is the No 1 reason for calling out the breakdown services. Modern car electrical systems put big demands on the battery. Usually the first sign of trouble is that the engine is reluctant to start when cold, with the starter motor turning more slowly than usual. This is then followed by a failure to start at all.
There are three main reasons for a battery to be flat. One is old age, in which case fitting a new battery will cure the problem. The second is leaving the lights on (or even the radio or alarm, if parked for a long time); generally a jump start will put things right. The third is a problem with the charging system; fitting a new battery won't cure that. See your Haynes Service and Repair Manual for details of simple tests.
What you'll need:
- A new battery of the correct size and capacity to fit your car. If you plan on using it straight away, it needs to be already charged - ask when you buy it.
- Some copper grease or special battery terminal corrosion protector spray is a good idea too.
1 Disconnect the leads from the old battery, negative (earth) first. Undo the clamp nut or bolt - fixing details will vary according to model - and lift out the battery. Be careful not to drop it, and keep it upright to avoid acid spillage.
2 Fit the new battery, secure it with the clamp and connect the leads, negative (earth) last. Protect the terminals against corrosion by smearing them with copper grease or using an anti-corrosive spray.
3 Check that the negative (earth) lead connection to the body is clean and tight. Unbolt it and clean it with a wire brush or abrasive paper if necessary.
Batteries contain sulphuric acid, which is poisonous and causes burns; they give off hydrogen gas, which is explosive when mixed with air; when charged they contain enough electrical energy to cause burns or fires if short-circuited. Follow all precautions specified by the manufacturer.
- A slack or broken alternator drivebelt can cause the battery to go flat.
- Recharging a failing battery will sometimes help it keep going for a little longer.
- Most modern batteries are sealed for life, so you can't check or top up the electrolyte level.
- Dispose of the old battery responsibly - don't just put it in the household rubbish. The lead it contains can be recycled.