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9.1. What Are the Four Stages of Battery Charging?

Three stages--bulk, absorption and float are normally used for wet car and motive deep cycle batteries with an optional equalizing stage. Three stages--bulk, absorption and float are normally used for AGM (Ca/Ca) and Gel Cell (Ca/Ca) VRLA car and motive deep cycle batteries. Three stages--bulk, float and equalization are normally used for wet stationary deep cycle batteries and two stages--bulk and float are normally used for VRLA stationary deep cycle batteries with an optional equalization stage is some cases.

9.1.1. The BULK stage is where the charger current is constant and the battery voltage increases, which is normally during the first 80% of the recharge. Give the battery whatever current it will accept as long as it does not exceed 25% of the 20 hour (expressed "C/20") ampere hour (AH) capacity rating, 10% of the Reserve Capacity (RC) rating, wet batteries do not exceed 125° F (51.5° C), and VRLA batteries do not exceed 100° F (37.8° C).

9.1.2. The ABSORPTION stage is where the charger voltage, depending on the battery type, is constant between 14.1 VDC and 14.8 VDC at 80° F (26.7° C) and the current decreases until the battery is fully charged, which is typically the last 20% of the recharge. For wet batteries, gassing (making a bubbling sound) usually starts at 80% to 90% of a full charge and is normal. A full charge typically occurs when the charging current drops off to 2% (C/50) or less of the AH capacity of the battery and each cell of a wet battery is moderately gassing equally. For example, end current for a 50 AH (C/20) battery is approximately 1.0 amp (1000 milliamps) or less. If the battery will not "hold" a charge, the current does not drop after the estimated recharge time, and a wet battery is hot (above 125° F (51.5° C)), then the battery may have some permanent sulfation. (Please refer to Section 16 for more information about sulfation and how to remove it.) Manual two-stage chargers that have a bulk and absorption stage must be turned off when the battery is fully charged to prevent overcharging.

9.1.3. The optional FLOAT stage is where the charge voltage, depending on the battery type, is reduced to between 13.0 VDC and 13.8 VDC at 80° F (26.7° C), held constant. It can be used indefinitely to maintain a fully charged battery to overcome the natural self-discharge of the battery. The current is reduced to approximately 1% (C/100) or less. Three-stage "smart" chargers usually have the bulk, absorption and float stages. (Please refer to Section 13 for more information about storing batteries and continuous float charging.)

9.1.4. The optional EQUALIZATION stage is a controlled 5% to 10% absorption overcharge to equalize and balance the voltage and specific gravity in each cell. Equalizing can reverse the build-up of the chemical effects like electrolyte stratification where acid concentration is greater at the bottom of the battery. It also helps remove sulfate crystals that might have built up on the surface or in the pores of the plates. The recommended frequency varies by motive deep cycle battery manufacturers from once a day to once a year. For stationary deep cycle batteries, some short daily (30 minutes or less) equalizations have proven to be beneficial and not require the longer equalization cycles. They are not as hard on a wet battery because they do not produce as much gas or heat the battery. You should equalize wet batteries when one or more of the following occur:

    Where the temperature compensated Specific Gravity reading difference between cells is .030 (or 30 "points") or greater

Where the temperature compensated Specific Gravity reading difference of a cell is .010 (or 10 "points") or more below the reading for a fully charged cell when the battery is fully charged

When one cell requires more water than all the other cells

When one cell does not require as much water as all of the other cells

When the SoC measured by a hydrometer does not materially agree with the SoC measured by an accurate (0.5% or better) digital voltmeter

Some AGM (Ca/Ca) VRLA batteries, like Concorde, can be equalized under certain conditions, but carefully follow the battery manufacturer's recommended procedures or you will damage the battery.

How Do I Equalize My Battery?

To equalize, please check that the electrolyte is covering the plates in each cell and fully recharge the battery. Then increase the charging voltage to the battery manufacturer's recommendation, or if not available, add 5% to 10% to the absorption charging voltage. Heavy gassing should start occurring in each cell. Do not allow the wet battery to get above 125° F (51.5° C) or a VRLA battery above 100° F (37.8° C). Take Specific Gravity readings in each cell once per hour. Stop equalizing when the Specific Gravity values no longer rise during the gassing phase and when every cell is gassing evenly. Insure that the plates are covered with electrolyte at all times, and add distilled, deionized or demineralized water if required, but do not overfill. Only equalize if the battery manufacturer recommends it. Four-stage "smart" chargers typically have the bulk, absorption, float and equalization stages.

Examples of Charging Algorithms

An excellent and easy to understand tutorial on battery charging basics can be found at The following graphs are examples of charging algorithms used by Deltran [Battery Tender ] for power sport, car and deep cycle batteries:

Wet Standard (Sb/Sb)

Wet Low Maintenance (Sb/Ca)

Gel Cell (Ca/Ca) VRLA

It is extremely important to use the battery manufacturer's recommended temperature compensated charging voltages and procedures whenever possible for optimum battery capacity, maintenance and service life. A good rule is not to use a charger (or charging setting) for batteries that is greater than 25% of the AH (C/20) capacity or 10% of the RC rating of the battery or battery bank being charged. For example, if the battery has RC of 100 minutes, do not use charger that will exceed 10 amps. The exception is when you are charging a large AGM (Ca/Ca) deep cycle battery bank. Due of the higher acceptance rate of the AGM (Ca/Ca) batteries a larger alternator to 33% of the capacity of the battery bank being charged can be used. Using a smaller alternator, unless temperature protected, can be damaged. The exception is not to use a charger (or charging setting) for SLA batteries that is greater than 10% of the AH (C/20) capacity.

9.2. Additional Words of Caution and Charging Tips:

9.2.1. Please prevent blindness and always wear glasses or safety goggles when working around a car or deep cycle battery in the event that it might explode.

9.2.2. Use the battery manufacturer's charging recommendations and temperature compensated voltages whenever possible for optimum capacity, maintenance and service life. MATCH the charger (or charger's setting) for the battery type you are recharging (or maintaining) and insure the charging voltages are compatible. Except for Gel Cell (Ca/Ca) VRLA batteries, a small overcharge is slightly better than an undercharge. Overcharging Gel Cell (Ca/Ca) VRLA batteries can cause voids between the plates and loss of capacity can result.

9.2.3. Lead-acid batteries should always be recharged within 24 hours after they have been used and the sooner the better. Before recharging, please check the electrolyte and insure that it is not frozen and that it covers the plates at all times to prevent sulfation and reduce the possibility of an internal battery explosion. Do not attempt to recharge or jump start frozen batteries because you will damage them. Allow them to thaw out first.

9.2.4. After recharging, recheck the electrolyte levels after the battery has cooled, top off with distilled, deionized or demineralized water as required, but do not overfill. (Please refer to Section 3.1. for more information about filling batteries.)

9.2.5. Reinstall the vent caps on wet (flooded) batteries before recharging and recharge ONLY in well-ventilated areas because explosive and toxic stibine or arsine gasses can be produced during the absorption or equalization stages. Insure the vent caps are not clogged. Do NOT expose lead-acid batteries to a lit cigarette, sparks or flames because they produce flammable gasses and could explode.

9.2.6. Please follow the charger manufacturers' procedures for connecting and disconnecting cables. Connect the positive (+) lead of the charger to the positive (+) terminal post of the battery to be charged and the negative (-) lead of the charger to the negative (-) terminal post. Operate in a manner to minimize the possibility of an explosion or incorrectly charging the battery. You should always turn the charger OFF or unplug it before connecting or disconnecting cables to a battery. Do not wiggle the cable clamps while the battery is recharging, because a spark might cause an explosion. Good ventilation or a fan is recommended to disperse the gas created by the recharging process for wet batteries. As a safety feature, some chargers are designed to not operate unless the battery has a partial charge or when the leads are reversed.

9.2.7. If a wet battery becomes hot. over 125° F (51.5° C), or if it

violently gasses or spews electrolyte, turn the charger off temporarily or reduce the charging rate. This will also prevent "thermal runaway" that can occur with AGM (Ca/Ca) and Gel Cell (Ca/Ca) VRLA batteries if the battery temperature is over 100°F (37.8° C). If an air cooled alternator becomes too hot during the bulk charging phase, stop and let it cool down or use an alternator temperature sensing voltage regulator, for example a Balmar. or a water cooled alternator, Bosch .

9.2.8. Insure that charging the battery with an external charger will not damage the electrical system or appliances with high voltages. If this is even a remote possibility, then disconnect the grounded battery cable from the battery before connecting the charger to the battery.

9.2.9. If you are recharging Gel Cell (Ca/Ca) VRLA batteries, the battery manufacturer's charging voltages are very critical. You might need special charging equipment. In most cases, standard deep cycle chargers used to recharge wet batteries cannot be used to properly recharge Gel Cell (Ca/Ca) or AGM (Ca/Ca) VRLA batteries because of their higher voltages or charging profiles. Overcharging Gel Cell (Ca/Ca) and AGM (Ca/Ca) batteries will significantly shorten battery service life or cause "thermal runaway" if the battery temperature is over 100°F (37.8° C).

9.2.10. If a battery is charged with a manual or defective charger and all the electrolyte is "boiled" out, some batteries can cause a FIRE or produce DEADLY CO (Carbon Monoxide) or other gasses.

9.2.11. Routinely tighten cable connections.

9.2.12. Never disconnect a car battery cable from a vehicle with the engine running because the battery acts like a filter for the electrical system. Unfiltered (pulsating DC) electricity sometimes exceeding 40 volts is produced by the alternator and can damage expensive electronic and electrical components such as the emissions computer, audio system, charging system, alarm system, etc.

9.2.13. Alternators are not designed to recharge dead (flat) or frozen batteries because the stator can be burned up or diodes could fail.

9.2.14. Wet battery gassing usually starts at 80% of a full charge during the absorption stage. A full charge normally occurs when the charging current drops off below 2% (C/50) of the AH capacity and the battery is moderately gassing (bubbling). For example, the end current for a good 50 AH (C/20) battery is approximately 1.0 amp (1000 milliamps) or less depending on the battery type.

9.2.15. Do not recharge batteries with cracked or leaking battery cases.

9.2.16. Recharge battery banks the same way you discharged them. For example, if you discharged two or more fully charged and identical batteries connected together such that all the batteries discharged the same, i.e. the same State-of-Charge (SoC) readings on all of the batteries, you should recharge them connected the same. If you discharged two or more fully charged and identical batteries not connected together such that the batteries discharged differently, i.e. different State-of-Charge readings on each of the batteries or banks, you should recharge them separately. When the batteries are connected together in a bank(s), it is a question for keeping the discharges and charges balanced ; otherwise, you will undercharge or overcharge one or more of the batteries or banks. Over time, undercharging will reduce capacity due to the accumulation of sulfation. The total time to recharge the batteries or banks together or individually is about the same because you have to replace the amp hours consumed.

9.2.17. Do not recharge batteries directly from a gas or diesel powered generator that does not have regulated DC voltage and most do not. A better approach to recharging batteries is to power a "smart" battery charger with the generator so the batteries are not overcharged or undercharged.

9.2.18. Continuous float charging or periodic recharging will prevent batteries from freezing. As shown in Section 4.4.1, an Electrolyte Freeze Points at Various States-of-Charge for a Wet Lead-Acid Battery Table indicates the SoC and temperature when the electrolyte will freeze.

9.3. Battery Charger Types

Basically, there are three battery charger configurations--single bank, multi-bank and multi-station. A single bank charger is one that is designed to provide a single voltage to recharge a single battery or bank of batteries. It is by far the most widely used configuration. A multi-bank charger provides single voltages to multiple banks of batteries by using an internal isolator. This type of charger can also act as a single bank charger and commonly used to recharge unbalanced two, three or four 12-volt batteries in series to power a motor. A multi-station charger is used to recharge more than one battery at the same time. It is functionally two or more single bank chargers in the same case.

Unless the charging system or charger has adjustable voltage settings, there is no one system that can recharge all battery types. For example, if the absorption charge voltage is set for a Low Maintenance (Sb/Ca) or AGM (Ca/Ca) VRLA battery at 14.4 VDC, the system would undercharge most wet Standard (Sb/Sb) or wet "Maintenance Free" (Ca/Ca) and overcharge some Gel Cell (Ca/Ca) VRLA starting batteries. This would reduce the battery's service life. Unless a charger is temperature compensating, it is assumed by the manufacturer to operate at 77° F (25° C). Some chargers are equipped with an electronic switch that senses battery voltage at some predetermined level before the charger will operate. For deeply discharged batteries, this gives the appearance that they can not be recharged. Please see the charger manufacturer's operator manual for instructions on how to override this "soft start" feature. A good quality charger used on a cheap battery is better than a bad quality charger used on a good battery.

9.3.1. Vehicle Charging System

A vehicle charging system is made up of three components, an alternator (or DC generator), voltage regulator and a battery. Occasionally when a vehicle needs to be jump started, it has NOT driven long enough to fully recharge the battery. The length of time to fully recharge the battery depends on the amount of discharge, the amount of surplus current that is diverted to the battery, how long the engine is run, engine speed (RPM), and ambient temperature. An alternator is sized by the vehicle manufacturer to carry the maximum accessory load and to maintain a battery and NOT to recharge a dead battery. For example, if 300 amps were consumed for two seconds to start a car from a fully charged battery, it will take an 80 amp charging system approximately 10 seconds at 3000 RPM to replace the .167 amp hours of power used. If 25 amps are available to recharge the battery, it will take 30 seconds at 1100 RPM and at least 10 minutes at 750 RPM at one amp. With a dead 120 minute RC (60 amp hour) battery, it would take approximately 90 minutes at 80 amps at 3000 RPM and at least five hours at 1100 RPM at 25 amps to fully charge (100% State-of-Charge) it.

More information can be found in Section 5 or Dan Landiss' Car Batteries Are Not 12 Volts on about vehicle charging systems. Some battery experts believe that some vehicle charging systems undercharge starting batteries and that the batteries should be periodically recharged with an AC "shore" powered battery charger to optimize their service life by removing accumulated lead-sulfate or electrolyte stratification.

If you have added after-market lights, winches, audio amplifiers, two-way radios or other high powered accessories to your vehicle and engage in stop-and-go driving, the vehicle's charging system might not produce enough current or voltage to keep your battery fully charged. You might need to increase the capacity of the charging system. If you are also recharging deep cycle battery banks, please see the temperature caution in Section 9.2.7 above. Ideally the combined load of all the accessories should be less than 75% of the charging system's maximum output, so that at least 25% is available to recharge the battery.


9.3.2. Manual Constant Current Charger

A manual constant current charger increases the energy stored in a battery at a constant current rate and the battery voltage will increase as the State-of-Charge rises. If you use an external constant current charger, set it to deliver NO more than the lesser of 1% of the CCA, 12% of the RC rating, or 25% of the C/20 rated AH Capacity of the wet battery and also carefully monitor the current flowing into the battery. C-rate is a measurement of the charge (or discharge) of battery over time. It is expressed as the Capacity of the battery divided by the number of hours to recharge or discharge the battery. For example, a 48 amp hour battery would have a charging (or discharging) rate of 4.8 amps for ten hours. With manual chargers, you need to determine how many amp hours have to be replaced and determine the amount of charging time based on the constant current output of your charger. Manual constant current chargers will overcharge a battery if not turned off when the battery is fully charged. Some constant current chargers have a timer that can turn off the charger and help prevent it from overcharging the battery. These types of chargers are not recommend to recharge VRLA or SLA batteries because the charging voltages are critical, especially for Gel Cell (Ca/Ca) VRLA batteries.

Category: Forex

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