How to Test a Car Battery
A - Visual Test
1. Does the car battery leak?
Yes - Continue with A2
No - Continue with A3
2. Is the external damage visible in vicinity of leak?
Yes - Car battery not faulty. Ascertain cause cause of damage
No - Car battery faulty
3. Are the cells evenly covered with electrolyte?
Yes - Continue with A4
No - Car battery not faulty. Not manufacturing fault. Possible causes: overcharging or extreme ambient temperature.
4. Is the car battery 12.5 Volts or over?
Yes - Proceed to B to confirm cell conditions.
No - Car battery will require recharging. Then proceed to B to confirm cell conditions.
Recharging - A car battery should be capable of accepting a recharge at a minimum of 1/20th of its capacity. ie: 50 Amp Hours = 2.5 Amp charge. The car battery should be charged until all cells are gassing freely (maximum 20 hours). If the car battery will not accept a charge at the above rate or if 3 or more cells fail to respond in a prolonged charge, the battery has failed due to aging, wear and tear or deep cycling discharge. A car battery with five cells gassing freely and one/two inert cells will be covered by B1.
B - Specific Gravity Test
Functioning car batteries should have same specific gravity (S.G.) in all cells. Maximum permitted tolerance in all 6 cells is 0.030kg/dm3 between lowest and highest S.G. reading (e.g. 1.28 max and 1.225kg/dm3 min would be permitted).
Specific Gravity at 27°C in kg/dm3
1,250 - 1,280 - Indicates fully charged, proceed with test
1,200 - 1,240 - Indicates half charged, charge required
Less than 1,200 - Indicates minimal charge, charge immediately
1. The battery has One/Two adjoining cells with exceptionally low specific gravity, remaining cells 1,250 or above.
Yes - Car battery is faulty
No - Continue to B2
2. Is the electrolyte discoloured (dark brown) and /or is there evidence of excessive water consumption?
Yes - Car battery not faulty - damage caused by overcharging - test voltage regulator on car.
No - Continue with B4
3. Is the S.G. in all cells equal or above 1.240kg/dm3 in all cells after charging?
Yes - Battery not faulty - damage caused by deep cycling discharge or undercharging. Check car electrics i.e. voltage regulator, alternator or fan belt.
No - Continue with B4
4. If the S.G. in all cells is equal or above 1.250kg/dm3 after charging - proceed to test C.
C - High Rate Discharge Test
This test can only be carried out if the voltage is equal to or greater than 12.5, otherwise charge the flat car battery first. Important: read the instruction notes on the battery tester.
1. Is the result 'defective' or 'not defective'?
Defective - Car battery is faulty
Not defective - Good car battery
For car battery testers with switchable resistances (Variable Load), discharge the battery at about 3 x the 20hr rate for 10 seconds. (e.g. battery capacity 45 ampere hour (A.H.) = 3 x 45 = 135 amp(s). The voltage during discharge should be stable at 9.6 volts or over.
Testing Sealed Car Batteries
The lead-calcium-silver technology used in some car batteries means that the conventional filler plugs have been replaced by a sealed labyrinth lid, for technical reasons, and due to consumer demand.
The only fully accurate method of evaluating the condition and serviceability of ‘sealed’ car batteries is through the use of recognised digital electronic test equipment which, together with a visual inspection will provide an extremely high level of accurate diagnosis.
In the absence of specialised test equipment, test steps A and C can be carried out.