A word of caution: Batteries contain a sulfuric acid electrolyte which is a highly corrosive poison and produces gasses that will explode if ignited. When working with batteries, you need to remove jewelry, wear protective clothing and eye wear, and exercise caution. Follow the manufacturer's instruction for testing, jumping, installing and charging. This FAQ assumes a 12 volt negative ground system. For six volt batteries, divide the voltages by two.

What is the bottom line?

A. Check specific gravity in each cell and remove surface charge before load testing,

B. Retest after deep discharges or jump starts,

C. Buy the freshest and largest reserve capacity, non-sealed car battery that will fit with a CCA rating for your climate that meets or exceeds the car's OEM cranking amps requirements, and

D. Perform preventative maintenance, especially during warm weather.

Why bother?

Because only the rich can afford cheap batteries....
A good quality car battery will cost between $50 and $100 and, if properly maintained, will give you four to eight years of service.

How do I test a battery?

A. Visually inspect for obvious problems, e.g., damaged case, corrosion, loose hold-down clamps or cable terminals, or low electrolyte.

B. If you have just recharged your battery or driven your car, eliminate any surface charge by one of the following methods; otherwise, go to the next step:

  1. Allow the battery to sit for two to three hours,
  2. Turn the headlights on high beam for three minutes and wait five minutes before further testing, or
  3. With a battery load tester, apply a 150 amp load for 10-15 seconds.

C. Using the following table, determine the battery's state-of-charge. The best way to measure the state-of-charge is to check the specific gravity in each cell with a hydrometer. A temperature compensating hydrometer can be purchased at an auto parts store for approximately five dollars. If the battery is sealed (maintenance free), the correct procedure to test it is to measure the battery's voltage without the engine running with a good quality digital DC voltmeter.

Some sealed batteries have built-in hydrometers. They are not good testing devices because they only measure the state-of-charge in one of the six cells.

If the state-of-charge is below 75% using either test, then the battery needs to be recharged before proceeding. If there is a .050 or more difference in the specific gravity reading between the highest and lowest cell or the battery will not recharge to 75% or higher, then the battery should be replaced.

Battery VoltageApprox. State-of-chargeAv. Cell Specific Gravity
12.66100%1.265
12.4575%1.225
12.2450%1.190
12.1625%1.155
11.8901.120

Note: If the temperature of the electrolyte is below 70 degrees F, then add .012 volts (12 millivolts) per degree below 70 degrees F.

D. If the battery's state-of-charge is at 75% or higher, then load test the battery by one of the following methods:

  1. Turn the headlights on high beam for six minutes,
  2. Disable the ignition and turn the engine over for 15 seconds with the starter motor,
  3. With a battery load tester, apply a load equal to one half of the Cold Cranking Amp (CCA) rating of the battery, or
  4. With a battery load tester, apply a load equal to one half the original equipment Manufacturer (OEM) cranking amp specification.

During the load test, the voltage on a good battery will not drop below 9.7 volts with the electrolyte at 80 degrees-F. (If the electrolyte is above 80 degrees, add.1 volt for every 10 degrees above 80 until you reach 100 degrees. If below 80 degrees, subtract .1 volt for every 10 degrees until 40 degrees.) After the load is removed, the battery should "bounce back" to the 50% state-of-charge level or above. If the battery drops below minimum test voltage, does not bounce back or will not start the engine, then you should replace it. If it passes this test, you should recharge your battery to restore it to peak performance.

How do I know if my charging system is OK?

When the charging system fails, usually the alternator light will come on. With a good battery and the engine running at 2000 RPM or more for two minutes, depending on the load and ambient temperature, the voltage will increase to between 13.0 and 15.1 volts. Other factors affecting the charging voltage are the battery's age, state-of-charge, and electrolyte level and purity. A loose alternator belt or bad diode will significantly reduce the alternator's current output. If the battery tests good after being externally recharged and you are still having problems keeping it charged, then have the alternator's output voltage and current and car's parasitic (key off) load tested.

What do I look for in buying a new battery?

A. The most important consideration is the battery's CCA rating (Cold Cranking Amps). CCAs are the discharge load measured in amps that a fully charged battery at 0 degrees F can deliver for 30 seconds and while maintaining the voltage above 7.2 volts. Batteries are sometimes advertised by their Cranking Amps (CA) measured at 32 degrees F or Hot Cranking Amps (HCA) measured at 80 degrees F, which are not the same as CCA. Do not be mislead by CAs or HCAs. To convert CAs to CCAs, multiply the CAs by .8. To convert HCAs to CCAs, multiply HCs by .69. In hot climates, buying batteries with double or triple the CCA ratings that exceed the OEM requirement is a waste of money. However, in colder climates the higher CCA rating the better, due to increased power required to crank a sluggish engine and the inefficiency of the cold battery. One of the major manufacturers, Exide, published the following table:

Available Power From BatteryTemperature Degrees FPower Required To Crank Engine
100%80100%
65%32155%
40%0210%
25%-32350%

If more CCA capacity is required, two 12 volt batteries can be connected in parallel.

B. The next most important consideration in buying a battery is the Reserve Capacity rating because of the effects of an increased parasitic or "key off" load produced by electrical devices, e.g., fans, clocks, computer, etc., that operate after the engine is stopped. RC is the number of minutes a fully charged battery at 80 degrees F can be discharged at 25 amps until the voltage falls below 10.5 volts. More RC is better in every case. For example, if your car has a 360 OEM cranking amp requirement, then a 450 to 500 CCA rated battery with 120 minute RC would be more desirable in a warm climate than one with 700 to 800 CCA with 90 minutes of RC. If more RC is required, two six volt batteries can be connected in series or two 12 volt batteries can be connected in parallel.

C. The two most common types of car batteries are non-sealed (low maintenance) and sealed (maintenance free). A sealed (maintenance free) battery will not allow you to test the specific gravity with a hydrometer or add distilled water when required. Sealed batteries are more prone to deep discharge failures, but require less preventative maintenance. Some manufactures have introduced a "dual" car battery that combines a standard battery with switchable emergency backup cells. For about the same cost, a better approach is to buy two batteries and isolate them. Car batteries are specially designed for high initial cranking amps (usually for five seconds) to start a car; whereas, deep cycle or "marine" batteries are designed for prolonged discharges at lower amperage. A "dual marine" battery is a compromise between a car and deep cycle battery. However, a car battery will give you the best performance in a car. For RVs, a car battery is used to start the engine and a deep cycle battery is used to power the accessories. The batteries are connected to a diode isolator and recharged by the RVs alternator.

D. Manufacturers build their batteries to an internationally adopted BCI group number (24F, 35, etc.) specification, which is based on the physical case size, terminal placement and terminal polarity. The OEM battery group number is a good starting place to determine the replacement group.

Within a group, the CCA and RC ratings, warranty and battery type will vary in models of the same brand or from brand to brand. Batteries are generally sold by model, so the group numbers will vary for the same price. This means that for the same price you can potentially buy a physically larger battery with more RC than the battery you are replacing. Be sure that the replacement battery will fit, the cables will connect to the correct terminal, and that the terminals will not touch the hood when closed.

The battery manufacturers publish application guides that will contain the OEM cranking amp and group number replacement recommendations by make, model and year of car, and battery size, CCA and RC specifications. Manufacturers might not build or the store might not carry all the group numbers. To reduce inventory costs, dual terminal "universal" batteries that will replace several group sizes are becoming more popular. The four largest domestic battery manufacturers are Johnson Controls (Interstate, Motorcraft, Energizer, older Diehards), Delco (Sears, newer Diehards), GNB (Champion) and Exide (NAPA).

E. Determining the "freshness" of a battery is sometimes difficult. Never buy a battery that is more than six months old. The date of manufacture is stamped on the case or printed on a sticker. It is usually a combination of alpha and numeric characters with letters for the months starting with "A" for January (skipping "I") and digits for the year, e.g., "F5" for June, 1995. Like bread, fresher is definitely better.

F. As with tire warranties, battery warranties are not necessarily indicative of the quality or cost over the life of the car. Manufacturers will prorate warranties based on the list price, so if a battery fails half way or more through its warranty period, buying a new replacement might cost less. The exception is the free replacement warranty period. This represents the risk that the manufacturer is willing to assume. A longer free replacement warranty period is better.

Technik
A battery FAQ, part 1
written by Bill Darden
edited by Greg Mierz
Zundfolge
BMW CCA Puget Sound Region February, 1996