jump starting a car with a flat battery

[macafee2]

Originally Posted by SD1too

Hi Ian, thanks for your reply.

Unfortunately your assumption may be unreliable. Unless your "spare" battery receives a frequent maintenance charge, it is likely to become sulphated. This means that although it may illuminate a "fully charged" indicator, many of the plates may be rendered useless so, in fact, only a proportion of the battery is fully charged. In this condition it is unlikely to start a car and particularly when placed in parallel with the vehicle's discharged battery.

If you put your voltmeter across the "spare" battery and operate the starter, a voltage of below approximately 10.5v suggests that the battery is sulphated if it previously gave a "fully charged" indication.

To guarantee that a "spare" battery will be useful whenever needed, leave it permanently connected to a smart maintenance charger. I recommend C-tek as they are well made and very compact but there are other choices.

Simon

Ok thanks Simon I will try that test in the next few days when the rain has stopped. The battery chargers are smart but I have known them not to be smart enough to charge an exceedingly flat battery. A good old fashioned charger was needed for a while before the smart one would operate

macafee2

[Davy]

The best way to check a battery is with a drop tester .Most garages will check it for you free of charge.