[feeblebob]

Mine did the same thing over Xmas, lot's of lack of use, I thought I was being careful, but when I went restart, nothing.

AA chap came out, jumped it and then tested the battery, apparently OK. I'm suspecting other drains from TM and FSU.

And yes, before the flames, I've noted the dangers of jumping the M5 (had to do it again yesterday), and the fact the battery can't really be tested without letting it go properly to sleep. The AA guy was aware of all this and spent longer telling me all the disclaimers of what his tester would say, than actually testing it.

Does anyone have an opinion on how long the battery should happily last without the car being started, maybe in cold temps, without it being declared a bad battery?

 

[Tiauguinho]

Well for what is worth, I daily drive my car, just that the past 5 days it rested in the garage. Battery is 10 years old, so I guess it was time for it to go.

 

[Leezamonkey]

I'm having a strange one at the moment Mr. T, mine is in the garage on a trickle charger while my beast sleeps, but the indicator on top still says it needs charging all the time, so I put it on a normal charger, still says it needs charging! I haven't actually tested it yet or put it back on the car but how reliable are those indicators?

 

[dan.]

just returned from the garage with a new battery,mine wont charge anymore 3 years and its a gonner.
just about to go drop it in and fire her up.

 

[RichardP]

10 years is pretty good going for a battery, but the original batteries fitted were very good.

If you ever get a flat battery, unless you know the cause, you should check the 'dark current' draw, i.e. the current draw when everything is off and the car is in sleep mode. This should be no more then 100mA, preferably more like 10mA.

Common culprits in our cars are :

- FSU
- The phone TCU
- Traffic Master (UK cars)
- Nav not shutting down due to errors reading the map disc or a failed software update
- Radio module not shutting down due to water from the rear light cluster gasket leaking and getting into the multi block connector

As a very rough calculation, a 100Ah battery with a 100mA drain would last about 500 hours to 50% capacity, which should be sufficient to start the car, that's over 20 days.

 

[josiahg52]

The indicators are fairly reliable. They consist of a density-calibrated ball that will float given a certain specific gravity of the electrolyte in the cells. A fully-charged battery has a specific gravity of around 1.300 80/60F. Anything less than that is not fully-charged. I do not know what the indicator is calibrated to for your battery.

A low specific gravity means that most of the sulfuric acid in the electroyte has been converted to lwater and that the lead at the neagative plate has been converted to lead dioxide and the lead sulfate at the positive plate has been converted to lead dioxide. Water has a lower specific gravity (of 1.000 @ 60F) than sulfuric acid.

Over time however deposits can gum up the cylinder that the ball is in and it can get stuck. Ten years is an EXCEPTIONALLY long time for ANY lead-acid battery.

Unless it is about -70F outside (approximate freezing temperature of electrolyte at a nominal specific gravity) and all things being equal with your M5's electrical system, i.e. no excessive parasitic drains, a few weeks should be expected but something marginally less than that should not be cause for immediate alarm. A tender is very cheap assurance and a 2A charge of several days can work wonders on a "dead" battery. It really depends on the nature of the failure, though.

Edit/add: Richard's post above makes me want to mention another thing.

Factors affecting battery capacity is electrolyte temperature, recent periods of nonuse, rate of discharge, the age of the battery and the specific gravity of the fully charged battery.

While using rated Ah and a given discharge rate can provide an approximate time to a low ICV, residual capacity depends on the efficiency factor at the given discharge rate.

The way we use SLI batteries is not really conducive to developing capacity. Cycles or equivalent cycles help form the battery and really sets up its characteristics throughout life.

The batteries we use are subject to a float charge when the engine running and a trickle discharge when the engine is off. The trickle discharge is never enough to really discharge the entire battery, especially in the case of daily drivers, and for winter stored vehicles, the discharge is usally maintained for much too long and permanent damge to the plates or cell reversal occurs.

As a result batteries are "dead" and by dead I mean not capable of accepting charge NOT that it has no charge. That is a distinction we make in the my field when it comes to batteries. As I have said before, even a fairly significantly discharged battery can be recovered to greater than 90% capacity after several days of 2A charging to full-charge level. Additional cycling will restore capacity to nearly 100%. Of course, it depends on individual battery condition and the nature of discharge.

Battery manufacture quality also comes into play. I know this because we have tested several different batteries from several different manufacturers (of there are only three in US, for example) and most tested well below their rated capacity even when new. If it is my money being spent I would choose a battery manufactured by Johnson Controls like the Interstate line. They consistenly tested higher than their rated capacity right from the manufacturer. Die-hard is another good choice.

Another to keep in mind is that retail stores, like Wal-Mart for instance, who have their own brand (Everstart in this case) may specify a different manufacturer depending on the location of the store to minimize shipping costs. That is why you may hear of some people singing the praises of the Everstart brand and others cursing their existence. The fact very well may be that they are different batteries.

-Josiah

 

[technician117]

I am unsure of the age of my battery, but I had a similar experience today! The car is daily driven, except for this past weekend. It sat from Friday afternoon until this morning (monday). The temps over the weekend were in the 20's (F) but went down to negative 5 this morning. Car would not start. I was forced to jump it as I needed to get to work. I then drove it was only a mile before the traction control light turned on, and 30 secs later the brake light came on with a "check brake fluid" message. <--- This has happened one other time, the car started but the temps were about the same, -5 or zero degrees F. After driving the car, shutting it off and restarting around lunch time all lights were off. I havn't tried it yet today to see if the same is true.

I think its safe to assume the battery needs replacing and is causing the gremlin. If I still get those messages on another very cold day after replacing the battery then I can assume there is another issue... maybe with the brake fluid itself.

 

[Jetpilot718]

I've got a 2000 MY M5, and it's on its third battery. The OEM one was gone when I purchased, in its place was a Bosch unit. The battery that came with the car when I bought it started going flat after 2-4 days of no use, so I purchased a new Bosch unit and all problems seem to be gone. I have gone almost a full week to 10 days without touching the car and she starts right up no drama.

These cars are very sensitive to voltage, and for $100 a new battery as PM is worth it. ****, an oil change runs more than that. Get the battery, your electrical components will thank you.

Oh, and when you swap out the battery... don't remove the old one, close the trunk, and drop it off at the counter at Pep Boys because the annoying teenage girl wouldn't give you the core credit unless "she sees the old battery". To make her happy, I did as she asked, took my new battery to the car only to realise I now had no way to open the trunk.

Had to do it the caveman way and go in through the back seat, I was not happy. There was almost one less snotty teen brat in the world that day.

 

[Slippery]

I fitted a new battery about 3 months ago, mine went flat after not using it for a couple of weeks,charged it but kept going flat after 3 days or so. When i checked the battery it was not upto the job,it was the correct size but incorrect battery for an M5. I bought a Bosch battery as per the thread on here,the correct one with more cranking amps. I left it for about 3-4 weeks between using it & its been perfect ever since new battery fitted.

Simon.

 

[Tiauguinho]

Got the new battery this morning at the local dealer, total cost came up to €205 (with 15% discount).

 

[Jetpilot718]

Dang man, that's pricey! Almost what, $400 USD?? BMW doesn't make the batt's btw, they are outsourced anyways. So I went with the Bosch. As long as it's the correct specs for CCA's, you'll be OK. AFAIK, it's 1000CCA's, but don't quote me.

 

[RichardP]

OEM is 850CCA, Bosch Silver Plus is 920CCA, both are 110Ah.