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 Angus and Rosemary's Miscellany

  of Malvern - Motoring

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Looking after your 12 volt car battery


Many cars have stood unused for weeks during the COVID-19 pandemic and owners have subsequently found their car battery flat.

Following retirement some fifteen years ago our car 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 charges at only a few amps so an occasional long run is needed to keep the battery topped up.

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.

Your vehicle ideally needs to be run for about 30 minutes every day in the winter to make good these losses and the power consumed by the starter motor.

Modern vehicles fitted with more electronics and start stop technology place an even heavier strain on the battery,

Battery Charging

If your car is only used occasionally it is advisable to check and if necessary charge the battery every fortnight 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 at about 1 Amp.

Of course this will only be possible if you are able to park on a drive close to a power outlet.

It is recommended to charge 'maintenance-free' car batteries with a modern smart 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.

Examples of such chargers are the CTEK MXS 3.8 and CTEK MXS 5.

The CTEK charger is small, splash resistant, and can, subject to precautions, be safely left connected. You can buy it from Halfords, on-line from Amazon, and other stores. Its case is made of plastic, so don't drop it.

CTEK MXS 5 battery charger

CTEK MXS 5 battery charger

The CTEK charger comes with a storage bag and the option either to connect the 12V connector into a short lead with eyelets, for example to bolt into a Classic Car, or crocodile clips for direct connection to the car battery. In our case we snipped off the eyelets and crimped on a cigarette lighter plug to enable charging through the cigarette lighter socket  - you can buy a CTEK adaptor ready made.

Other makes of charger are available.

Extension leads

We thought about putting the charger indoors and running a long 12V cable out to the car, but calculated the resistance of the lead would probably upset the smart charger. Better to run a mains extension lead from a Residual Current Circuit Breaker (RCCB) out to the car; but take care water does not run down the cable into the mains socket and that the cable is not trapped in the door or window.

If you want to try a 12V extension lead we recommend you use a flexible cable rated at 20 Amps, for charging the battery 1 to 5 Amps; this is to minimise the voltage drop in the cable.

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; slightly less when cold. This 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. This too can be plugged into the cigarette lighter socket with a suitable plug, so you don't need to lift the bonnet.

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

Diesel cars have a bigger battery as the engine is harder to start. Sometimes the bigger battery can be fitted in the Petrol version of the car.

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 newer batteries should last longer they may be more difficult for Classic Cars 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 Calcium 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 smart charger.

Spiral cell technology batteries such as the Optima and Exide Maxxima, and Absorbent Glass Mat (AGM) batteries are more robust but much more expensive so only likely to be justifiable for specialist applications, for example vehicles with stop start technology and 4 x 4 vehicles bumping over rough ground.

Case sizes, filler caps and terminal positions vary. 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.

Our old Ford Focus owner manual stated use only a 'silver' battery which has a slightly higher terminal voltage than batteries obtainable from many  tyre and battery centres. Some later Fords are fitted with 'Calcium' batteries.

Our current Ford Ecosport has a box of electronics bolted above the battery making it difficult to replace - what idiot thought of that!


Click for more photos of the CTEK charger:-

CTEK MXS 5 charger

Home made lead to plug into cigarette lighter socket

Front of packaging

Rear of Packaging

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