Battery Testing
There are four steps to follow in testing an automotive battery
- Inspection
- Removal of Surface Charge
- State-of-Charge Test
- Load Test
Inspection
Visual inspection is important for the detection of obvious problems
- Loose Generator Belt
- Low Electrolyte Level
- Corroded Cable or Terminal Clamps
- Loose Hold-Down Camps or Cable Terminals
- Damaged Battery Case
State-of-Charge Test
Using the measured battery voltage, the state of charge can be determined. Use the table to determine the battery's state-of-charge.
| Open Circuit Battery Voltage | Approximate State of Charge | Average Cell Specific Gravity |
|---|---|---|
| 12.65 + | 100% | 1.265 + |
| 12.45 | 75% | 1.225 |
| 12.24 | 50% | 1.190 |
| 12.06 | 25% | 1.155 |
| 11.89 | 0% | 1.120 |
BATTERY VOLTAGE CHART
Pay special attention if the DVOM measurement of OCV is equal to
- 0 volts -Indicate an open cell.
- 10.45 - 10.65 volts -Indicates a shorted cell.
For non-sealed batteries, check both specific gravity (SG) in each cell with a temperature compensated hydrometer and battery OCV, without the engine running.
For sealed batteries (installed in all MINI's), measuring the battery's OCV (without the engine running) with an accurate DVOM is the only way to determine the state-of-charge.
Batteries with a built-in hydrometer measure the state-of-charge in one cell only. If the indicator is clear or light yellow, the battery has a low electrolyte level and should be refilled before proceeding or replaced.
A state-of-charge reading BELOW 75% using SG, voltage measurement or dark indicator in batteries with a built-in hydrometer, indicates the battery must be recharged before proceeding.
Replace the battery if one or more the following conditions are met
- More than 0.050 difference in the specific gravity readings between the highest and lowest cell (There is a weak or dead cell).
- The battery will not recharge to 75% or greater state-of-charge or the built in hydrometer does not indicate good (green indicates 65% or better).
- DVOM reading indicates 0 volts (Open cell).
- DVOM reading indicates 10.45 - 10.65 volts (Shorted cell).
Load Test
A battery which has a state-of-charge of 75% or greater or has a "good" built-in hydrometer indication may be load tested.
With a battery load tester properly installed, load the battery for 15 seconds to one of the following
- One-half (1/2) the CCA (Cold Cranking Amps).
- Three (3) times the AH Rating (Amp Hour Rating).
The voltage on a good battery will NOT drop below 9.7 volts during the battery load test. After the load is removed, wait 5 minutes, the battery should bounce back to 50% or greater state-of-charge. If a battery drops below 9.7 volts during the load test, does not bounce back or fails to start the engine, the battery should be replaced. Batteries which pass this test should be recharged to restore peak performance.
Load Test Conditions
- Tests assume electrolyte temperature of 80°F, 26.7°C.
- If the electrolyte temperature is above 80°F add .1 volt for every 10° up to 100°F.
- If the temperature is below 80°F subtract .1 volt for every 10° to 40°.
Testing a Battery for Sulphation
A battery which fails the load test should be tested for sulphation. To test a battery for sulphation, place it on a battery charger for three minutes with the charger set on 40 amps.
After three minutes check the OCV, if the reading is greater than 15.0 volts the battery is sulphated. Batteries which indicate a sulphated condition should be recharged slowly and retested before being discarded.
Closed Circuit Current Draw Testing
Increased closed-circuit currents may occur permanently or intermittently and cause the battery to discharge prematurely. The increase in closed circuit current may be caused by a faulty control unit or by the installation of a non-approved accessory.
Minimal amounts of parasitic draw can be overcome by the chemical reaction that takes place inside the battery. A problem occurs when the average vehicle current consumption higher that the battery can produce chemically.
In a situation where a vehicle has broken down due to a discharged battery, for diagnostic purposes it is important not to disconnect the battery. The control unit may reset if the battery is disconnected. Following a reset, a faulty control unit may start functioning correctly again, making accurate diagnosis impossible.