Having completely forgotten that the OEM battery has a condition indicator on it, I decided to take it to Advance Auto Parts to have it checked since it is going on 6 years. I don't want the thing to leave me stranded.

The battery checked out like brand new, measured at over 600 cca @ 98*F.
However, the kicker is the alternator. At idle it read 14.01 volts, but under load (turn the AC full blast, high beams) and the motor revved at 2K rpm he alternator read at 12.84V and he told me I need a new one.
Not trusting this, I went to Autozone. They only tested the system at idle and once again battery checked out great at 668cca and alternator at 14V.
Wanting to recheck the alternator under load, I went to yet another Advance Auto Parts.
This time however while performing the test under load, I revved the engine higher to 3.5K rpms. This time the alternator checked out, read 13.13V and the battery at 698 CCA.
I really don't think I have a problem but figured I'd check in. Is it normal for our alternators to read slightly low under load and low rpms? The first time I had it tested and it read low, I had only revved up to about 2K rpms. The guy initially told me an alternator should read 14v no matter what
If the alternator was flaky I am sure it would have killed the battery by now.
I have a lumpy/somewhat rough start in the mornings if I don't hold the key in the ON position for about 10 seconds. I am suspecting, as I have read around here, that I might need to replace the fuel filter. Car is an 06 S and has 76K miles.
So let this be a lesson that having the battery/alternator tested is not foolproof I guess.

 

You don't need to go to autozone -- get yourself a simple cheap multimeter at Radio shack (always nice to have a simple tester.


Set the multimeter to voltage and turn off all lights and accessories on your vehicle while the engine is running. If the car has been warmed up, the voltage output and normal idling range should be between 13.8 and 15.3 volts.

Your first reading was 12.84V which is bad.
The 2nd test is 14V which is good and normal.
The 3rd test was 13.13V still within the good side but not the best.

I say, keep an eye but you mentioned you are worrying being stranded.
So, if you are handy and be able to change it yourself, it will save you a lot.

 

From what I know the alternator is a PITA to change. If it's not bad I'm not going to mess with it. A multimeter is definitely something that I need to buy.

Again, the reading of the alternator was 14v consistently across the board. It was inconsistent under load with the motor revved up, so I guess my question was, how much of an impact do RPMs have on the kind of voltage the alternator should churn out. Are there ever conditions in which it is normal to be under 14.

 

So you asking "inconsistent reading under load with the motor revved up"


The best I can explain is this:
Some cars (mostly BMW) it has a design feature in the ecu that tells the alternator how much excitation field to apply to the alternator.
At hi-rpms, you don't need to maintain battery voltage (especially in WOT which is a only a few seconds). The ecu will kill the alternator and you will lose the counter torque caused by electrical load, then free some horsepower and torque to the engine and drivetrain. Similar to the aircondition unit, @ around 5,500 rpm the ecu will cut the AC.

But...this design mostly in BMW & MINI is BMW right?

 

That makes sense. I dunno, I may go in for a 3rd test and rev that sucker to 4K rpms and see if it makes a difference. Going from 2K to 3K did. Either way it would be one hell of a coincidence that I'd go and have it tested JUST as my alternator is flaky. I have driven the car in all immaginable conditions with a flaky alternator and the 6 year battery checks in as brand new? Unpossible. Heck, I even had to use the MINI to jump-start my Legacy twice.

 

3rd test at 13.13 volts is NOT good. It sucks. In order to maintain a charge, a certain voltage threshold must be exceeded. It depends on a few variables, but is generally around 13.6 volts. Anything less than that and you ain't gonna charge your battery. Somethins ain't right with your charging system.

Or.....you may have a funky battery and it's drawing too much current when the alternator ramps up. I'd do a temporary swap with another known batter and run your voltage tests again. At idle and also at 3 to 4000 rpm.

Battery testers at auto supply stores are notoriously crappy. Most especially if they're the electronic type where it simulates a load on the battery rather than applying a real load. Those kinds of testers, while in common use, are practically useless. I had Kragen's Auto Supply keep telling me nothing was wrong with the battery I bought from them in my Toyota 4Runner. I said ******** but couldn't get anywhere with them. I went out and bought a new battery from another source and presto, problem solved.

 

I would not keep any battery for 6 years, those plates can pretty much break at any time. I am wondering if having a pulley makes any difference in how the Alt charges. My readings on mine stay about 12.8 on the highway with lights and ac on, never drops below 12.3. Mine has been like this since 19K when i bought the car, now it has 97K on it, I changed the battery once 2 years ago and as I said never had any problems

 

For me dropping voltage under load seems perfectly normal. If a person only takes short trips at night with wipers, defroster etc on, a marginal battery will slowly go bad since it never gets a good charge. I am guessing you don't do that and as such the battery is doing better than many. Also the S with the battery away from heat helps with their life.
With that said, batteries tend to die a fast death, but my experience says that long trips are better.
I wouldn't worry, but then again, I don't have run flats. When I pay the piper, I try to remember that I sort of asked for it.

 

A supercharger pulley, no. But a 2% crank pulley, absolutely. With a fully-functioning charging (and A/C) system, a 2% slow-down isn't an issue. But if your alt is already getting tired or your battery isn't taking/holding a charge like it used to, then a 2% larger crank pulley will have a substantial impact.

 

I ditched the runflats a while ago, I now carry a set of Dynaplugs AND quick spair AND tire inflator and on long trips a spare tire. I pretty much got that covered.
I don't think anything is wrong. I have a multimeter so I'll try my own test. I have no reason to change the battery since both the indicator on it says it's good and it also tested out great. But I will keep an eye on it due to age alone. The car is mostly driven at the revs I tested the battery at and now that hot weather is here, AC on etc. It would have killed the battery and showed symptoms long ago, I think.

 

So how did you test that battery? If it's with one of those electronic load testers that auto supply stores have nowadays, you can't trust the resutls. Period. You need a load tester that applies a real, resistive load.

12.3 to 12.8 volts in your charging system while driving is way, way, way too low. Assuming, of course, that is the actual and true reading. Which it isn't if you haven't had any charging system or battery problems. 12.8 volts is only at the level of a fully charged battery. If the voltage level in your system really never exceeded that number, you would have had a dead battery long ago. This is because it takes a minimum (approximately) of 13.6 volts to actually charge a lead acid battery. Since you haven't had any battery problems, that means your reading is highly inaccurate. This it usually due to one or the other or both of two reasons. First, often there are significant voltage drops in the wiring path from the battery or charging system output to the volt meter. (Voltage drops can occur on the ground side as well.) There are various reasons for this. The second reason it may be reading low is that the meter itself is inaccurate.

The bottom line is that there's no way you could keep driving your Mini for any length of time if the charging system was only putting out 12.3 to 12.8 volts.

 

Stick with the current alternator....it is likely better than many of the rebuilds...if you do get another, get a quality remanfactured or new denso (oem) alternator....
denso has factory remans....for about 100 less than new...same as new parts for the most part.
I would probabbly replace the battery....that way it is convient, and you pick the one you want, and the price....no downtime. The current one is likely on borrowed time...many folks have gone for a quick trip to the store...just to have a no crank 20 minutes later in the parking lot...
besides, badttery is cheap and easy diy....alternator, not so much....and if you can keep the current alternator going ( if you do lots of night driving, fan on hi, rear defrosrer on, it might be getting weak) and just monitor it....the voltage drop of should be pretty slow...
Denso makes a good product...many denso's in toyota' and honda makes routinesly go 150,000 miles plus......
The proof if it is working is, does the car crank good, bright lights, and do what you want the car to do. Imo the Worst thing you could do is drop in a $100 auto-zone lifetime warrenty or other web deal....a few folks have done 2 or 3 "rebuilt" mini alternaors before getting a good one ( or getting the oem one out of frustration, like one mod here).

 

PS the "indicator" on the battery that glows does not mean the battery is "good"...it just means it is sufficently charged to be tested....I have a battry (oem) sitting in my garage that will glow when I charge it....but won't crank a car...(having a spare to use as a core when you buy a new one saves one trip to the store).

 

Good point. Proper voltage (12.6 to 12.8 for a new battery) does not a good battery make.

 

Thanks, I agree. But just for clarification, it never read 12.3. On the low side it read 12.8 and then 13.1
Would a multimeter get a more accurate reading?

Are you sure you guys are reading carefully my initial post? I realize the meter on the battery is not foolproof, but the battery read as being over 660cca consistently. With a flaky alternator, that would simply be impossible.
I am open to suggestions as to how to properly test it. Or, better yet, perhaps leave it alone. I just wanted to test out the battery as I have never had one last 6 years.

 

The only REAL way to test an alternator is to remove it, and bench test it....likewise, a battery is best tested on a bench with a true resistive load....the in car quickie tester many places are what they are....a tool that just gives a general idea if a part is ok, bad, or failed.
It sounds like the car is running fine....
Proactive mantance is good...replacing a worn compotent is fine....but if you look to deep, it becomes a bit like chaseing a ghost...like renovating a house..you never know what you will find, and much of what you find that is not 100% perfect NEVER AFFECTED YOU....
Not to say ignor anything, but until you have an issue, slow cranks, dim lights, etc, i'd enjoy the ride....the specs for a "new" part is often high enough an inplace, worn compotent that is mostly good is fine....
Most folks don't get more than 4-5 years out of an oem battery....they tend to have the plates fail from vibration and sulfation....it is something that can be calculated by the manfacture with some certainity based upon the construction, and quality compotenselect hense the selection avalable at price points and warrenty length...
Also on battery voltage...the checks should be long enough after the start that the battery has recovered....a big drop on.a battery with little reserve capicity (an old or small one is common) is expected, and even indicidive of an impending failure...

 

When you check the voltage under load, where are you measuring from?
If you measure "downstream" from the battery or alternator terminals very far,
you can see a lower voltage than at the source, because of resistance
in the wiring causing some voltage drop under load (lights, A/C, etc.).
This was very noticeable in my old car that had a volt meter under the dash.
It always dropped voltage much more than at the battery when the lights or fans were on.

 

Lumpy start issue..
slow crank? Better after a long drive? Worse after a long park?seems the reason for the post got sidetracked by a autozone part-timer trying to upsell from a test...kinda like them selling sensors from an obd2 code check...

 

cristo, all tests were done right at the battery terminals, in the trunk. This is a Mini S as you've already guessed.