Acceptable resting battery voltage for new alternator?

mtthmpsn · #1 08-27-2018, 12:13 PM

So, I got the blinking battery light a week or so back, recharged the battery one morning, and confirmed the alternator wasn't charging (11ish volts w/ car on).

Finally got around to replacing the alternator (reman Bosch AL9413X) last night (along w/ CCV, OFH gasket, and Vanos oil ine). In the alternator instructions it stated you need a fully charged battery (12.6v @ 100% or 12.4v @ 75%) or you risk limiting the life of the alternator.

So I made sure the battery was again charged before I started it. Success! No battery light and voltage in the low 14s with the vehicle on. But this morning, I checked the resting battery voltage and it was 12.1. Started just fine, drove around for a while, and now it's back to resting @ 12.1. I'm sure it'll be fine going forward, but don't want to ruin my newly installed alternator.

Should I look into replacing the battery given that it's only charging to about %50 (per chart) even if the car starts fine? Should I really be concerned about this or is the 12.6v per Bosch instructions just to cover their behinds? Thoughts? Thanks.

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oldskewel · #2 08-27-2018, 12:53 PM

No doubt you are just fine. Nice work on all that other stuff "while you were in there."

Does anyone have any theories about how using a new alternator to charge a weak battery could "risk limiting the life of the alternator." The only one I can think of is that if you try it on a dead battery and it won't charge because the battery is dead, and the owner incorrectly thinks the alternator is bad, and takes it back to the store, yes that would limit the life of the alternator. But other than that ... ???

Also, whenever you measure voltage, especially in these cars, it does matter where you measure it. Measuring at the battery jump ports in the engine bay should equal V at the battery posts. But that voltage will be higher than any system voltage read off a wire, or by the computer or any gauge you may have installed. Usually I think about 0.3-0.4V lower on the BMW displayed voltage vs. the jump ports. And that's for battery (engine off) or alternator (engine on).

andrewwynn · #3 08-27-2018, 12:58 PM

How old is the battery? I've replaced a couple batteries lately that started the car right up (until they didn't) and even when they tested fine on a load tester they were below 40% capacity.

If it's old and can't hold a reasonably full charge it's time to replace.

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mtthmpsn · #4 08-27-2018, 01:17 PM

Quote:

Originally Posted by oldskewel

No doubt you are just fine. Nice work on all that other stuff "while you were in there."

Does anyone have any theories about how using a new alternator to charge a weak battery could "risk limiting the life of the alternator."

Thanks for the quick reply. Yeah, the alternator install was a breeze compared to the other stuff. I did take my voltage reading from the dash, but confirmed it was in the low 12's with a voltmeter at the battery.

I read up a little more and the theory anyway is that a weak battery will cause the alternator to "work harder" to get it back to required voltage. Apparently, the same reason you shouldn't use a running car to recharge a dead battery - what I read recommended always recharging first with the alternator's only objective to maintain charge. But also reviewed some posts and 12.1v isn't uncommon among BMW owners.

The battery is only about a year old. That said, it did die due to the alternator issue and I had to recharge to check the alternator so may have hurts some cells. Might try and take it back anyway since it's still under some type of warranty.

StephenVA · #5 08-27-2018, 04:32 PM

Quick questions (all readings at the terminal posts of the battery itself)
Battery voltage w/o engine running is .....
The reading at the jump points under the hood...
With engine running at idle...
With engine at 2500 RPM....

These test will expose battery issues and charging issues.
On the running test perform with all accessories off, then everything on including defroster. Report back your results.

Note: When batteries fail, some will collapse internally and short out causing a constant drain on the battery. When you charge it it will over heat to the point of KABOOM. Never charge a DEAD battery (one below 10 volts) in a car.

Your battery showing 12.1-12.5 at rest is good but the real question is what happens when you load it w/o engine running. This test shows capacity of the battery to store and hold amps. If you turn on the headlamps and HVAC fan to high, where does the voltage reading go? Down below 11.0V? Lower?

If you have one of these you will be able to see the actual amp capacity of the battery and the alternator. Without it is all a big SWAG.

andrewwynn · #6 08-27-2018, 05:19 PM

How old is the battery? I've replaced a couple batteries lately that started the car right up (until they didn't) and even when they tested fine on a load tester they were below 40% capacity.

If it's old and can't hold a reasonably full charge it's time to replace.

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StephenVA · #7 08-27-2018, 05:54 PM

Quote:

Originally Posted by andrewwynn

How old is the battery? I've replaced a couple batteries lately that started the car right up (until they didn't) and even when they tested fine on a load tester they were below 40% capacity.

If it's old and can't hold a reasonably full charge it's time to replace.

Sent from my iPhone using Tapatalk Pro

This is answering the common question of "How long should my battery Last?"
Answer: It depends....insert lots of anecdotes and classic real world examples. Bottom line: when it is dead/dying/and gives you trouble.
Example 1: My own X5 Sears DieHard AGM. At the 5 yr mark it was discharged due to a power outage so the charger did not reset. Batt Voltage fell to 7V as the battery discharged. Recharged at 3-4 amps for 48 hrs went to 12.5 volts, but as soon as a load was presented it fell off the scale. Dead unit, replaced with another Sears AGM unit. $200/60 months not bad.

In the old days of open cell batteries, you could drain the old acid, wash out the plates, install fresh acid, and recharge, extending the battery until the first real cold snap. Those days are long gone.

OP: when it gives you enough trouble replace it. Most batteries last 3-7 years depending on charge vs discharge rates. Sits on a battery tender when not in use longer than three days? Last longer.

andrewwynn · #8 08-27-2018, 06:03 PM

If your car is new enough to have a registered battery, you can use a level two scanner like nt-510 and it will show the charge history of the battery, example the past five days each day the average charge

The battery I just replaced worked perfectly fine until one day the car wouldn't start. When I scanned the battery module it showed the past five days average of less than 45%. The total life histogram also showed way too many hours in the 20-40% range.

If you can read that info it make it painfully clear the state of the battery

StephenVA · #9 08-27-2018, 06:10 PM

Yep good call

andrewwynn · #10 08-27-2018, 10:45 PM

Case in point regarding the comment about damaging your alternator with a bad battery: there are no current limiting devices between alternator and battery. A battery when damaged can act almost like a dead short which will cause so much strain on an alternator it could easily destroy it.

My personal example of my "seems fine" battery that I recently replaced and tested over 500cca on a load test: after 8 minutes of hazard blinking, not only could I not start the car, my 900A jump starter could not overcome the battery and start the car.

That same jump starter started a 6L diesel truck in the dead of winter!

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