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Newer batteries often fail with little or no warning. It is good to carry a set of jumper cables.  If you start to notice the engine turns over slowly, you could have a weak battery, or a weak charging system. Some auto parts stores can check the battery or charging system for you free of charge

Tools Required

Inspect a Battery

Clean a Battery Case, Tray, and Cables

Test Battery Specific Gravity

Test Battery Open Circuit Voltage

Charge a Battery

Remove and Replace a Battery

Tools Required

Safety glasses 
Box of baking soda 
Adjustable pliers with insulated handles 
Screwdrivers with insulated handles 
A small, stiff-bristled brush (such as an old toothbrush)
Assorted small open-end and box wrenches: usually 3/8, 7/16, 1/2, and 9/16 inch 
Small metal or plastic scraper
Special battery tools, including a cable puller and cable terminal cleaning brushes 
A turkey baster or small funnel 
Commercial all-purpose household cleaner in a spray bottle 
One or more sponges or clean cloths 
A source of ample clean water such as a garden hose or a large bucket and sponge 
Rubber gloves to protect your hands 

Inspect a Battery

The battery is a common source of breakdown on the road. A battery that fails will prevent the owner from cranking and starting the car. You should perform a visual inspection on the battery anytime you have the hood up.

Preventative Maintenance Check List
Keep a Maintenance Record

CAUTION: Always wear eye protection when working on a battery. Protect yourself from electrolyte splash, which can injure skin and eyes. Never smoke or create any spark around a battery or it might explode.

The first step in inspecting a battery is to look for obvious damage


You should look for: 

Cracked or bulged case or cover

Signs of electrolyte leakage

Frayed insulation on battery cables

Corrosion buildup on terminals and posts

Loose or missing holding hardware

Electrolyte level (if the battery has cell vent caps)

Visually inspecting a battery

Any physical damage to the battery indicates it must be replaced. Broken or damaged cables should also be replaced. Corrosion can be cleaned off the post and terminals, as explained later.

Inspect the top of the battery for dirt or electrolyte. Too much electrolyte on the top of the battery may be the result of overfilling. If the top of the battery is not clean, current can flow across the foreign material. This will cause the battery to discharge by itself when the vehicle is idle. Check the battery hold-down to be sure it is tight. A loose hold-down could mean the active material has vibrated off the plates.

The electrolyte level should be checked monthly on older style non-maintenance-free batteries. During warm weather driving, these batteries lose water out of the cells. The higher the battery temperature and the higher the charge rate, the more gas that is developed and the faster the water loss. Long trips in hot weather subject the cells to high temperatures and high charging rates. These conditions mean the electrolyte level must be checked often. If the water drops too low, the battery will fail to function.

Older style batteries have cell vent caps. Low maintenance batteries have a pry-off cell cover. Remove the cell vent caps or use a screwdriver to pry off the cell cover to inspect the electrolyte level.

The electrolyte level should be well above the plates. Most manufacturers have a guide ring built into the top of the case above the cell. The electrolyte level should be filled to the level of the guide ring. If the electrolyte level is low, add water to the cell. Because regular tap water may have a high salt and mineral content, you should use distilled water. The water should be added with a plastic or rubber tool made for battery filling, not a metal funnel. A metal object could cause a short between the plates. Be sure not to overfill the cell. This would dilute the electrolyte strength and cause an acid buildup on the outside of the battery.

Clean a Battery Case, Tray, and Cables

You should clean the outside of the battery when you observe a buildup of dirt or corrosion. Dirt, moisture, and corrosion on the top of the battery provide a path for current flow between the battery terminals. This can cause the battery to self-discharge. If the battery has cell vent caps or a pry-off strip, be sure they are installed tightly before cleaning the battery.

CAUTION: Always wear eye protection when cleaning a battery or battery cable.

CAUTION: Before you begin to disconnect any hold-down hardware, you must disconnect the battery from the circuit. If you do not, you could accidentally touch a wrench between the positive terminal and ground and create a direct short. The short could cause a burn or a possible battery explosion. Always disconnect the ground cable first because this cable is less likely to cause a spark when it is connected or disconnected.

To do a good job of battery cleaning, remove the battery from the battery tray. Use the correct size wrench or battery pliers to loosen the negative terminal. 

Side terminals are removed with a wrench. Bolt-type clamps are removed by holding the bolt end and driving the bolt end with a wrench. Spring-type clamps are removed by squeezing tabs on the terminal to release the terminal from the post. A puller is often required to pull the terminal off the post after it has been loosened as shown below

Using a puller to remove a battery cable.

Follow the same procedure to disconnect the positive terminal. 

CAUTION: Use a carrier to carry the battery. Never hold the battery against yourself. Electrolyte can get on your clothing and skin and cause injury.

Remove the battery hold-down hardware and any heat shields. Use a properly adjusted battery carrier to lift and remove the battery from the tray

Using a carrier to carry a battery

Mix a mild cleaning solution of one tablespoon of baking soda and one quart of water. This mixture will neutralize the battery acid. Brush the solution over the battery and flush the case off with water.

Dry the battery with paper towels and then dispose of them properly. 

Use a scraper and wire brush to remove all the corrosion and flaking paint from the battery tray and hardware. Brush the same baking soda mixture over the tray and parts; allow them to dry. When dry, paint the parts with a rust-resistant paint. Again allow them to dry.

A good electrical contact is necessary between the battery terminals and the battery cables. Corrosion on the terminals increases the resistance between the cable and the battery. This will eventually cause starting difficulties. In the long run, the corrosion will completely destroy the cable connections.

The cable terminal and battery terminal post may be cleaned with a special wire brush. One side of the brush is designed to clean inside the cable terminal. The other side is designed to fit over and clean the battery terminal post. A solution of baking soda and water can also be used to neutralize corrosion. Make sure you get all the surfaces clean and bright

Reinstall the battery back in the tray. Be sure to lift it with the proper carrier. Attach all the hold-down hardware and shields. Tighten all hold-down hardware to the torque value specified in the shop service manual. A loose hold-down can allow the battery to vibrate and lose active material from the plates. Proper torque on the hold-down bolts is necessary to prevent excessive strain on the battery case.

WARNING: Do not overtighten the battery hold-down hardware. An overtightened hold-down can bulge or break a battery case.

CAUTION: Always connect the negative terminal last when connecting a battery to a circuit. This procedure will prevent sparks and a possible battery explosion.

Install the positive cable terminal on the positive battery post. Reinstall the negative cable terminal on the negative battery post. You can coat the terminals and hardware with special protective sprays to resist corrosion buildup. If you use these materials, be sure you do not get any between the terminal and post; this will cause excessive resistance. Test your connections by starting the car.

Test Battery Specific Gravity

When a battery fails to crank the engine, or it cranks too slowly, the technician must find out if the battery is charged. If the battery has cell vent caps or a pry off strip, the way to find out if it is charged is to measure the battery electrolyte specific gravity. As the battery is discharged, sulfuric acid in the electrolyte combines with the active material on the plates. As the sulfuric acid leaves, the remaining electrolyte is nearly pure water. The specific gravity test tells the amount of sulfuric acid and of water in the electrolyte. This is a measure of whether the battery is charged or discharged.

The tool used to measure specific gravity is called a hydrometer. This is a glass tube with a squeeze bulb at the top. Place the hydrometer into the cell through the vent cap hole. Squeeze the bulb and release to draw electrolyte about halfway up the glass tube as shown below

Using the battery hydrometer

If the electrolyte has a high sulfuric acid content, the float in the tube will not sink very far below the electrolyte level. If the electrolyte is mostly water, the float will sink deeper

The float sinks or rises depending on specific gravity of the electrolyte

The reason for this is that water is less dense or has a lower specific gravity than water mixed with sulfuric acid.

Numbered marks on the float allow the technician to note its level. On the specific gravity scale, pure water is 1.000. Numbers bigger than 1.000 mean a higher specific gravity or more sulfuric acid. Most hydrometer scales read from about 1.140 to 1.265 in gradations of 0.005.

The electrolyte level of the battery must be normal before a specific gravity test. If water is added, it must be mixed thoroughly with the electrolyte by charging the battery before a reading is taken. To read a hydrometer, hold it vertically. Then sight directly across the electrolyte level to the marks on the float that lines up with the electrolyte level. Note and record the number on the float. Repeat this procedure for each of the other cells.

The specific gravity of the electrolyte changes not only with acid strength but also with temperature. Hydrometer floats are made to be read at a temperature of 80°F (27°C). As its temperature rises, the electrolyte's specific gravity is reduced. As temperature falls, the electrolyte gets thicker and its specific gravity increases. Specific gravity changes caused by temperatures above or below 80°F (27°C) must be corrected to a true reading.

Most hydrometers have a thermometer built into the side. When measuring the specific gravity, also note the temperature on the thermometer. If it is above or below 80°F (27°C), the reading must be corrected. Add 0.004 specific gravity for every 10° over 80°F (5.6° over 27°C). Subtract 0.004 specific gravity for every 108 under 808F (5.68 over 278C). A correction chart can be used to quickly make the correction.

A temperature correction chart for specific gravity readings

When you have recorded and corrected the specific gravity reading from each cell, you are ready to determine the battery state of charge. Compare your readings to the chart.

Percentage of charge at specific gravity readings

As this scale shows, a specific gravity of 1.265 or above in all the cells shows a fully charged battery. A reading of 1.140 and below shows a completely discharged battery. A variation of 0.050 or more between the highest and lowest cell indicates internal damage in the battery. The battery must be replaced.

Many maintenance-free batteries have a built-in hydrometer.

Maintenance-free battery built-in hydrometer

 It consists of a plexiglas rod with a green ball in a cage at the bottom of the rod. When the charge is 65% or greater, the ball rises, touches the bottom of the rod, and the window in the top of the battery appears green. At a lower charge, the ball sinks and the window is dark. If the electrolyte level is low, it will be below the end of the rod, and the window will be yellow.

Test Battery Open Circuit Voltage

You cannot measure the specific gravity of a maintenance-free battery because you cannot get access to the cell electrolyte. An open cell voltage check can be used to determine the state of charge for maintenance-free batteries.

CAUTION: Always wear eye protection when testing a battery for open circuit voltage.

Before you do this test, make sure the battery temperature is between 60° and 100°F. Do not do the test if the battery has just been on a charger. Wait at least 20 minutes after charging to allow the battery voltage to stabilize.

The first step in the test is to disconnect the ground battery cable from the negative battery post. Turn on the digital volt-ohmmeter and select the correct voltage scale to measure 12 to 13 volts.

Connect the red (positive) test lead from your meter to the positive battery terminal. Connect the black (negative) test lead to the negative battery post. Observe the reading on the meter and compare your reading to the chart shown below.


Open Circuit Voltage

State of Change

12.6 or greater

100 %

12.4 to 12.6

75 - 100 %

12.2 to 12.4

50 - 75 %

12.0 to 12.2

25 - 50 %

11.7 to 12.0

0 - 25 %

11.7 or less

0 %

Open circuit voltage indicates battery state of charge.

If you find a reading of 12.65 volts or higher, the state of charge is 100%. If you find a voltage of 11.70 volts or lower, the battery is completely discharged.

Charge a Battery

CAUTION: Always wear eye protection when charging a battery. Never try to charge a battery with frozen electrolyte. This could overheat and explode. Do not exceed the manufacturer's battery charging limits. Never charge a battery with a built-in hydrometer showing clear or light level. Replace the battery.

When the specific gravity reading or open circuit voltage reading shows that the battery is at less than a full charge, it must be recharged before it will perform satisfactorily. A battery may be charged in or out of a vehicle. If it is charged in the vehicle, both cables must be disconnected prior to connecting a charger. Failure to do this may damage the vehicle's charging system. Battery charging involves applying a current through the battery to restore its chemical potential. A battery must be charged with the correct amount of current, called rate, and for the correct length of time.

A fast charger charges at the high rate of 40-50 amperes

Battery connected to a fast charger

This charger is used when there is not enough time for a slow charge. A fast charge will provide enough charge in about an hour for the battery to handle the vehicle electrical load. The fast charger may also be used for an emergency boost charge to crank start the engine. Batteries that do not have uniform specific gravity readings and batteries that have been discharged for a long time should not be fast charged.

To use a fast charger, connect the positive lead of the charger to the battery positive terminal and the negative lead of the charger to the battery negative terminal. Most fast chargers do not have a rate selector. The charging rate of 40-50 amperes is preset. Set the voltage selector to either 6 or 12 volts, depending on the battery voltage. Some chargers have a timer that can be set according to the battery's specific gravity.

CAUTION: The temperature of the battery electrolyte during charging must never be allowed to get higher than 125°F (52°C). If the temperature is too high, cool the battery by reducing the charging rate or turning off the charger. An overheated battery can explode. Do not fast charge a battery longer than an hour.

High charging rates are not good for completely charging a battery. To completely charge a battery or to charge a completely discharged battery, a slow charger with a rate of 2-10 amperes is required. Slow chargers are made as separate units or as a part of a fast charger.

A discharged battery may often be brought back to satisfactory condition by slow charging. Connect the slow charger to the battery by hooking the negative lead of the charger to the negative battery terminal and the positive lead of the charger to the positive terminal. Set the voltage switch to 6 or 12 volts, depending on battery voltage. Many slow chargers have a rate switch.

The battery should be tested with a hydrometer or an open circuit volt-ohmmeter at regular intervals throughout the charge. The average length of time necessary to slow charge a battery ranges from 12 to 16 hours. The battery is fully charged when there is no further rise in specific gravity or open circuit voltage after three successive readings taken at hourly intervals.

Remove and Replace a Battery

When testing shows a defective battery, you will have to install a new battery. A battery's electrical rating must at least equal to that of the original equipment battery. The automobile manufacturer's service manual specifies the recommended battery rating as a capacity rating, a wattage rating, or both. An undersized battery may result in poor performance and early failure. A battery with a higher rating may be used if the electrical load has been increased through the addition of accessories.

Remove the old battery following the procedures described earlier. Compare the old and replacement battery for physical size, electrical rating, and group number . When removing a battery from the circuit, always remove the grounded terminal connection first. During installation, always connect the grounded terminal last.

After the defective battery is removed, clean the carrier assembly with baking soda and water as described previously. Make sure the bottom of the carrier is clean and in good condition so that the new battery will rest properly in it. Clean and tighten the cable connections and protect them with corrosion-resistant spray as described earlier. Follow recommended tightening torques for the terminal studs and hold-down hardware.

TIP: Anytime you look under the hood of an automobile, you should be constantly performing a visual inspection of critical components. Catching a problem before it ends up causing a breakdown on the road can be very satisfying.


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The objective of this Web Page is to familiarize you with basic auto maintenance
-  in some common emergencies -
not to make you an expert in auto mechanics

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