Car Batteries

Preamble
The invention of a ‘Re-chargeable Cell’ by the 1920’s revolutionized the way cars work today. Until then, they had engine driven ‘Magneto’s which became a source of electrical power for the car’s ignition and lighting systems only when the engine was manually cranked to a ’start’. Consequently, every thing electrical in it went dead the moment its engine stopped running.

So here’s some more on the subject…

Car batteries are ‘Lead-Acid’ type ‘Secondary-Cells’ i.e. re-chargeable and today besides the entry level ‘regular’ ones, they come in variants such as low maintenance types to totally sealed types - requiring minimal to no topping up at all during their useful life of 3-5 years.

There is no National or International Standard which lays down the criteria for ‘Power’ of a Storage Battery - coz of the very nature of the ‘device’ - since ‘Power’ is defined as ‘Rate of doing Work’ which in turn is measured in Units of ‘Watts’ or ‘Hp’ continuously or indefinitely – say like a Ceiling Fan.

Therefore, Storage Batteries are universally given an ‘Ampere-Hour’ rating at a ’specified’ rate of ‘current’ discharge against their designated terminal voltage.

In other words, a ‘C5/38AH’ Capacity of a Car Battery means that it can sustain a discharge rate of ‘5 Amps’ for 38/5 = 7.6 hrs, before it gets discharged for all practical purposes BUT is good enough for a re-charge. This level for a 12 V Car Battery occurs when its terminal voltage sinks to ~ 10.8 Volts - from a ‘fully charged’ one of 13.2 Volts.

On Cars fitted with Alternators, adequate charging is easy to test by using a DC Voltmeter and here’s how the Battery Service people do it -

i) The Battery terminal voltage with engine idling with all loads off should not be < 13 volts and at an engine rpm of 3000, it should not be > than 15 Volts.

ii) Likewise, with all loads on, it should not be < 12 volts at idling and < 13.0 volts @ 3000 rpm.

iii) The Battery Terminal voltage should not drop to < 9 Volts when the starter is cranked.

iv) The Specific Gravity of 'each' Battery 'Cell' should not be < 11.80 the first thing in the morning. If it does on an otherwise healthy battery, then it’s logical to suspect that there’s unhealthy ‘leakage’ in the Car’s electrical system.

v) The correct way to check for leakage is to - a) Take off the Batt (-)ve Terminal and b) insert a 0-1000 mA range Ammeter between it and a good body earth point - such as the GB Housing where the earth cable is normally clamped - with all the loads switched off. Any current indication of > 25 mA would be a cause for concern.

vi) Likewise, a supplementary Alternator charging check is done by introducing a 0-30A Range Ammeter in the ‘Main Fuse’ ckt and see that it kicks upto 30A soon after a self-start and then gradually tapers down to 5A or below after a while.

vii) A precaution one should take would be NOT to sw off the engine at every traffic light and then restart - remembering that after every self-start, a Car needs to be driven for atleast 10 mins non-stop for the Battery to fully re-coup. This way, one can considerably enhance not only the Battery life but also that of the Starter Motor.

viii) Similarly, ensure that your cabin roof/boot light doesn’t remain on overnight by any chance - whether visible or not. Faulty door switches are often the cause.

With all these things in place, there’s no reason for a new Battery to be running down frequently. There has to be a leakage or lack of adequate charging somewhere – unless ofcourse the battery is faulty. The average battery life of a well maintained car in our climate is ~ 3-yrs. If inspite of the Car’s electrical/charging system and starter motor being OK, the engine doesn’t crank, in all probability the battery is half dead or has lived its life.

An authorised battery service station or a competent garage can further confirm this by checking the specific gravity of the electrolyte in each one of it 6-cells, followed by a ‘heavy current discharge test’ using a gadget for the purpose. If the battery voltage sinks below 9 volts during this test, the battery is either discharged or dead, depending upon its age.