Originally Posted by
Rupert Prior
small batteries will work but they are working flat out and have no reserve power when they start to fail, a bigger battery has more capacity to soak up the highs and lows but still give a stable voltage.
the battery problem have been arround for ages and just keeps being made worse, with more and more demands for power. rover say that the problem on LM range rovers is so bad they have to be put on charge when they are in for any service. this is because a voltage below 12.6 will start causing systems to shut down (this is a 12V system remember).
other cars need stable voltages of around 12V to function properly which is possible with smaller batteries but everything must be in top condition and the battery fully charged, otherwise the starter will drag the voltage down to 10 and below (which rattles relays and gives Becms a headache).
the charging system is also being pushed over its limit and so does not keep the battery fully charged, the temperature of the battery is too high (you cannot fully charge a hot battery), the charge time from the alternator is often too short and the voltage is too low (especially for calcium type). this is all i can remember of a long lecture i got on a course at rover. there are some more articles about this online if you are interested.