A customer with a "bad" battery typically expects something to be done about it immediately. The customer may not understand that the battery may be "good" but simply discharged. Customers who want to get back on the road quickly may demand a new battery under warranty.
There are two things wrong with this approach. First, if the battery is otherwise serviceable, it may only need a recharge to return it to health. Second, unless the cause that discharged the battery is resolved, neither a new battery nor recharging the original battery will solve the real problem.
The technician's job is to recharge the battery quickly and safely, and determine and repair the cause of the battery becoming discharged.
Testing a battery using a conductance-type tester can determine if the battery is good and can be recharged. (Fig. 9)
Charging Ahead
Battery charging involves applying sufficient voltage to the battery to cause current to flow through the battery. Charging causes a chemical change in both the battery's plates and the electrolyte. If the battery is simply discharged, but otherwise good, applying a suitable current for enough time will eventually fully charge the battery as much as is chemically possible. However, continued application of current can lead to overheating, loss of electrolyte and shortened battery life.
The charging process requires time and current. Multiply the charging rate in amperes by the number of hours and the result will be the ampere-hours of charge applied to the battery. To bring a battery to full charge will require roughly the same number of ampere-hours at a low charge rate as it does at a high rate. Put another way, a higher rate will take less time.
Since most customers like to have their vehicles returned quickly, it's desirable to get the charging job done quickly by using a high charge rate.
State of Charge
It's not possible to measure a battery's state of charge in ampere-hours, so another method must be used. Open circuit voltage (OCV), measured across the battery's terminals, relates to the battery's state of charge. The voltmeter must be capable of reading to the nearest 0.01 volt.
If the battery has not been charged or used (the engine started or the vehicle driven) in the last 12 hours, a reading may be taken.
If the battery has been discharged and recharged or used within the last 12 hours, connect a carbon pile and load the battery at 300 amperes for 15 seconds. Remove the load. Wait 15 seconds and then take the reading.
The OCV reading is accurate +/- 10%. A battery with a state of charge of 65% or greater is marginally charged enough to be returned to service. However, if the vehicle will be used in slow traffic or short drive times, or in very cold or very hot conditions, the battery should be at least 90% of full charge before returning to service.
|
State
of Charge |
|||
|
OCV (conven-tional flooded-cell battery |
OCV (AGM battery) |
% charge at 32°F (0°C) |
% charge at 75°F (25°C) |
|
12.75 |
12.8 |
100% |
100% |
|
12.70 |
-- |
100% |
90% |
|
12.60 |
-- |
90% |
75% |
|
-- |
12.6 |
100% |
75% |
|
12.45 |
12.4 |
75% |
65% |
|
12.20 |
12.3 |
65% |
45% |
|
12.00 |
12.1 |
40% |
20% |
Automatic Battery Chargers
Advancements in battery charging technology now combine diagnostics and charging control systems that monitor and automatically regulate the charging process. These controls maximize the chargers output based on the battery status, while protecting the battery from overcharge. This means you can use chargers with the highest output ratings, so long as the charger makes the necessary adjustments as the battery approaches full charge.
Automatic battery chargers such as the Midtronics GR1, Associated Intell/Matic Pro or the Cristie PDQ can return a good, but discharged, battery to at least 85% state of charge in 20 to 40 minutes. (Fig. 10)
Manual Battery Chargers
Non-automatic battery chargers do not offer the self-limiting features of the latest equipment. It is the operator's responsibility to control the charging process.
Monitor both charging current and voltage every 30 minutes. Because voltage is not held constant, it will rise as the battery becomes charged.
When voltage reaches 16 volts, reduce the charging current to 5 amperes. When the voltage reaches 16 volts again, at the 5 amp current, the battery is fully charged.
Dead Cold
A completely discharged battery will take more than twice as long as a half-charged battery to bring the battery to a usable state of charge. The electrolyte in a discharged battery is largely water, which is a poor conductor. The battery may accept such a small amount of current at first that it appears not to take a charge.
Also, chemical reactions in a battery are slower at low temperatures, so a very cold battery will take longer to charge than one at room temperature.
AGM Batteries
The Absorbent Glass Mat (AGM) battery uses absorbent glass mats to hold a small amount of electrolyte in contact with the plates. The battery uses a gas recombinant technology, where the gases produced at the plates are recombined to form water before they escape. This battery design offers high power for lower weight and is more resistant to high temperatures, vibration and cycling.
The AGM battery may be charged using an automatic battery charger, as long as the charger automatically adjusts the charging current as the battery nears full charge.
When using a conventional charger, limit the rate to 10 amperes or less. Do not allow the battery to charge at 3 amperes or more for longer than 10 hours, or 10 amperes for longer than 5 hours. When these limits are reached, allow the battery to stand overnight, and then resume charging if further charging is required.
Check the voltage every 30 minutes and remove the battery from charge when the voltage reaches 15 volts at 3-10 amperes. The state of charge should be at 90-95%.
Refer to GM bulletin #02-06-03-009C for additional information.
- Thanks to Mike DeSander
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