Battery - why was it low?

I drive 2 lots of 25miles each day. yesterday I stopped 1 mile from home to get some milk and when i tried to restart the car (1.6 1999 weekend) there was nothing just a few clicks etc. The radio had reset, time was flashing 0:00. OH came down and after trying to jump the car for a few mins we gave up and towed it home.
I bought a multi meter today and found the battery was dead so I bought a new one for £35 (not bad price IMO - Bridgend motor factors) Once fitted car started straight up.

Now the dead battery was only a 18 months old so I'm thinking there must be another underlying reason for the sudden lack of starting ability.

Any ideas what and how i should investigate?

i was told at last MOT that the Aux. belt tensioners had some play in them and the belts sometimes sequel at lower revs.

you seem to have the most likely answer tbh
loose alternator belt
tighten it up and monitor charging voltages

its gotta be alternator or drain (unless your belt is slipping so much that it isnt turning the alternator, but that is highly unlikely).



simple way to test which it is using a multimeter.

start engine, switch off all electrical devices, then read voltage acros battery it should be 14.4v. if it is less then alternator is suspect.
now switch on all electrical devices (radio, lights demister etc) and the voltage across the batery should measyre 13.99v, any less and alternator is suspect.
now switch engine off (remove key) and switch all electrical devices off. disconnect battery earth lead and connect meter between lead and battery set tyo measure current. there should be no current flow, or only a very small flow if there is one. that current is how quickly your battery is getting drained, to find out what is draining it you can reomve one fuse at a time, the fuse you reomve that stops the current is the fuse for the circuit that is draining the battery so you can do further tests on that circuit to see where the fault is (often an aftermarket stereo for example)

Jugs advice is 100% as ever. No mention of the alternator warning light being on so this points to drain or the battery. I did have batteries go on the MGB once at 2 years old. I fitted new fan belt re-calibrated the control box, took the dynamo apart to check the brushes and commutator. Eventually decided it had to be the batteries put 2 new ones on (different supplier) and they are still working fine 10 years on. I do use a conditioning charger which is not something you can easily do on a daily driver. I also lost a battery at 2 years old on a Morris Marina(company car) the car just suddenly died no electrics at all. The battery had lost a lot of the lead filling from the plates this had fallen to the bottom of the case and it had shorted internally crowbaring down the volts. Again, new battery and away we went. This had to be a manufacturing defect. I have carried out the current drain test Jug recomends and you should see nothing to a few microamps.

Any battery can fail within about 24 months on a modern car with all it's electronics, nothing uncommon.
Just buy a new one from a well known brand, they (may) last longer.