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I've been doing some battery testing lately on both my cars. (2010 A8L and a 2010 RS5). I don't seem to have any starting issues and both cars run fine.

Both of these have a resting voltage of around 12.1 V

I charge them up with a 6 stage charger every month and they get upto 12.6V after a charge which usually only take about 5-6 hours at 3.9amps, (23AH) but always settle back to 12.1V after 12-24 hours. This would indicate on a 90AH battery every month I lose about 1/4 of the battery capacity.

Did a Cold Crank test on the RS5 (outside temp was chilly 10 deg C) and the voltage drop was 12.1V down to 10.3V and then within 3 seconds went to 13.9 (charging from alternator).

All these stats bar the resting voltage of 12.1 seems to say my batteries are in good condition. But everything I read says 12.1 volts is almost completely discharged and it should be 12.6V resting voltage.

A8L battery is a brand new Bosch AGM DIN88L
RS5 battery is possibly orginal, but doubt it at 10 years...not even sure they would last that long.
 

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You could purchase a battery tester that's designed to check condition of your battery under load. The resting voltage of AGM batteries are higher than traditional flood batteries.

Are you checking the battery voltage at the battery terminals? Or are you checking it somewhere else in the car? There are lots of electronic things going on in the background even if you don't have in the car running, so you may see a different voltage elsewhere.
 

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12.6V is the normal open circuit measurement for a typical fully-charged car battery. Even close to the terminals it will be less than this when connected to the car's electronics which draw an idle current. If the battery is in the boot/trunk and the front terminals are used then the drop could be more depending on the wiring. My S5 is 6 years old on the original battery and although I have never had a starting problem or low battery message, since I don't use it much at the moment, I feel the need to use a charger on it weekly-ish. I check the voltage before charging and note that this is usually just as the lights switch back off after the timeout, which means that it is probably less than true "resting" voltage. After a week it is typically 12.1V but occasionally I have left it longer and I once saw 11.9V before charging. I haven't bothered doing a full test of my battery - I'm just waiting until I see the first "low battery" warning on my DIS before I get a new one.
 
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Well guys I use my old S4 only from time to time but she lives in a lovely garage and I use a CTEK battery conditioner every month or so and she always fires up quickly. Dont know the numbers but I know that my motorbike batteries (Yuasa) last 4 years or more using the CTEK instead of 2 years without.....
 
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+1 as I decided to maintain rather than replace mine- even though its still the original, so I first got one of these Ancel testers to check it's still ok, and then optimised it with a Ctek 5.0 on a regen cycle:
ANCEL BA101 12V Auto Battery Tester 100-2000 CCA 220AH AGM GEL Battery Analyzer
CTEK MXS 5.0 12v Car Bike Caravan Smart 8Step Fully Automatic Battery Charger
Both great pieces of kit & well worth the investment.
 

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Yes, the CTEK advanced technology allows this and protects the electronics so no need to disconnect from the vehicle:

The MXS 5.0 is an advanced microprocessor controlled battery charger which gives unrivalled performance for lead acid batteries from 1.2-110Ah. It comes with automatic temperature compensation built in which ensures the best charging performance, even in the most extreme conditions. The MXS 5.0 also includes battery diagnosis to establish whether your battery can receive and retain a charge, patented automatic desulphation program, an AGM option which maximises the performance life of most Stop/Start batteries and a special Recond mode for reconditioning deeply discharged batteries. The MXS 5.0 uses CTEK’s patented float/pulse system, which is the most efficient maintenance mode when a battery is connected for long periods.
 

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I have

Even does lithium batteries and 3-year guarantee
and

Both seam to work great and niot a lot of ££
 
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