Ben, instead of all this frustrating guesswork and speculation you really just need to buy yourself a basic digital multimeter, they're not expensive...
Even a cheap one (£9.99) like this would do what you need:
http://www.maplin.co.uk/ut-132a-digital ... tor-518506" onclick="window.open(this.href);return false;
Personally I would go a bit more expensive in the £30 range, but the one I linked has voltage readings with an accuracy of 0.5% (important for judging the state of a battery by its voltage) and a 10 amp range for checking for excessive current drain when the car is turned off, so it has what you need.
I know its harder to find stuff like this where you are but even if you get a forum member to post one to you it won't be much! Once you have one you can diagnose the problem pretty quickly, and have a tool that will help you diagnose any future electrical faults...
You don't need min/max capture because without context (you don't know when and under what conditions the min and max occurred) the figure is useless!
The first thing to do is find out if your alternator is charging correctly. This is very easy to do - let the car run for 5-10 minutes for the battery to stabalize and then measure the terminal voltage with the car running. It should be 14.2 volts +/- 0.2 volts. If it is within this range the alternator and charging regulator is fine!
Less than 14 volts with the engine running means that the battery will not get fully charged, and this is likely the source of your problem, try measuring it again with the rpm increased to 1500 or so and if its still below 14 volts you definitely have a charging problem. More than 14.4 volts will damage the battery from overcharging, so if you see this it could also be the problem due to the battery being damaged by the charging system. (The battery will loose capacity to hold a charge)
If your reading was 14 - 14.4v and the charging system seems ok the next step is to check for excessive current drain with the car off. In a modern car like a C5 there will be a lot of electronic devices that may draw power when they shouldn't, for example the suspension pump...it takes a while for some of these systems to settle down so I would leave it turned off and leave all the doors closed (but the bonnet open and ready) for at least an hour for everything to go into economy / low power mode.
Now put your meter in 10 amp mode and connect it to the positive battery terminal and positive battery post (positive of the meter to the battery post) before disconnecting the battery terminal. What you need to do is keep a constant connection so that the devices in the car don't lose power as you slip the terminal off the battery post, after the terminal is off the post the current is flowing through the meter which is connected in series with the positive terminal lead. You might need a couple of alligator clip leads or a second pair of hands to do this successfully.
I'm going to take a guess here and suggest that normal idle current drain on a modern car with everything turned "off" should be below about 200ma. 200ma would discharge a 60 amp/hour battery in approximately 12.5 days, certainly not in one or two days. Let us know what the current figure is. Do NOT turn on the ignition or attempt to start the car with your amp meter connected or you will fry it...
Assuming that the current drain is not excessively high when the car is sitting that points the blame at the battery. To confirm this we want to take a voltage reading after the car has sat overnight, or in conditions where you strongly suspect the car will have difficulty starting based on previous behaviour. Before starting the car or turning anything on pop the bonnet and measure the battery voltage, a fully charged idle battery will be 12.6 volts, fully discharged is 11.7 volts, and there is a relatively linear relationship between those figures, for example 12.05 volts would be approximately 50% charge. (Remember these figures only apply when the battery is not charging or discharging)
If the charging system was fine, there was no excessive current drain while the car is idle, yet this voltage is indicating a low state of charge your battery is faulty. Replace it.
There is some good reading material on Lead acid batteries here:
http://en.wikipedia.org/wiki/Lead_acid_battery" onclick="window.open(this.href);return false;
Good luck.
