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 Angus and Rosemary's Miscellany

  of Malvern - Motoring

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Looking after your car battery


Following retirement, one of our vehicles was only used occasionally and the battery gradually became discharged. It was discovered that whilst the alternator can supply up to 80 Amps to power ancillaries, the battery typically charged at only 2 Amps.

The immobiliser, clock and radio were found to draw about 26 ma on standby, and although that is small, it is enough to discharge the battery by about 50% in a little over a month.

The vehicle would ideally need to be run for about 30 minutes every day to make good these losses and the power consumed by the starter motor.

Battery Charging

If your vehicle is only used occasionally it is advisable to check and, if necessary, charge the battery every month or so. Likewise if your vehicle is garaged for the winter it is advisable to disconnect the battery plus trickle charge it every one to two months.

It is apparently recommended to charge 'maintenance-free' car batteries with a modern three state electronically controlled charger. These charge at a relatively high rate to start with, then switch to a lower rate and finally trickle charge using a regulated DC supply which reduces gassing compared to older unregulated chargers.

One example of these chargers is the CTEK 3600.

The CTEK charger is small, splash resistant, and can safely be left connected. You probably won't find it in the shops but it can be obtained by mail order.

Black and Decker updates its models frequently. They are usually available from both Argos and Halfords. Typically bulkier than CTEK and not weatherproof, but on some models you can display voltage and current so you can see what is going on.

Battery Monitoring

At 25 deg C the open circuit voltage of the battery should be about 12.7 V when fully charged and about 12.4 V when 50% discharged, so can be checked with a digital multi-meter, eg first thing in the morning - before turning on the ignition.

National Luna in South Africa manufacture a Battery Monitor with a row of LEDs which shows the state of battery charge quite accurately and can be used to indicate when the battery is likely to benefit from a charge.

These are also available for dual battery installations for 4 X 4 and caravan installations where a separate battery is provided eg to run a winch or fridge.

The National Luna Battery Monitor can be purchased in England from APB Trading near Kidderminster and in Scotland from Quothquan Workshops.

Battery Types

12 V car batteries are typically designated by,

Amp Hour Capacity (AH)

Cold Cranking Amps (CCA)

Terminal orientation

Due to Health and Safety legislation, in 2012 many car batteries were being made in Pakistan and 're-badged' in the UK.

There appear to be very few comparative tests of automotive batteries, and though manufacturers advertise premium versions you will probably find that your local dealer offers only a standard battery with a three year guarantee. Buy from a reputable supplier, checking that the battery was manufactured recently, keep the guarantee, and charge the battery when you get home as it is unlikely to be fully charged if it has been on the shelf for a couple of months.

Older batteries had Lead Antimony plates but newer batteries are more likely to be Lead Calcium possibly with Silver added. There is speculation that although the latter batteries should last longer they may be more difficult for older vehicles with regulators set for 13.8 V to charge. It may be that a Lead Antimony battery will be better for a classic car than the latest Silver technology. In any event the advice is the same - the voltage of infrequently used car batteries should be monitored, and ideally the battery occasionally trickle charged with a microprocessor controlled charger.

Spiral cell technology batteries such as the Optima and Exide Maxxima, and Absorbent Glass Mat (AGM) batteries are more robust but are two to three times more expensive so only likely to be justifiable for specialist applications.

Case sizes, filler caps and terminal positions vary but typically, for older Land Rovers, a Type 72 battery is called for and a Type 69 battery can sometimes be used as a fallback - consult your handbook to check which battery is recommended for your vehicle.

The Ford Focus owner manual states use only a 'silver' battery which has a slightly higher terminal voltage than batteries obtainable from most  tyre and battery centres.