Battery Wiring and boot trim questions

[DamianPinder-George]

Hello Everyone,
With the childebeasts back at school, I had the chance to get under the car and start faffing about getting to know it a little. Predictably, its left me with a few questions that the last couple of hours with RealOem hasn't really cleared up.

Firstly, the battery wiring looks like to have been buggered about a bit (a slow starter motor yesterday flagged my interest today - the battery is showing a 'Good-Charge' result on my tester that I use here but its only 12.1v and I'm going to replace it with an AGM anyway). To continue, there are 2 main battery feeds heading out of the battery box. One is through the rubber grommet heading down-under to the front and looks totally fine but the other one kind of skulks off under the boot carpet from a connection that s been wrapped in electrical tape like it doesnt really belong in there *at all*. I don't like the look of it one bit. Check the photos for a picture. I guess my question here is whether that cable is OEM or not and what does it do? And what does that electrical connection do? I'll rip into it tomorrow but any info would be helpful. And yes I know the battery negative looks gash - its a horrible DIY item - I've already ordered a replacement.

Secondly, the boot floor seems to be remarkably devoid of sound insulation. This one seems far noisier in terms of road and wind noise from the back than my last one (it was few years back mind). An when I lifted the boot carpet get to the battery, I seem to remember a second layer of underfelt should be there. Is that right? Or is my mind making it up?

Thanks in advance for any info, I suspect that the questions are just getting started.

P.S. I know the boot is pretty grimy. The whole car needs a bloddy good clean. Bits of it are, er, 'left wanting' on the cleanliness side. The interior is spotless though. Really nice. It'll get addressed but there are bigger fish to fry. I *will* save this one - it's got a spark in it somewhere, the little bit of civilised-spitefulness that I remember from a few of my old Bmw's. I guess you could call it 'Spirit'. I like that. My current BMW track bike has it in spades - Its crotchety, recalcitrant and grumpy as hell - its just a horrible old git when its cold but with a heart of gold it you push it just right. This Z4 isn't anywhere near as 'nice' as my old SE, but that was more sophisticated. This one... isn't. I like that. Like an old Suzuki, it's sceptical and wants to know that you know what you are doing.

 

[Mr Tidy]

I wouldn't bother with an AGM battery - the E86 doesn't need it, and my only experience of them is that they cast more! After all, the OE battery in mine lasted 10 years.

Anyway just because your Coupe is Montego Blue ( :roll: ) I've just been out to look in my boot and the sound insulation in your photos is the same as mine has.

But your positive battery lead doesn't look quite right to me - this is the best photo I could get in the dark (and my car lives outside) but I hope it's of some use. I'll try to have another go tomorrow in daylight.

 

[DamianPinder-George]

Thank you sir. I'd appreciate that. I think the actual battery positive might ok - your two cables seem to be heading out roughly the same as mine, but to see where the second one goes (the one thats heading 'down and away under the battery clamp on your pic) would be good. It could all be fine and I just need to replace a plastic cover or something to bring it back to OEM spec - in truth I'm finding a *lot* of covers missing on it. Almost every time I look into something I notice *another* plastic cover thats not there.
I've gone the AGM route specifically as the z4 may well sit for while between each outing - it's an occasional car for my wife so she may use it a lot one week and not at all the next. I look after a few vehicles here (some mine - some not) ranging from a 1948 Ford Tractor, some 80's plant, a few motorbikes and up to more modern stuff and almost all of it (except our dailies that stay on lead acid and one track bike thats on Lithium Ion) fares a bit better when I swap the powerpack for an AGM. It helps me keep my weekly checklist workload down too - I have only just so many battery tenders and dont really want to buy any more :lol: .
It's interesting that yours only has the carpet layer on the boot floor too. It might be that my road noise in the back is caused by something else. I read that the window seals can be noisy if the door adjustment isn't spot on, I guess I need to check that next just in case I'm missing something there.
Anyhow, thank you for getting back to me, if you could take a couple of pics in the daylight, that would be grand....

 

[Newbers]

If you have a slow starter watch out for the junction box on the bulk head under the bonnet (big red wires going in to it). This failed on mine (starter was always slow even with a good battery). BMW spotted and said it was so hot it was smoking.....

 

[Mr Tidy]

I'll see if I can get a decent photo tomorrow.

Do AGM batteries hold charge better? I'm just wondering because I had a facelift E87 1 Series that sat on the drive for 5 weeks when I went to Oz and it started just fine when I got home. It had an AGM because it had stop/start, but the battery only lasted 4 and a half years and cost over £250 from an Indy by the time it was coded back in 2012! BMW wanted over £300!

My E86 was a slow starter when I bought it, sometimes worse when it was hot - but after 6 months it had a new battery and has been fine ever since.

 

[DamianPinder-George]

If you have a slow starter watch out for the junction box on the bulk head under the bonnet (big red wires going in to it). This failed on mine (starter was always slow even with a good battery). BMW spotted and said it was so hot it was smoking.....

Nice Call! I'd not thought that far ahead. I'll check that tomorrow. Thanks for that!

Do AGM batteries hold charge better? I'm just wondering because I had a facelift E87 1 Series that sat on the drive for 5 weeks when I went to Oz and it started just fine when I got home. It had an AGM because it had stop/start, but the battery only lasted 4 and a half years and cost over £250 from an Indy by the time it was coded back in 2012! BMW wanted over £300!

They do indeed. In the days before stop-start, expensive leisure batteries were often early AGM ones. Sometimes referred to as Maintenance-Free to as they were sealed and can't be topped up. Before the advent of Lithium-Ion, I always had Odyssey AGM batteries on the track bikes (and a couple of my Adventure Bikes too) as they had more punch for the same size and could be ignored for longer when left in the workshop under a dust sheet for months on end. The Odyssey batteries could take a bit of cold too and still spin the starter over after few weeks of studied indifference.

My understanding is that AGM batteries pre-date stop-start. It's just a different internal construction that gives a better 'occasional usage' profile. Typically, AGM batteries will hold a shelf-charge for months and months on end. So, assuming no parasitic load exists, they will hold a decent terminal voltage for far longer than a lead-acid battery. In addition, they can support a better on-load voltage (or cranking voltage) when in a lower state of charge. So for vehicles with an occasional use profile, they can spin the starter a bit faster when they've been left for longer without a maintenance charge. There are downsides though, they don't really like trickle charging, and don't like conventional charging profiles for extended periods of time when they are already at 12.6 or so volts (fully charged). Best to charge them up and then just leave them alone as a 'standard' battery tender will do more harm than good. Some of the more modern battery tenders know about AGM's and change the way they maintain the charge - they're fine - but older ones just keep dribbling in 150mAh or so and that's no good for an AGM. More modern cars too, it seems, might create a problem with an AGM if you don't *tell* the car you have an AGM (or worse, buy a cheaper lead-acid battery to replace an OEM AGM one). Can-Bus can control the charging profile and will often modify how long, how often, how hard and much to power to feed back to the battery in order to recover it after each start and support its general charge state as the power demands of the car ebb and flow during driving. I would say that if an AGM battery fails its because the charging profile isn't *quite* right. They are remarkably resilient to high loads too. I have recovered AGM in the past by charging them at 15 or even 20amps which is heading towards the 'bonkers' side for standard lead-acid and will buckle the plates if left for even a minute too long. I will admit that the last one at 20Amps and about 27volts had me on top-line for a few minutes until I saw the current start to peak, you couldn't have paid me to turn my back on it. But even that pails into insignificance when you start talking about Lithium-Ion. But that subject can be left for another time.

In short, AGMs have more power density, more cranking voltage, less internal parasitic drain, cope with lower temperatures better and can be batter for occasional use cars. The downsides are that they are expensive and can be made to fail early if exposed to a poor charge profile.

I let my inner-nerd show there and for that, I apologise. I'll go back to asking dumb questions any minute now..

 

[Mr Tidy]

I let my inner-nerd show there and for that, I apologise. I'll go back to asking dumb questions any minute now..

Actually that's really interesting - thanks.

My Z4 often sits for a week or two when the weather gets too wintry, so maybe an AGM battery would be a good idea. Shame it had a new ordinary one 3 years ago!

And you post reminded me I was going to post a photo of battery terminals for you.

The joys of advancing years - hopefully I'll remember tomorrow. :roll:

 

[Mr Tidy]

Delayed update, but I finally managed to get a photo in daylight and with no rain!

It's not a great shot, but hopefully it'll give you an idea of how the cables should be.

 

[DamianPinder-George]

Thank you Chap. I'd say that mine is the same as yours by the look of it. The only thing that ropey on mine is that blob of something or other that wrapped up in electrical tape. I suspect that its a fuse holder of some kind. The replacement AGM Battery, new battery box, new earth lead and battery strap arrived today so I'll be removing all of that and getting it back into an acceptable state while the wheels are away for refurb. Thanks again!