Low amperage battery effect

[yesterjay]

The initial message was deleted from this topic on 07 November 2023 at 13:13

[Smint]

Once running with the alternator doing its business everything should be fine.
I'd be more worried about poor starting, the starter struggling to turn the engine over swiftly enough once the weather turns cold, putting strain on the starter itself which will be turning over slower and longer, possibly triggering low voltage reactions throughout the rest of the car during the start procedure, ie triggering random faults and maybe putting the eml on.
I've always replaced a dying battery with the highest capacity that will fit in the holder.

[Caddyshack]

Smint said:

Once running with the alternator doing its business everything should be fine.
I'd be more worried about poor starting, the starter struggling to turn the engine over swiftly enough once the weather turns cold, putting strain on the starter itself which will be turning over slower and longer, possibly triggering low voltage reactions throughout the rest of the car during the start procedure, ie triggering random faults and maybe putting the eml on.
I've always replaced a dying battery with the highest capacity that will fit in the holder.

Agree with all the above.

I tend to find modern batteries have done their best around 5 years and would replace at the slightest hint of failing. You can get good battery testers that suggest charge or replace.

Some modern alternators only seem to charge back up to where the battery started at, worth a good charge off the car sometimes.

[Baldchap]

Certainly in more modern stuff, 'weird things' tend to accompany a low battery. Odd problems like electronic stuff behaving strangely or erratically.

[Caddyshack]

yesterjay said:

Stalling at low speeds and on full lock with lights and air con on?

Does the car have limited slip diffs?

[Krikkit]

Baldchap said:

Certainly in more modern stuff, 'weird things' tend to accompany a low battery. Odd problems like electronic stuff behaving strangely or erratically.

Often induced by low voltage during the start cranking - the low voltage resets some circuits and confuses others.

Under very heavy electrical load and low idle speed it can also cause odd running when the battery is topping up the alternator's performance temporarily

[HelldogBE]

yesterjay said:

Caddyshack said:

yesterjay said:

Stalling at low speeds and on full lock with lights and air con on?

Does the car have limited slip diffs?

E39 530

No LSD.

I feel like it's electricity related. And low amp suggests the gremlins may be at play.

Maybe fuel pump I guess but it's more an electronic load that invokes issue. Particularly when the autobox changes or idles.

Got it on a recon charge now so we'll see how that goes...

As stated smaller capacity battery might have trouble starting in winter, causing excess currents through the whole system (esp. starter) as the voltage dips. Also slower recharge if you're going with a tiny battery as it will take less amps. Once it's running you should be fine with a 2/3rds capacity (unless it's bricked but then it probably wouldn't start either).

Doubt battery is the fault as it doesn't have EPAS so doesn't put extra electrical strain on the battery causing a voltage dip.
Those 6-cylinder petrol don't need much to stall at idle (if it also idles at 700rpm) though and hydraulic steering is quite a mechanical load, but you'd think the ECU would raise the revs in an auto.

If you put the lights on in the dark and point it to a wall you should be able to notice if the voltage dips before it stalls. You can also measure the voltage in the cigar socket but a digital multimeter might be too slow to pick up on it.

[Caddyshack]

yesterjay said:

Caddyshack said:

yesterjay said:

Stalling at low speeds and on full lock with lights and air con on?

Does the car have limited slip diffs?

Implying... dry diff?

No, my Cossie can stall when on a tight circle as the diffs pick up, it’s like putting the brakes on, if the engine is on a low tick over I wondered if the load of the power steering and the diff were enough to stall the engine.


E39, I would call Richard Barker at Barkers auto works in HASLEMERE surrey and I suspect he will know what to do.

[ingenieur]

When batteries die they lose a cell at a time. So whilst the voltage is maintained by the other cells keeping the voltage up the 'capacity' of the battery is reduced. Meaning it has less load bearing ability. That's why a bad battery can have a reasonable voltage.

You may also be surprised to learn that the difference between a good and bad battery is hard to determine just with a simple voltage test.

The voltages look a bit like this:

12.60v = 100%
12.45v = 75%
12.30v = 50%
12.15v = 25%

In case you've not done it before the way to test your alternator is to connect the volt meter to both battery terminals. With the car running and nothing switched on (turn off AC, lights, stereo) you should see around/over 14v. When you turn on everything you should see the same number. Turn on heated rear window, heated seats, stereo, full beam headlamps, etc etc....

If the voltage drops to a lot less than 14v... say somewhere in the 12s or low 13s then your alternator is kaput. So at minimum you must change the alternator but 9 times out of 10 it'll take the battery with it due to the alternator fault creeping up slowly and wearing the battery out over a long time. You can try keeping the existing battery and seeing if it'll maintain a charge once the new alternator has been fitted but it doesn't usually work out.

[ez64]

yesterjay said:

E39 530

No LSD.

I feel like it's electricity related. And low amp suggests the gremlins may be at play.

Maybe fuel pump I guess but it's more an electronic load that invokes issue. Particularly when the autobox changes or idles.

Got it on a recon charge now so we'll see how that goes...

This is a known problem with the M54 and automatics. As they wear the software values for idle recovery usually when pulling up to a stop and having strain on the system like full lock turning will cause a stall on higher mileage engines\gearboxes .


BMW fixed this in a tech bulletin which is in a software update for the engine ECU, this is a bh of a job DIY if you lose power during the upgrade. I cannot be bothered to risk this so I raise the base idle by always running the A/C system and you won't get the stall if your AC actually works.

Edited by ez64 on Tuesday 27th September 13:36

Edited by ez64 on Tuesday 27th September 13:42

[TonyRPH]

yesterjay said:

Currently on, but reading as an exhaust sensor only

EML on and stalling...

Have you checked all the hoses under the inlet manifold?

What is the idle speed like? Does it idle smoothly?

[ez64]

EML light could be related I did not realise that was on as well. Just for reference i have the same problem though even with an engine that is otherwise perfect and no errors. The risk you are right is the risk of bricking the ECU so I would just fix the eml light with a smoke detector if you don't have part codes and then fix the A/C and call it a day but your choice but I will bet money it's the software problem even after the eml light.

[Draxindustries1]

It won't be a lamda sensor with those symptoms it'll have an air leak under the inlet manifold. The sensor will show on the code reader as the oxygen content will be all over the place.

[bolide]

ingenieur said:

When batteries die they lose a cell at a time.

The cells are connected in series, so the bad cell is required to pass the full current demanded when starting. Bad cells go high resistance, restricting the current available, so the starter turns more slowly.

Once started, the car will still run OK as the voltage drop due to the bad cell is much lower when the current demand is low

The only reason lead-acid batteries work for starting cars is that their internal resistance is incredibly low. So they can deliver 1000 amps or so for a brief period