Battery Charging General Information:
All information contained pertaining to battery chargers and the charging process is for general, informational use only. Please refer to your battery charger for specific product information, instructions and features.
- Never test or charge batteries without wearing wraparound safety glasses and protecting your skin from battery acid. Battery acid is extremely caustic. It can burn you and blind you.
- Always keep a first aid kit and eye wash solution handy at all times. If eye wash solution is not available, flush any battery acid that enters the eye(s) with clean water. Do so immediately! Seconds are precious.
- Never smoke near a battery. Batteries give off explosive gasses, especially when charging.
- Turn off a battery charger before disconnecting the charger clamps from the battery posts. If the charger is still on when you remove the clamps, you may create a spark that ignites the battery gases.
- Never pound on a battery..
- Do not add electrolyte to a battery after it is in service. If the acid level in a removable cap battery is low, add only distilled water. Do not overfill the cells.
- Properly dispose of batteries and battery acid. Batteries are among the most recycled of all products. Battery contents and acid pose significant health hazards and are environmentally harmful. They must be disposed of according to federal, state, and local regulations. Failure to do so is illegal and dangerous.
- Check your eye wash station and first aid kit before you need them..
- Make sure your first aid supplies and eyewash are fresh and ready to go in an instant.
- If eye wash solution is not available, flush any battery acid that enters the eye(s) with clean water. Do so immediately! Seconds are precious.
Visual Inspections of Your Battery Prior to Charging:
Perform a thorough preliminary inspection of your battery.
- Loose/corroded battery terminals.
- Replace temporary terminal ends with permanent cable ends.
- Replace batteries that have loose posts.
- Replace broken or damaged cables.
- Replace batteries with severely bloated, cracked, or leaking cases.
- Plan on replacing frozen batteries. Few frozen batteries survive.
- Replace missing battery caps.
- Add distilled water to cells that are low on electrolyte in removable cap batteries. Rule of thumb: cover the cells with 1/2 inch of water. Do not overfill the cells, especially in a cold battery: the electrolyte level will rise when battery temperature increases. Recharge the battery if necessary.
- Replace a sealed top battery that has one or more cells with a low electrolyte level. Some sealed tops have translucent battery cases that allow you to see the electrolyte level. Never attempt to open a sealed top or alter the case by drilling or cutting.
- Repair, replace, or tighten loose or damaged battery hold downs. This is a very important and often overlooked aspect of battery service. A loose battery is more apt to sustain internal damage from vibration and impact than one that is properly secured to the battery tray.
Battery Rating Systems
There are several battery rating systems used: we will concentrate of the three you will see most often. These rating systems are designed to tell us how much power the battery was designed to deliver when new and fully charged.
Here are the common ratings:
Cold Cranking Amps rates the battery's ability to deliver amperage for 30 seconds at 0 degrees F while maintaining a post voltage of 7.2 volts (1.2V/cell). For example, an 800 CCA battery can deliver 800 amps for 30 seconds at 0 degrees F and still have 7.2 volts, measured across its posts. This is a very important standard for vehicles in cold climates. CCA tells us how well we can expect a battery to crank the engine in cold weather.
Cranking Amps is a less strenuous measurement of the battery's cranking power. The CA rating is the current (in amps) that a battery can deliver for 30 seconds at 32 degrees F while maintaining a minimum voltage of 7.2 volts (1.2V/cell). This is not to be confused with CCA, especially when choosing a battery for use in cold climates.
Reserve Capacity (RC) measures the battery's ability to provide sustained current. This rating is important for batteries installed in vehicles with high key-off battery drains. Reserve Capacity is the time (in minutes) required for a steady 25 amp draw to pull the battery voltage below 10.5 volts at 80 degrees F. RC is an important consideration for vehicles that are frequently driven for short distances in stop-and-go traffic or those with high key-off loads.
Battery State Of Charge: Tools And Testing:
Battery State of Charge - Hydrometers
Is the battery bad? Or is it simply discharged? To be sure, we need to measure the Specific Gravity of the electrolyte in each of its six cells. This test tells us the battery's State of Charge.
As a battery discharges, the electrolyte contains more and more water and less acid. Since water is lighter than acid, the weight of electrolyte decreases as the battery discharges.
As the battery charges, the acid content of the electrolyte increases. Electrolyte in a charged battery weighs more by volume than electrolyte in a discharged battery.
We can use a hydrometer to test Specific Gravity and State of Charge in batteries with removable caps. Here's how:
- Remove the battery caps. Check the electrolyte level. Make sure all cells are covered but not overfilled.
- Insert the hydrometer into each cell and draw electrolyte into the glass cylinder with the squeeze ball. Draw just enough acid into the cylinder to make the float rise. Hold the hydrometer vertical as each sample is drawn. Note the exact level at which the fluid level intersects the measurement scale on the float and record it.
- Repeat the test at each of the remaining cells. Record the reading for each cell. Compare readings to the chart on this page to determine state of charge.
- If the battery is below 75 percent state of charge, recharge it before load testing.
If the temperature of the electrolyte is below 80 degrees F, subtract .004 (4 points) from the actual reading for each 10 degree change.
Add 4 points for each 10 degree change above 80 degrees F.
For example, if the actual specific gravity reading is 1.265, but the electrolyte temperature is only 30 degrees F, then the true, corrected specific gravity is 1.245 (1.265-.020 = 1.245).
Why? Because the higher 1.265 specific gravity reading at 30 degrees F is the result of the increased density of colder electrolyte, not because the sample contains a higher concentration of acid.
Testing State of Charge in Sealed Batteries
The Open Circuit Voltage Test
If the battery has a sealed top, you cannot perform a specific gravity test to determine the battery state of charge since there is no way to take samples of the electrolyte.
To determine battery state of charge in sealed-top batteries, you must perform an Open Circuit Voltage test across the battery posts using an accurate digital voltmeter; analog voltmeters are not accurate enough for this test. Open Circuit Voltage (OCV) refers to voltage measured across the battery posts with no electrical loads turned on.
Adjust your digital multimeter to the 20 or 40 Volt SC scale and place the meter leads across the battery posts.
Take the voltage reading and compare it to the chart below. The chart combines specific gravity and OCV test standards.
There is a direct relationship between specific gravity and open-circuit voltage measured across the battery posts. Batteries below 75% of full charge must be recharged before performing a load test.
Determining The State Of Charge
State of Charge tells us if the battery is sufficiently charged to undergo load testing. It does not tell us if the battery can deliver both voltage and current at the same time. That's what the load test is for. Once it is determined that the battery state of charge is 75% or greater, the load test measures the battery's ability to provide POWER. Power is measured as wattage, or volts multiplied by amps.
As batteries age and deteriorate, call material degrades or falls off and plates become less powerful. A battery that passes the specific gravity or open-circuit voltage tests may still have a hard time maintaining its voltage when electrical loads consume large amounts of current.
This is very important, so we will repeat it: Batteries must provide power. They must provide enough electrical current to operate all vehicle loads and still maintain the correct voltage.
What Is a Load Test?
A Load Test is exactly what the name suggests! A measured electrical load is applied to the battery for a specific time using a device called a Load Tester.
The Load Tester is commonly referred to as a Volt-Amp Tester, or VAT for short. Its main components include:
- A voltmeter connected to the battery terminals by large cables and clamps.
- An ampmeter, connected around either the VAT main cables with an inductive current clamp, also referred to as an amp probe.
- An adjustable carbon pile inside the load tester that creates an electrical load. The amount of load applied to the battery is adjusted with a knob on the front of the load tester.
What is a Load?
A Load is any electrical consumer. During normal vehicle operation, many types of loads consume electrical power. Examples of loads include: the starter, headlights, ignition system, fuel pump, comfort and convenience items, and heater blower. Even the vehicle computer is a load.
Performing the Load Test - Part One
- Before you perform the load test, make sure the battery is at 75% state of charge or better.
- Connect the load tester to the battery, and clamp the amp probe around either of the battery cables.
Performing the Load Test - Part Two
- Rotate the knob on the load tester to apply a load equal to: 1/2 the battery's Cold Cranking Amps (CCA) rating.
- Apply the load for exactly 15 seconds. At exactly 15 seconds, note the battery voltage and turn off the load.
- A battery at room temperature should have 9.6 volts across its posts at the end of the 15 second load test. If the load tester has an analog voltmeter, connect your DMM to the battery during the test to get an accurate battery voltage reading. The analog meter is not accurate enough.
Rotate the load tester knob to apply the correct load as quickly as possible. This ensures accurate test results. Measure the battery voltage at exactly 15 seconds, once again to ensure an accurate battery evaluation. Do not apply 1/2 of the CA rating. Use only the CCA rating.
Evaluating the Load Test
Battery voltage should be at least 9.6 volts at the end of the 15 second load test with the battery temperature at 70 degrees F. Slightly lower voltage values are normal when temperatures are below 70 degrees F.
Refer to the charts below and to the right to make the corrections for battery voltage minimums when the battery temperature is below 70 degrees F.
Batteries that barely fail when tested at 75% may pass when fully charged. (For example, the battery voltage drops to 9.4 volts.) Recharge to 100% and retest.
Batteries that pass with room to spare when tested at 75% have a better than average reserve capacity. (For example, battery voltage is 9.8 volts at the end of the load test.)
If the battery fails the load test, continue to watch the open circuit voltage for several minutes. If the battery open circuit voltage bounces back and rises above 12.45 volts, don't be fooled into thinking it is good. It is probably on its last legs.
Temperature and Relative Post Voltage
Temperature affects performance during the load test. Compensate for changes in battery temperature as follows:
Bounce Back Voltage
Here's why the battery is bad if it bounces back above 12.45V after failing its load test: While the battery acid is strong enough to generate the high no-load voltage, the battery doesn't have enough good plate material remaining to provide both voltage and current during the load test.
The battery is a pretender and will probably fail to provide enough power under high electrical load conditions.
Do not use the vehicle starter to simulate a load test. One of the main benefits of the load test is that it uses a controlled, carefully measured load to evaluate battery capacity. Starter current is too unsteady to provide accurate test results. Use a dedicated volt-amp tester with a true, adjustable carbon pile instead.
A battery is fully charged when the specific gravity reaches 1.270, or when the charging current drops to a few amps and holds steady for approximately one hour.
Batteries can withstand a limited number of discharge/charge cycles. Therefore, it is recommended that all stored batteries be maintained at a full state of charge. Special chargers are available that monitor battery voltage and apply enough charge to maintain the battery during extended periods of nonuse. Chargers that maintain full charge constantly are preferable to those that recharge the battery when the battery falls below 80% of full charge.
Batteries accept a charge better when warm. Cold batteries have higher chemical resistance to charging that decreases gradually as the battery warms during charging. On the other hand, batteries self discharge more slowly when stored at cooler temperatures.
Battery charge acceptance decreases in cold temperatures. Battery resistance is greater when the battery is cold. It takes more charging voltage to push the same charging current through the battery at low temperatures.
Battery Charging Checklist & Quick Tips:
Battery Charging Checklist
- Put away your flammable objects. Anything that might cause flame or sparks near the battery are very dangerous. Battery gasses are highly explosive.
- Check the battery case for signs of damage. If the case is cracked or severely warped or bulged, it is not safe to charge and/or test. If the battery is frozen, get a new one. Frozen batteries seldom recover and can explode when charged. NEVER CHARGE A FROZEN BATTERY!
- Check and correct the battery electrolyte level. If the electrolyte in a cold battery just barely covers the cells, add nothing. The electrolyte level will rise as the battery warms during charging.
- If the electrolyte level is low enough to expose plate material, add just enough water to cover the cells. If you can see that the electrolyte level is below the tops of the cell material through the translucent case of a sealed battery, replace the battery. There is no approved method for adding water to a sealed battery.
- Check the battery vents to be sure they aren't clogged with debris. All batteries gas as they charge, and the gas needs to escape gradually through the vents. Leave the battery caps in place to prevent acid from bubbling over to the battery exterior. If the top of the battery has become wet enough from this spray, the electrolyte may create a conductive path between the posts that increases self-discharge.
- Determine the battery rating and state of charge before selecting a charge rate. Refer to the battery charger documentation.
- Connect the charger to the battery before turning it on.
- Turn the battery charger off before disconnecting it from the battery.
- A slow charge is better for the battery than a fast charge.
- Don't let the battery get too hot. Cut back the charging rate if the battery case gets hotter than 125 degrees F.
- If there is excessive gassing or bubbling, stop charging immediately.
- Use lead adapters, not steel bolts, for side terminal battery charger connections. Lead adapters are much more conductive.
Battery Charging Overview
There are two things needed to charge a battery: A voltage source strong enough to move current through the battery - and time. The time required to charge a battery depends in part on the battery state of charge when the charging begins.
The more current we can push into the battery, the faster we can charge it. Unfortunately, faster is not always better when it comes to battery charging. Charging at too high a rate can overheat and damage the battery.
When in doubt, charge at a slow rate. This reduces stress on the battery and chances of plate damage caused by overheating.
The time needed to recharge a battery also depends on the battery state of charge when charging begins. At a given charging rate, a totally discharged battery takes more than twice as long to recharge than one that is half-discharged. Why? Electrolyte in the totally discharged battery contains much more water than acid, and water is a poorer electrical conductor than acid. As water changes back to acid during the initial charge, the conductivity of the electrolyte increases, but this takes additional charge time.
Low temperatures slow chemical reactions. Cold batteries take longer to charge than warm ones.
Check battery temperature periodically during charging. If the battery case is hot to the touch (125 degrees F or more) stop charging immediately until the battery cools. Resume charging at a lower rate to prevent battery overheating. If the electrolyte bubbles, cut back the charge rate immediately. Severe bubbling indicates overcharging.
A vehicle with a battery that is too discharged to operate the starter may be jump started in an emergency. Jump starting connects a charged battery in parallel with the discharged battery to provide enough power to start the engine.
Whenever possible, a safer alternative to jump starting is to remove the battery from the vehicle for charging and testing. This reduces the chances of personal injury or vehicle damage associated with the jump starting procedures.
- Jump starting a vehicle is a potentially dangerous procedure. Battery gases are explosive. Gloves, protective clothing, and goggles should always be worn when jump starting a vehicle.
- Never smoke or create a spark that could ignite explosive battery gases.
- Never make the final cable connection at the battery, it may create a spark.
- Make sure the dead battery is not frozen. Do not attempt to jump start a frozen battery. It could explode.