Automotive batteries come in many shapes and sizes, but their operating principles are remarkably similar. In this Tech Center installment, we'll look at the basic automotive battery. Then we'll close by discussing ways to maintain your battery to ensure a long life.
A Shocking Expose: Your Car's Battery
The modern automotive battery is a lead-acid storage design. In short, it's an electrochemical device that converts chemical energy into electrical energy. When the battery is placed under a load, such as when the ignition is switched on, the device converts stored chemicals into electricity, and the current flows through the wires to its destination.
A standard 12-volt, lead-acid battery is made up of six cells connected in series. Each cell produces approximately two volts.
The cells are filled with an electrolyte. An electrolyte is an ionized bath — in this case, sulfuric acid (H2SO4) diluted with water — that generates an electrical current when called upon.
Each cell also contains plates (grids of active material), which are both positive and negative. Typically, the positive plates contain lead dioxide (PbO2), while the negative plates are composed of straight lead (Pb).
The plates are formed into a plate group, which holds a number of plates of the same polarity (for instance, all positive or all negative). The like-charged plates are welded to a post strap. The plate groups are then alternated within the battery — positive, negative, positive, negative. There is usually one extra set of negative plates to balance the charge. To ensure that the different plate groups don't touch each other, non-conductive sheets called separators are inserted between them.
The battery case, which holds all these components, is made from molded polypropylene.
A maintenance-free battery, similar in design to a conventional automotive battery, is really just a heavier-duty version of the same arrangement. Many of the components have thicker construction, and different, more durable materials are typically used. For example, the plate grids often contain calcium, cadmium or strontium, to reduce gassing (which causes water loss) and self-discharge. This design is called a lead-calcium battery. The heavier-duty parts ensure that fluid loss is kept to a minimum and that components have a much longer life, making it a closed system.
The charge of a conventional 12-volt automotive battery is actually 12.6 volts. This can vary slightly, depending on the concentration of the electrolyte in solution. Ideally, for optimum battery performance, the sulfuric acid/water bath should have a specific gravity of 1.265 at 80 degrees Fahrenheit. Specific gravity refers to the weight of a solution, with water having a reference rating of 1.000. The electrolyte in an automotive battery is therefore just slightly over one-and-a-quarter times the weight of regular water.
Battery power rating is measured by two standards. The more popular of these, cold cranking power, determines the amount of current (amps) a battery delivers for 30 seconds at zero degrees Fahrenheit while maintaining a minimum terminal voltage of 7.2 volts. The higher the number, the stronger the battery. Think of those old Diehard commercials, with all those non-Sears batteries giving up the ghost in the cold and snow, and this will perhaps give you a visual image.
The second standard is called reserve capacity rating. This is a warm weather rating (80 degrees Fahrenheit), which estimates the amount of time it takes the terminal voltage of a fully charged battery to dip below 10.2 (or 1.7 volts per cell) at a continuous discharge rate of 25 amps. The rating is expressed in minutes. For example, a rating of 120 means the battery will run for two hours (120 minutes) before ceasing to function.
A battery has two main cycles, the charge and discharge cycles.In the discharge cycle, a chemical reaction takes place inside the battery in which the lead (Pb) of the negative plates combines with the SO4 of the sulfuric acid to produce lead sulfate (PbSO4). In this cycle, the electrolyte becomes weaker — specific gravity lessens — and the positive and negative plates become more like one another. Since the voltage, or charge, of a battery depends on the difference between the two plate materials and the concentration of the electrolyte, and since this difference decreases during discharging, the battery loses power. To anyone who has sat in a non-starting car and cranked away as the battery grew weaker and weaker, this scenario will be immediately recognizable.
In the charge cycle, the reverse is true. Electrical current, generated by the car's alternator, passes through the plates, forcing SO4 back into the electrolyte bath and elevating specific gravity. Voltage increases.
Sulfuric acid is highly caustic. Caution should used when dealing with electrolyte or automotive batteries. Sulfuric acid can burn clothing and the skin and even cause blindness. There is even a slight danger of explosion. Most battery professionals wear gloves and goggles or a face shield when working. If for some reason electrolyte gets on your clothes or body, it should immediately be neutralized with a solution of baking soda and water. When working on a battery or handling sulfuric acid, here are some safety tips:
- Always wear goggles or a face shield.
- Use a lead-lined or nonmetallic container to hold electrolyte.
- Always pour acid slowly into water, not water into acid.
- Stir as you add small amounts of acid.
- Never lean over a battery when charging, testing, or jump-starting an engine.
- Always disconnect the negative cable first and reconnect it last.
- Charge batteries only in well-ventilated areas.
- Never charge or jump-start a frozen battery; let it warm to at least 40 degrees Fahrenheit.
Speaking of charging, there are several different methods used to recharge a depleted battery. The most gentle of these is called trickle-charging. Other methods include quick-charging and slow-charging. See your battery's owner's manual to decide which method may work best for your situation, or talk to your mechanic.
One advantage of a manual transmission is the ability to start a vehicle with a depleted battery. If the charging system (alternator and voltage regulator) are in proper working condition, simply push-start the car and kick it over, then drive long enough to let the charging system do its work. A good half-hour drive should give it a solid charge.
Here are a few tips to quickly restore a battery using this method.
- Drive at a constant speed (highway driving) versus stop-and go (city driving). This will give the alternator an opportunity to charge more evenly.
- Turn off all accessories (radio, air conditioner, etc.).
- If possible, drive during the day. Even headlights use power. Having them off increases the amount of electricity going to the battery.
Remember, this does not replace charging the battery. A car's alternator is not designed to fully restore a depleted battery, but rather to maintain a healthy one. As soon as possible, put your battery on a battery charger such as the Battery Tender, and a give it a full charge for a day or two.
http://www.batterytender.com/ If you have an automatic transmission, you can jump-start the car instead. If the charging system is in proper working condition, it will recharge quickly. Follow the same steps as above to ensure that the car won't need another jump, then, as soon as possible, have the battery fully charged by a mechanic or by yourself if you own a high-quality battery charger .
We've included an Edmunds.com link below as a refresher course on jump-starting.
How To: Jump-Start Another tip: If parking a car for long periods of time (weeks or months), it's best to disconnect the battery to prevent discharging. Use a crescent or open-ended wrench to loosen the strap from the negative terminal on the battery, then remove the connector. Make sure the connector is tucked away from the terminal, where it cannot come into contact with the post.
We promised a few maintenance tips on standard-duty batteries.
- Check the water level every couple of months. It should be just touching the bottom of the refill hole.
- Refill the battery, when needed, with distilled water. Don't use tap water, which produces corrosion on the terminals.
- Don't overfill the cells. Just to the bottom of the refill hole is perfect.
The following tips apply to all batteries, including maintenance-free.
- To ensure good connectivity, clean the terminals periodically with a wire brush.
- When removing a connector from a terminal, twist it from side to side and pull gently upward. Refrain from excessive tugging or prying.
- When reconnecting a connector to a terminal, seat it down firmly on the post. A few gentle whacks from a rubber mallet will do it. Don't overtighten and strip the nut.
- After securing the connector, coat the whole post with high-temperature grease. This will reduce corrosion and rust.
- If you keep having electrical problems (battery dies, car won't start, power is intermittent or weak), it's not necessarily the battery. It could be in the charging system, normally either a bad alternator or voltage regulator. A mechanic can test the system to isolate the problem.
These general maintenance tips will extend the life of your battery.
One final note. News has come to light recently that automotive manufacturers are experimenting with 36-volt batteries in prototype cars. Because of the increasing number of electrical gadgets finding their way into modern vehicles, it's likely we'll be seeing a more advanced electrical system in coming years.