I can't offer a specific guide, but let's go back to basics. There are three and a half reasons why a battery would fail:
- Not enough current coming in from the alternator
- The battery can't retain its charge
- Too much current going out through the electrics
- The battery is actually charged,, but can't deliver that energy where you want it.
You really need a multimeter, and a mate, to work through the following (the former's fairly cheap at Halfords/Maplins/etc., and both are rather useful to have handy)
1) Current In
13.5V sounds OK, not great but OK. So in principle this isn't the problem. However, perhaps there's a flaky connection. So have a nose around the connections. Then start the car (by whatever means necessary), and use the multimeter to measure the voltage across the battery terminals. It should read 13.5V or more. Try waggling the connections from underneath (careful with the fingers) and see if there are any fluctuations. If not, you can be fairly confident the alternator's OK.
2) Battery Fails To Retain Charge
Batteries do fail of old age. But not this quickly. in your case, this is a symptom, not the cause.
3) Current Out
This is the most likely cause. It can be due to too much current, or too long a time. Puntos certainly do drain their batteries rather quickly, so you might find that it's your lack of use that's causing the problems.
You say that failure has occurred overnight, which would suggest it's not time, but it might still be this. There is a misunderstanding that 15 mins running (a normal drive) will charge a battery fully. This is not correct - it will put a healthy dose in, but won't fully charge it. So if you are not using the car much, it might seem OK one day (chugging into life), but be flat the next morning. Worth considering.
The other aspect, more likely, is that too much current is draining. First, remove, or disconnect the battery. Check the voltage across the bare terminals - if it's below 12V that battery is probably failing, and you should really buy yet another.
Now charge it for at least 12 hours. You should now see about 13V.
Now leave it overnight and recheck (and note the value). If it's dropped notceably, that battery's dying.
But if it's OK, reconnect it, and recheck. You should see perhaps a 0.1V drop. Any more, and you probably have a dodgy earth. If so:
- Whilst your mate watches the multimeter, start removing each fuse, one-by-one (remember, there are three different sets of fuses to check)
- If you pull a fuse, and the voltage reading jumps back up, that's the/a circuit with a dodgy connection.
Quite what you do with that information is where the fun starts. You need to check through the wiring for items that shouldn't be on it, or can be switched off, or have badd connections. At the very elast, you can at least give the garage something to aim for.
4) Battery Can't Deliver The Power It Has
This is an odd one that came up recently, and it will depend on the symptoms you actually have. You would see all the electrics (lights, etc.) working fine, just the starter refusing to turn over or making a very flabby attempt to engage.
The issue here was that the battery was fine, but that the connection to the starter motor was damaged. In this example's case, the earthing strap was badly corroded. Inspect the cables and fixings for tightness and integrity. If there's any suspicion, you could jury-rig up a temporary earthing strap from some spare battery cable and see it it solves the problem.
If the symptoms fit, and the multimeter tells you the battery looks fine, this could be a route to look down.
So sorry, no definite answers, but at least some ways of trying to isolate the cause of the prblem.
