Battery voltage mystery

[Goran]

Today I decided to check my battery voltage (or alternator voltage) with engine running.
I have a new multi-meter which I assume is accurate. It agrees with my old cheaper Maplins multi-meter.
According to the new multi-meter my battery voltage at terminals is a healthy 14.1V

But checking via VCDS shows battery voltage to be 13.86V a bit low.

Which voltage should I trust? What is VCDS actually measuring, voltage at battery terminals or at some other location?

[notorious]

I'd trust your multimeter. VCDS doesn't measure voltage on battery poles.

P.S.

You can also check voltage without VCDS. Press and hold the recirculation button. Then press the manual flow control up arrow on left hand side of the vehicle.

HVAC will go into 'engineering mode'. Select channel 36 with + and - buttons. HVAC will display voltage.

To exit this mode press Auto.

[Goran]

Thanks Sergey, very cool hidden function I will have to try that.
I'm going to trust my multi meter, in this case it looks like all systems go for the lithium-iron battery. It arrived last week, shockingly light and small for a 59Ah battery. I just need to confirm sleep mode current consumption. Or maybe I should keep it safe in the house until spring, and let the lead acid battery suffer the winter temperatures

[ainarssems]

On VCDS each module will show power supply voltage that it gets. So ECU can show one voltage, HVAC slightly different, ABS different ect.
It's all down to losses in wires, connections, fuses.

[awolfinsheepsclothes]

Goran
Tell us more about the new battery?
Dave

[Goran]

Thanks, so the VCDS is not voltage at battery terminals.

Here is more info about the lithium battery. Its put together by a company called Voltphreaks, and used in motorsport. Also a few people on the M5board have been using them for a few years.

Nominal voltage is 13.3V, fully charge it should be 14.4V, it is made up of 4 cells in series, 3.6V max each.

Weight is 4.9kg for the 59Ah version. Supposedly it will reduce the charging load on the alternator because it holds higher voltage than a Lead Acid battery (12.2V 50% discharged), so in theory it will release a miniscule amount of horsepower.

The other main advantage appart from weight is that it has a low-voltage cut off, preserving enough charge to start the car when you come back to it even if you left the headlights on, etc. Also it has a built in on-off button.
Apparently it should last 5-10 years, although to be honest my original Audi Lead Acid 95Ah battery in my previous S8 lasted 7 years before it no longer held a charge.

I managed to get the 59Ah model they just discontinued at a discount, "only" £880 shipped. I guess you have to be really serious about performance to spend this kind of money. Probably not worth it and not noticable, but now I started this weight loss madness I feel I have to see it through to the end

http://www.voltphreaks.com/technology.php

[ainarssems]

You could buy 60 of these and make one yourself :

http://www.ebay.co.uk/itm/UltraFire-...item51b3097a81

[awolfinsheepsclothes]

Nice piece of kit I'd proberly buy one if I had the spare cash and everything else that needs doing was done. the price didn't really surprise me to be honest they are a lot more in demon tweeks! I think you need carbon bucket seats,carbon propshaft and polycarbonate windows next
Cheers Dave
Ps and a full titainium or inconel exhaust!

[Goran]

Originally Posted by ainarssems

You could buy 60 of these and make one yourself :

http://www.ebay.co.uk/itm/UltraFire-...item51b3097a81

Thats a great idea! thanks for the link. The charging circuitry would have been a problem for me I dont know how to build electronics (beyond the odd resistor or zener diode).
Now I paid the (hopefully) one-off cost for the electronics, its good to know I can replace the actual batteries for only £60.

Originally Posted by awolfinsheepsclothes

Nice piece of kit I'd proberly buy one if I had the spare cash and everything else that needs doing was done. the price didn't really surprise me to be honest they are a lot more in demon tweeks! I think you need carbon bucket seats,carbon propshaft and polycarbonate windows next
Cheers Dave
Ps and a full titainium or inconel exhaust!

I was thinking about the titanium exaust too but I think it would cost close to £3000 if I went somewhere professional like Primary Designs. Its just too much for me at the moment.
I think for now I will have to satisfy myself with just chopping out the mid and rear boxes and replacing with some long cherry bombs / silencers (whatever they are called). For the sound as much as for the weight reduction.
Oh and for the seats I have 2 very comfy Audi TT seats lined up, 22kg each, and same rail spacing as D2 seats.

[awolfinsheepsclothes]

What does the std battery weigh in at? 23kg ish ?
22kg for your seats! I thought you would have gone for the 3.5kg each carbon buckets! How much weight have you shaved of your car so far?
Cheers dave