then this would explain why, if i leave the car for a week, there will be nothing there to turn the engine over with.

thanks


MJ

Battery tester will just say you've got drained battery, but will tell you no useful information as to why. Measure actual drain amount:

1. Set voltmeter to 0-10 amp range
2. Disconnect negative battery cable
3. Connect negative lead of voltmeter to negative terminal of battery
4. Connect positive lead of voltmeter to negative battery cable
(use test-cables with alligator clips for hands-free in #3 & 4)

What is current flow from battery? By inserting voltmeter in-line with battery circuit, any and ALL flow must go through meter and can be measured precisely.

5. Disconnect fuses one-by-one and see which circuit causes drain to stop

6. Inspect all wires and components of offending circuit from end-to-end for frayed exposed wires which may be shorting

One very common source of drain is the dome-light wires where it goes across the top of windshield.

The alternator can also cause a drain if some of the diodes are bad. Does anyone know what the acceptable current draw from the alternator (key off) is?

-Joel.

When the dome light switch is tripped when you exit the car it shuts off the window relays and the radio. If your dome light switch is bad then that window relay and radio will stay energized and that will drain your battery down. When you check the current drain at the battery as described you won't find this. This allows you to run up your windows after you turned off the engine.

Dome light switch has no relation to controlling door-relays or radio. It's the door-switches that provides that function to turn off dome-light, door-relays and radio. Frayed wires to the dome-light will cause short to the body regardless of the position or condition of the dome-light switch.

Uh, if you disconnect your battery cable, where is the power that the stuck window relay and radio is draining coming from?

Or to put it conversely, if you have an unknown short that's draining the battery and you disconnect the battery, will the battery still run down in the course of a week?

By disconnecting the battery cable, and bridging the gap with the voltmeter, ANY and ALL current flow MUST go through the meter. If there's a short, regardless of where it is, that current will register on the meter.

i guess it comes down to how often you drive car and battery's capacity. I've seen figures from 55 to 73-ah, so let's take 64-ah as a ballpark figure. And assuming you drive the car at least once a week. And we don't want to drain the battery more than halfway. Let's do some math:

7-days = 168 hrs
max-drain = 32 ah

Max drain-rate = 32 ah / 168 h = 0.190 amps flow

So you can measure up to 190 mA of flow in order not to drain battery halfway in a week. I usually measure about 20-50mA flow at rest on car with no problems. I'm not exactly sure on that halfway mark, i'm sure if battery is in good condition, you can have it drained more than that and it'll still start car.

I meant door switch in my post above. And it turns off the relay when you close the door. And if you disconnect the battery the relay will turn off and it won't turn back on so you won't be able to detect a current draw. I think they call that a catch-22

Simple way to catch and test that kind of door-switch problem is to see if you can still roll up & down windows after you open door. If door switches are working properly by unlatching relay, then drain is somewhere else.