RS246.com Forum :: The World's #1 Audi R, S and RS Enthusiast Community

My battery seems to go flat quite a lot, for example if I am working on the car and turn power on to the car (but not starting it) a few times while testing things over an evening it will go flat ...

I turned my car off on friday night, when I went to start it on Sunday it was flat ...

I took it for a long drive (few hours), disconnected the power to the battery and tested it ... it read 12.57 volts (started at 12.55 volts and over 2 mins went up to 12.57, not sure if it was going to continue to climb?) anyway, that was 10pm last night, by 9am this morning it was reading 12.45 volts.

Signs of a end of life battery??

If I need to replace it, what are my best options? Can I get a lightweight battery at all for the car????

Cheers guys ...

Oh, ps, I got some blade fuses and rewired the gadgets I have on the car, much better now, every wire is fused

Sounds like a dead/dodgy cell to me. My battery kept going flat - could barely hold enough charge to start the car after 4 days. The root cause was a phone kit badly installed that made it go flat whilst parked at an airport and the battery never was the same after that. Got the phone kit sorted out and a new battery and now everything is great. No idea about a lightweight battery, I just put a standard one on mine.

Mind you, with the re-wiring you may have ensured that all the "gadgets" are off when the ignition is off (that was the problem with the phone kit - the kit and bluetooth was on 24/7 - poxy professional installers!). Any chance you can leave the car for longer than overnight to see how the battery holds up over say 48 hours?

Paul

I did think about that, but I can't think of any device that might be left on (now) ... but, I did the same and connected a device to the permenant 12v feed, which was left on overnight, the batter was not flat the next day but very low ... Does this sort of stuff kill batteries then??

What I might do when I get home tonight is leave the headlights on full for 5 mins or so (this should not drain the battery I assume) and see how I get on.

Is it possible to run the voltmeter over the battery and see if anything is drawing current? What kind of volts should I be looking at when the battery is connected to the car? (and nothing drawing a current for example)

If you put the voltmeter across the terminals you might see it gradually fall.

If something is left permanently connected and it drains the battery flat then (in my experience) that can cause harm, expecially in cold weather.

You could disconnect the battery and stick an ammeter in line to see what the drain is. But make sure the ammeter can take the expected current drain otherwise it'll melt. I don't know what a normal current drain would be on one of these (to run alarm system, clock etc) so couldn't tell you whether you have an excessive drain or not!

I suppose your idea of leaving the lights on for 5 minutes would be a good test of the battery's ability to hold charge (not with an ammeter or voltmeter connected). If you test the voltage as you get home with everything off and then after the 5 minutes, it shouldn't have dropped too far.

Hopefully someone will be along who can advise what a normal current drain would be.

Paul

I think what I will do is take a reading from the battery tonight, take it out and put my caterham battery in (its a lot smaller)

So, few reasons, 1) I can see if it looses charge over a few days, plus, I will get to see if my caterham battery works in the Audi ...

Also, I get to weight them both ... see where I am going here??!?!

Check this out ... http://uk.youtube.com/watch?v=LvTpO6KGhsg

Took the Audi battery out (18kg) and put in my caterham battery (8kg)

Car starts fine!

I can now sit the audi battery and see if it holds the charge after a few days ...

Is this correct?

Open Circuit Voltage ~ State-of-charge
12.65 V = 100%
12.45 V = 75%
12.24 V = 50%
12.06 V = 25%
11.89 V = 0%

Dunno about the table you show, but if the battery drops it's voltage considerably over a few days with no load then I'd have thought it was time to replace.

Can't see how a votage reading of 11.89v would indicate a knackered battery though - it's too close to 12v (nominal) to be readable (even using a digital meter - which may be inaccurate). Mine was dropping to about 9 volts over 3/4 days, and if we had a very cold day, the car would struggle to turnover in the evening at work.

Hey, you've lost 10Kg, now fold those seats down and hold on tight!
Seriously, is the Caterham battery of some special construction - otehrwise I can't see how it would be able to cope with the demands of the car...surely the manufacturer wouldn't have bunged an 18Kg battery in when an 8Kg one will do the job just as well? The Caterham battery is only designed to turn over a "puny" 4 cylinder engine and also not have the electrical demands that these cars have.


Paul

Yeah, I think I will do what you say and just leave the battery till the weekend and see what it comes back with ...

I know exactly what your saying with the caterham battery, but, if it works, it works! I mean, its not something I will keep in the car all the time, because I am sure it will drain quickly if the engine is not on, but for events like the ring, or hill climbs/tracks days its perfect, another 10kg lost!

I am assuming there are no "risks" by running a this better other than the fact it will go flat quicker???

Have you checked that you haven't got any other (excessive) drains on the car that could cause the Caterham battery to go flat?

The Caterham battery may not be designed to cope with the demands placed on it when starting the car - huge current drain - and so I guess this may harm it at some point (but no idea when).

Paul

Once the engine starts it should be self sustaining by the alternator. IMO

Worthwhile saving

derdle wrote:Have you checked that you haven't got any other (excessive) drains on the car that could cause the Caterham battery to go flat?

The Caterham battery may not be designed to cope with the demands placed on it when starting the car - huge current drain - and so I guess this may harm it at some point (but no idea when).

Paul

I have not checked yet, I dont have an ammeter, but I think re checking my wiring is not going to be a bad idea ...

The "caterham" battery, I believe is just a lawn mower battery!!! So thankfully not the end of the world if it cant cope (but seems to be doing not too bad so far, few short journeys today and still turning over no problem)

Mark, all these little savings are going in my spreadsheet, I am sure I can get the car down to 1500 kg

Once the engine starts it should be self sustaining by the alternator

Quite agree - but the battery itself may not be able to cope with the starting current being drawn from it - bit like trying to draw 30 amps through a piece of 5 amp cable...somthing will go phut sooner or later. iirc there is a "cranking current" marking on batteries and for these cars it's something 450/480amps.

Personally, I wouldn't like to run the risk of the lawn mower battery letting me down when I'm miles from home!

Paul

derdle wrote: Personally, I wouldn't like to run the risk of the lawn mower battery letting me down when I'm miles from home!

Paul

Very true, I intend to put a proper (hopefully working) battery in for my trip to the UK over the festive period ... the thought of sitting broken down at the side of the M6 to save 10kg is not be idea of fun!!!

But, for events, I can see an attraction!

Are there any good/bad car batteries that I should look out for? ie good makes./ cheap bad makes etc?

I should have said A14, I am going to be down your neck of the wood for xmas!

Ahh...the lovely A14...awful road! So many prangs. Don't use it myself as it is north of where I work (Royston) but a couple of guys do have the unfortunate pleasure of either using it or being caught in tailbacks caused by prangs on it. If you are down this way before 21 Dec then pm me before and we can hook up....am off on holiday on 22nd. Christmas in the sun!

The first battery I got for my RS4 was a Halfords jobby (£90 ish)- which lasted about 3 years, and then I bought an "unbranded" one from the local motor factors (50 metres up the road) for £65. Didn't fancy paying the Audi price - but I beleive the OEM is a rebadged Bosch battery.

Paul