[HighlandPete]

Originally Posted by Chelsea_Tractor

Whatever you do don't buy a Banner battery. They are shockingly bad and have a notoriously short lifespan. The quality varies so much from most lasting 2 years to a few making it 5 or more. .

I obviously got one of the few good ones, BMW/Banner battery. 8-years old in July.

[HighlandPete]

Originally Posted by teaston

Yes I understand if it has started to decrease in performance, but if it's still 12.6v or more then it's working just as well as a new battery.

With AGM batteries in our BMWs, even 12.6V is not a reliable gauge of battery health, as energy management doesn't keep a battery (even a good one) ~100% SoC. May only be 75 - 80% SoC (or even less), with a lower voltage, due to our driving profile.

[teaston]

Originally Posted by HighlandPete

With AGM batteries in our BMWs, even 12.6V is not a reliable gauge of battery health, as energy management doesn't keep a battery (even a good one) ~100% SoC. May only be 75 - 80% SoC (or even less), with a lower voltage, due to our driving profile.

Yes I think you need to check the voltage after a long drive iirc?

[Hooded]

Originally Posted by HighlandPete

I obviously got one of the few good ones, BMW/Banner battery. 8-years old in July.

^This is exactly why it's wasting money to change a battery at 5 years old if it's still working as it should.

[HighlandPete]

Originally Posted by teaston

Yes I think you need to check the voltage after a long drive iirc?

Even that doesn't necessarily reflect the condition of the battery. If we don't get much over-run energy harvesting during a longer trip, the SoC may not increase much at all. If we were to look at the charging history we may find over many hours driving the SoC doesn't even get in the 85 - 100% range at any time.

I know for myself, driving 120 miles in the same trip, the battery may not increase the SoC very much at all. Age of the battery doesn't make any difference, simply charges during the trips under intelligent alternator control, (IGR) to keep within the 70 - 80% range.

The charge status of the battery must be within certain levels that permit charging. A fully charged battery (100 % charged) cannot accept energy and is therefore avoided as part of the intelligent alternator control strategy.

[HighlandPete]

Originally Posted by Hooded

^This is exactly why it's wasting money to change a battery at 5 years old if it's still working as it should.

We usually get signs that a battery in coming to the end of its life. Sluggish cranking, being a common clue.

In my almost 50 years of driving, I've only had one battery which actually failed, went open cell from working fine in the morning. Went to start at lunch time and it was dead, no voltage at all.

[teaston]

Originally Posted by HighlandPete

Even that doesn't necessarily reflect the condition of the battery. If we don't get much over-run energy harvesting during a longer trip, the SoC may not increase much at all. If we were to look at the charging history we may find over many hours driving the SoC doesn't even get in the 85 - 100% range at any time.

I know for myself, driving 120 miles in the same trip, the battery may not increase the SoC very much at all. Age of the battery doesn't make any difference, simply charges during the trips under intelligent alternator control, (IGR) to keep within the 70 - 80% range.

Oh I see, it’s because of the the brake regen needing space in the battery to put the regen electric into! I guess my knowledge is going back to cars before brake regen.

So is there any sure fire way of testing one of these modern batteries?

[Hooded]

Originally Posted by HighlandPete

We usually get signs that a battery in coming to the end of its life. Sluggish cranking, being a common clue.

In my almost 50 years of driving, I've only had one battery which actually failed, went open cell from working fine in the morning. Went to start at lunch time and it was dead, no voltage at all.

Agree, .. the last time I changed a car battery was when it had got to the stage where it was cranking really slow and struggling every time the weather dropped to around zero over night.

[HighlandPete]

Originally Posted by teaston

Oh I see, it’s because of the the brake regen needing space in the battery to put the regen electric into! I guess my knowledge is going back to cars before brake regen.

So is there any sure fire way of testing one of these modern batteries?

Yes, AGM batteries with energy recuperation typically run at 70 - 80% SoC, to allow capacity for the over-run regeneration. Therefore it messes with conventional ways of perceiving a battery is on the way out.

Even testing the voltage without disconnecting the battery, we have to ensure the car is asleep, or it's a bit hit or miss. I've put the charging leads on the charge terminals, closed up and locked the car and checked later in the day, or the next day.

There are ways to load test the battery, plus there is a history kept in the DME/ DDE.

For F10 and later vehicles, a battery test is built into the vehicle (through the IBS), and is accessed using ISTA.

[teaston]

Originally Posted by HighlandPete

Yes, AGM batteries with energy recuperation typically run at 70 - 80% SoC, to allow capacity for the over-run regeneration. Therefore it messes with conventional ways of perceiving a battery is on the way out.

Even testing the voltage without disconnecting the battery, we have to ensure the car is asleep, or it's a bit hit or miss. I've put the charging leads on the charge terminals, closed up and locked the car and checked later in the day, or the next day.

There are ways to load test the battery, plus there is a history kept in the DME/ DDE.

I see, could explain why bimmerlink detects my brand new battery at 12.0v at a standstill then! I thought it was incorrect, but maybe not then.

Best to just keep using the battery until the car starts to behave as though the battery is reducing in performance then.

[HighlandPete]

Originally Posted by teaston

I see, could explain why bimmerlink detects my brand new battery at 12.0v at a standstill then! I thought it was incorrect, but maybe not then.

Best to just keep using the battery until the car starts to behave as though the battery is reducing in performance then.

I've physically checked my voltage from the front terminals and the battery, both can read 0.3 - 0.5V low, compared with a locked car that is asleep.

A voltmeter in the front power socket, can be the same low voltage reading.

My E91 always appeared to be low on sitting voltage, but never got worse with the age of the battery. Was sold at 6.5-years on the original battery.

[ByTheSea]

I traded in my MY2000 3er after 12 years, 75k miles. Aside from tyres, bulbs, window motor, suspension rods/bushes at MOT everything was original. Never had a failure that impacted. Car would be left for 6 weeks, battery flat, charged and good to go.

[Pequin]

I've never changed a car battery in the last 37 years of car ownership but if I needed to this seems a good offer!

[330XDave]

Why would you ever pay £320 for a car battery in the first place!?