9. HOW DO I CHARGE (OR EQUALIZE) MY BATTERY?

Last Updated on July 16, 2004

INDEX:

9.1. What Are the Four Stages of Battery Charging?

Charging Algorithms

9.2. Additional Words of Caution

9.3. Battery Charger Types

9.3.1. Vehicle Charging System

9.3.2. Manual Constant Current Charger

Current Charging Table

9.3.3. Manual Constant Voltage Charging

Constant Voltage Charging Table

Constant Voltage Charging Temperature Compensation Table

9.3.4. Manual Tamper Current Charger

9.3.5. Automatic Constant Voltage or Taper Charger

9.3.6. "Smart" Microprocessor Controller Charger

9.3.7. Float Charger and Battery Maintainer

9.3.8. Trickle Charger

9.3.9. High Rate Fast, Boost or Starting Assist Charger

9.4. How Long Does It Take to Recharge a Good Battery?

9.5. How Do I Know When My Battery Is Fully Charged?

9.6. How Do I Know If My Battery Is Overcharged?

9.7. Battery Charger Buying Tips

9.8. Is Opportunity Charging Worthwhile?

9.9. Is Gassing Good For a Wet Battery?

9.10. What is the Difference Between a Converter and a Charger?

9.11. What Are Charge Controllers or Voltage Regulators?

9.12. How Long Will a Deep Cycle Battery Last On a Single Charge?

9.13. How Can I Reduce Recharging Time?

9.14. How Can I Adjust the Specific Gravity?

9.1. What Are the Four Stages of Battery Charging?

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. You can 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) or 10% of the RC rating and wet batteries do get over 125° F (51.5° C) and VRLA batteries do not get over 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. Gassing usually starts at 80% to 90% of a full charge. A full charge normally 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. Two stage chargers normally have the bulk and absorption stages.

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, and is used indefinitely to maintain a fully charged battery. The current is reduced to approximately C/100 or less. Three stage chargers usually have the bulk, absorption and float stages. Please refer to Section 13 for more information about storing batteries and float charging. Three stage chargers usually have the bulk, absorption and float stages.

9.1.4. The optional EQUALIZING stage is a controlled 5% to 10% absorption overcharge to equalize and balance the voltage and specific gravity in each cell. Equalizing reverses the build-up of the chemical effects like stratification where acid concentration is greater in the bottom of the battery. It also helps remove sulfate crystals that might have built up on the surface or in the pours of the plates. The recommended frequency varies by battery manufacturer from once a month to once a year. 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 up. 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

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

To equalize, 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%. 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 or demineralized water if required, but do not overfill. Only equalize if the battery manufacturer recommends it. Four stage chargers typically have the bulk, absorption, float and equalization stages.

An excellent and easy to understand tutorial on battery charging basics can be found at http://www.batterytender.com/. The following graphs are examples of charging algorithms used by Deltran [Battery Tender] for car and deep cycle batteries:

Wet Standard (Sb/Sb)

Wet Low Maintenance (Sb/Ca)

Wet Maintenance Free (Ca/Ca)

VRLA Absorbed Glass Mat (AGM)

VRLA Gel Cell

[Source: Deltran]

It is extremely important to use the battery manufacturer's recommended charging voltage and procedures whenever possible for optimum battery capacity, maintenance and service life. A good rule-of-thumb 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 batteries being charged. For example, if the battery has RC of 100 minutes, do not use charger that will exceed 10 amps.

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9.2. Additional Words of Caution and Charging Tips:

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

9.2.2. Use the battery manufacturer's charging recommendations and 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 VRLA Gel Cells, a slight overcharge is better than an undercharge.

9.2.3. Lead-acid batteries should always be recharged within 24 hours after they have been used. Before recharging, check the electrolyte and insure that it is not frozen and that it covers the plates at all times to prevent sulfation and to reduce the possibility of an internal explosion.

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 over fill. (Please refer to Section 3.1. for more information about filling batteries.)

9.2.5. Reinstall the vent caps before recharging and recharge ONLY in well-ventilated areas. 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. Follow the battery and charger manufacturers' procedures for connecting and disconnecting cables. Operate in a manner to minimize the possibility of an explosion or incorrectly charge the battery. You should turn the charger OFF 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.

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 VRLA (AGM or Gel Cell) 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, like a Balmer or a water cooled alternator, Bosch for example.

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.

9.2.9. If you are recharging Gel Cell batteries, the battery manufacturer's charging voltages are very critical. You might need special recharging equipment. In most cases, standard deep cycle chargers used to recharge wet batteries cannot be used to properly recharge Gel Cell or AGM batteries because of their charging profiles or voltages. Overcharging Gel Cell and AGM batteries will significantly shorten battery service life or cause "thermal runaway" if the battery temperature is over 120°F (48.9° C).

9.2.10. If a battery is overcharged 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 cables 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 and can damage expensive electronic and electrical components such as emissions computer, audio system, charging system, alarm system, etc.

9.2.13. Alternators are not designed to be dead (or flat) battery chargers and the stator can be burned or diodes go bad if used in that manner.

9.2.14. Gassing usually starts at 80% to 90% of a full charge. 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. 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 type.

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

9.2.16. Do not recharge frozen batteries. Allow them to thaw out first.

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9.3. Battery Charger Types

9.3.1. Vehicle Charging System

A vehicle charging system is made up of two components, an alternator (or DC generator) and voltage regulator. Usually when a vehicle is jump started, it is 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, 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 nine seconds to replace the power used. If 25 amps are available to recharge the battery, it will take 30 seconds and twelve minutes at one amp. With a dead 120 minute RC battery, it would take approximately 45 minutes at 80 amps, 2.4 hours at 25 amps, or 60 hours at one amp to obtain a 90% State-of-Charge (SoC). More information can be found in Dan Landiss' Car Batteries Are Not 12 Volts on http://www.landiss.com/battery.htm.

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 alternator might not produce enough current to keep your battery fully charged. You might need to increase the capacity of the charging system. If you are also recharging battery banks, please see the 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 for float applications like a vehicle or "liveaboard" charging system after the engine has been started and the battery has been initially recharged.

VEHICLE CHARGING VOLTAGE

[Source: Bosch]

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 battery at 14.4 VDC, the system would undercharge most Standard (Sb/Sb) or Maintenance Free (Ca/Ca) and overcharge some Gel Cell starting batteries. 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 charger used on a cheap battery is much better than a bad charger used on a good battery.

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9.3.2. Manual Constant Current Charger

A manual constant current charger chargers the 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 lessor of 1% of the CCA, 12% of the RC rating, or 20% of the C/20 rated AH Capacity of the battery and also carefully monitor the current flowing into the battery. C-rate is a measurement of the charge or discharge of battery overtime. 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 will help prevent it from overcharging the battery.

For fully discharged batteries, the following table lists the recommended battery charging rates and times using a constant current charger:

CONSTANT CURRENT CHARGING

[Source: BCI]

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9.3.3. Manual Constant Voltage Charger

A manual two stage (bulk and absorption) constant voltage charger applies a regulated voltage to the battery at a constant level during the absorption stage. The current drops to below 2% (C/50) of the battery's capacity when it approaches 100% State-of-Charge. The recommended charging method using a constant voltage charger is to slowly recharge the battery using a charger sized to recharge the battery over a ten-hour period (C/10). To prevent damage to a fully discharged battery, the current should be less than 1% of the CCA (Cold Cranking Amps) rating during the first 30 minutes of charge. The charger (or DC power supply) should be adjusted to the battery manufacturer's absorption voltage recommendations or, if not available, to typical charging voltage ranges in the table below with the electrolyte at 80° F (26.7° C) or temperature compensate, if required. Manual constant voltage chargers can overcharge a battery if not turned off when the battery is fully charged.

BATTERY CHARGING VOLTAGES

Most vehicle charging systems are temperature compensating; however, if the external charger is NOT temperature compensating, you should adjust the charging voltage from the table below to correct for the temperature of the battery. For example, if the electrolyte temperature is 20° F (-6.7° C), then increase the charging voltage to 15.788 volts for a Wet Low Maintenance (Sb/Ca) battery if the normal charging voltage is 14.6 at 80 ° F. If 100° F (43.3° C), then decrease the charging voltage to 14.204 volts for the same battery.

CHARGING VOLTAGE
TEMPERATURE COMPENSATION

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9.3.4. Manual Taper Current Charger

The taper current chargers and have no controlled current and voltage and are dependent upon the internal resistance of the battery. The current starts high and tampers off as the voltage increases when the battery approaches 100% State-of-Charge (SoC). With a taper charger, a high current (up to C/2), can be only applied to non-sealed batteries for 30 minutes maximum or until the battery heats up to 125° F (51.7° C). The current is then regulated downward by the battery until the charge state reaches 100% where it is at a minimum. A better approach to recharge the battery with a tamper charger is to size the charger to recharge the battery over a minimum of a ten-hour period (C/10). This technique allows the acid more time to penetrate the plates and there is less mechanical stress on the plates. Manual taper current chargers can overcharge a battery if not turned off when the battery is fully charged.

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9.3.5. Automatic Constant Voltage or Taper (Ferroresonant) Charger

The next step up is an "automatic" two stage charger that will stop charging when the battery has a full charge by turning itself off at some predetermined current or voltage cut-off points. If the battery manufacturer's recommended absorption voltage are used, there is less chance of overcharging a battery than with a manual charger. A 10-amp automatic starting battery charger will cost approximately $50 at an auto parts or battery store and is suitable for most simple non-gel cell car battery recharging charging applications with battery capacities to 100 amp hours (C/20) or 250 minutes of RC. If left connected, when the voltage drops to predetermined point (normally 90%-95% SoC) due to self discharge, some better automatic chargers will turn itself back on and recharge the battery. Better automatic chargers will also include temperature compensation, which is critical if recharging occurs in temperatures other than 80° F (26.7° C), do not produce sparks if the polarity of the clamps are reversed, and will help prevent VLA battery "thermal runaway".

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9.3.6. "Smart" Microprocessor-Controlled Charger

The best chargers for wet and some AGM car and deep cycle batteries are four-stage "smart" microprocessor-controlled temperature compensating chargers. They will automatically switch between bulk, absorption, float, and equalizing charging and have adjustable voltage set points or switches for the different wet battery types. The best chargers for Gel Cell or AGM car batteries are a less expensive three-stage temperature compensating versions that have bulk, absorption and float charging capability (or settings) especially designed for these types of batteries. They will also help prevent VRLA battery "thermal runaway". The microprocessor based chargers can continuously charge a starting battery to keep it fully charged. A one to two-amp three-stage version costs less than $60. They are ideal for for Low Maintenance (Sb/Ca) and some Standard (Sb/Sb) and AGM deep cycle batteries to 75 Ah (C/20) or 188 minutes of RC that are used once per week or less. Good examples are ATVs, Jet skis, fishing boats, motorcycles, snowmobiles, RVs and antique vehicles.

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9.3.7. Float Charger and Battery Maintainer

If you have a tamper or two stage constant current or voltage charger, a temperature compensating voltage-regulated "float" charger or battery maintainer cost less than $50 can be continuously used after a starting battery has been fully charged to maintain it at a 100% State-of-Charge with a C/100 rate to offset the battery's internal self discharge.

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9.3.8. Trickle Charger

Trickle charger is typically a cheap, unregulated voltage (C/100) charger used to maintain a starting battery after it has been fully charged typically costing less than $20. Do NOT use these types of chargers because they can easily overcharge and destroy your battery.

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9.3.9. High Rate Fast, Boost or Starting Assist Charger

High rate fast, boost or starting assist chargers (or settings) are high rate chargers that are designed to provide high current to for up to 15 seconds to start your engine when the battery is discharged. These types of chargers (or settings) to recharge your battery are NOT RECOMMENDED because they can easily overcharge and destroy it with the excessive current or voltage. If you use use one, please do it with extreme caution.

Hyperlinks to battery chargers, "smart" chargers, float chargers and battery maintainers can be found in the Battery References and Information Links List at http://www.uuhome.de/william.darden/batlinks.htm
Please remember to match the charger to the battery manufacturer's recommended charging voltages for that type of battery or match the batteries to the charger capability. The better the match, the longer the service life and more capacity the battery will have.

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9.4. How Long Does It Take To Recharge a Good Battery?

When a battery is discharged, the same amount of power has to be replaced. However, some of the power is converted to heat and lost due to the resistance in the cables, connectors and elements within the battery. For most starting batteries that are discharged less than 10% of their full capacity, an estimate of time is amp hours to be replaced divided by 90% the current output of the charger. For example, a 40 amp hour battery with a 5% discharge would require approximately 2 amp hours to be replaced. Using 5 amp charger, it would take approximately .44 hours (2/(.9x5)) to recharge the battery. A 10 amp charger would take approximately half the time or 13 minutes. For batteries that are deeply discharged battery, an estimate of time is two times the number of amp hours to be replaced divided by the current output of the charger. For example, a 40 amp hour battery with a 95% discharge would require approximately 38 amp hours to be replaced. Using 5 amp charger, it would take approximately 15.2 hours ((38x2)/5) recharge the battery. A 10 amp charger would take approximately half the time or 7.6 hours.

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9.5. How Do I Know When My Battery Is Fully Charged?

In descending order of accuracy, one of the following three methods is normally used to determine if a battery is fully charged. After the battery has cooled to room temperature, recheck the electrolyte levels. The plates must be covered at all times to prevent an internal explosion or sulfation.

9.5.1. According to IEEE 450-2002 Annex B Recommended Practice, "The pattern of charging current delivered by a conventional voltage-regulated charger after a discharge is the most accurate method for determining state of charge. As the cells approach full charge, the battery voltage rises to approach the charger output voltage, and the charging current decreases. When the charging current has stabilized at the charging voltage, the battery is charged, even though specific gravities have not stabilized." It should be less than two percent of the capacity (C/50) of the battery. For the average sized car battery (BCI Group 24), that would be less than two amps. With non-sealed wet batteries, you will also see the cells gassing freely and evenly.

9.5.2. Remove the surface charge by one of the methods in Section 4.3., measure the cells with a hydrometer, temperature compensate, and compare the average of the readings with the battery's manufacturer's Specific Gravity definition of a cell in a fully charged battery.

9.5.3. Remove the surface charge, measure the Open Circuit Voltage (OCV) with an accurate (.5% or better) digital voltmeter across the terminals, temperature compensate, and compare the reading with the battery's manufacturer's OCV definition of a fully charged battery.

If the battery will not "hold" a charge, the charging current does not drop below 2% (C/50), and the battery just gets warm or hot, then it might have some permanent sulfation. Please refer to Section 16 for more information about sulfation and how to remove it.

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9.6. How Do I Know If My Battery Is Overcharged?

Normally, overcharging will consume more water for from the battery than normal and the electrolyte levels will be low. Other signs of overcharging are a "rotten egg" oder, violent gassing, spewing of electrolyte, black "tide-marks" on the inside walls of the cells, or black deposits on the bottoms of the filler caps. Other signs of overcharging are lumpy brown sediment or muddy red or brown electrolyte.

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9.7. Battery Charger Buying Tips

The following are some tips for consumers on buying battery chargers car and deep cycle lead-acid batteries. Please see Section 7.1 for definitions of the battery types. An excellent and easy to understand tutorial on Battery Charging Basics can be found at http://www.batterytender.com/.

9.7.1. Always wear glasses when working around a battery in the unlikely event that it might explode.

9.7.2. MATCH the charger's output voltages to the battery type and manufacturer's recommended absorption, float and equalization (if required) charging voltage requirements. A mismatch can easily overcharge or undercharge the battery. Some charger manufacturers state that their charger to able to recharge all or most battery types. There are differences in the charging voltages and profiles for each battery type, so one charger setting can NOT possibly fit all types of batteries because of the differences in plate chemistries and alloys used. If the documentation that came with the battery or charger or the manufacturer's Web site does not state voltages, contact one of their Customer Service representatives and ask. If you do not charge your batteries at 80 degrees F (26.7 degrees C), temperature compensation needs to occur on the charging voltages to properly recharge the battery. A recent study has shown that cell equalization will significantly increase the life of wet (or flooded) Standard (Sb/Sb), Low Maintenance (Sb/Ca), Maintenance Free (Ca/Ca) batteries, but is NOT recommended for most AGM or Gel Cell batteries.

9.7.3. Size the charger based on the discharge amount and how fast you need to use the batteries again. Slow recharging is recommended, so chargers that are sized 15% to 20% of the capacity of wet or 10%-15% of the capacity of AGM or Gel Cell batteries should be used. Fast or "boost" charging batteries can kill batteries because they can warp the battery's plates. Do not exceed the battery manufacturer's charging current or voltage limitations. For most car batteries, a charger output of five amps or less should be sufficient and for motorcycle and power sports batteries, two amps or less.

9.7.4. Determine special features you want, for example, "automatic shut off" (two stage), "smart" microprocessor controlled, automatic temperature compensation, "soft start", portability, waterproofing, indicators, ammeter, lead reversal protection, short circuit protection, high temperature protection, etc.

9.7.5. Determine the total cost of ownership. Shopping on the Internet by using search engines, like http://www.google.com/ or http://www.yahoo.com/ to find the best prices. A charger as a long term investment and a good charger used on a cheap battery is much better than a bad charger used on a good battery.

9.7.6. If you have two stage charger, use a float charger (or battery maintainer). After you have fully charged your battery with a two stage charger or the vehicle's charging system, you can continuously maintain the full charge with a voltage regulated, one-half to two amp float charger matched to your battery type while the battery is not being used. This will prevent sulfation from occurring while the battery is not being used. Cheap, unregulated "trickle" chargers can overcharge your battery.

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9.8. Is Opportunity Charging Worthwhile?

Opportunity charging is recharging in between the normal charging cycle. An example is a electric fork lift truck being recharged when not in use during the workday and during meal breaks. Some experts will argue that a deep cycle battery should be sized so that the average Depth-of-Discharge (DoD) should not fall below 50% (or 80% depending of the plate chemistry) and the battery should be charged only once per day. Other experts will argue that opportunity charging significants lowers the average DoD and causes multiple, shallow cycles per day, which is better that a higher average DoD and one deep cycle with a lower average DoD. The answer to this question probably lies somewhere in the middle. You will need to compare the effects of lower average DoD and multiple cycles vs. greater DoD and one cycle using the battery manufacturer's data to determine the break even point. Generally, opportunity charging is good, especially when the average DoD is below 50% and you can fully recharge battery at least once during a 24 hour period.

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9.9. Is Gassing Good For a Wet Battery?

When a wet 12-volt lead-acid battery reaches approximately 13.8 VDC at 80° F (26.7° C) during a charge, it will start to gas and is a normal part of the charging process. Gassing is the electrolysis of water into two parts Hydrogen gas and one part Oxygen gas. The gas bubbles given off by the plates will help to mix the electrolyte as they rise to the surface. This will help to prevent electrolyte stratification. Electrolyte stratification is acid concentration that is greater at the bottom of a battery than at the top, especially within larger batteries. Normal charging should produce moderate amount of even gassing of all cells, which is good. Overcharging a battery or rapidly charging with high voltage will produce heavy gassing, heat, consume excessive quantities of water, accelerate positive grid corrosion, warp the plates, and is NOT recommended. Ventilation is required for all lead-acid batteries and good ventilation is mandatory for wet batteries to dissipate the explosive gasses produced during charging.

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9.10. What is the Difference Between a Converter and a Charger?

An AC-DC Converter (or AC-DC Power Supply) is used to convert 120 or 240 VAC power to filtered 12 to 13.8 volt DC power to run DC appliances while connected to "shore power" instead of running on battery power. Converters are normally voltage regulated to provide a constant supply of DC power. A manual battery charger is designed to recharge a battery and typically produces higher voltages. An automatic or "smart" battery charger is designed to stop charging when a preset current is achieved or produce different voltages, depending on which charging cycle it is in. Battery chargers typically do not have the degree of filtering that a converter has.

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9.11. What Are Charge Controllers or Voltage Regulators?

Charge controllers and voltage regulators are devices used to control the level or levels, in the case of three and four stage units, of DC voltage from a source of power to the battery or batteries. A good example is to control the output of solar panels, DC generators or alternators.

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9.12. How Long Will a Deep Cycle Battery Last On a Single Charge?

Discharging, like charging, depends on a number of factors such as the initial State-of-Charge, average Depth-of-Discharge, condition and capacity of the battery, load and temperature. To determine the amount of discharge time (T) for a fully charged battery at 80° F (26.7° C), the simple formula is ampere-hour rating (C) divided by the average load in amps (I) or T=C/I is often used. So, 100-ampere hour battery with an average 5-amp load should last approximately 20 hours (100/5). The total number of amps that are produced when a battery is discharged over a 20 hour (C/20) period is the most commonly used specification for expressing the capacity of deep cycle batteries used in most RV and Marine applications; however, six hour (C/6)for Golf Carts or eight hour (C/8) rates for RV/Marine batteries might be more realistic.

For example, if cycle battery's capacity is rated at 100 ampere hours (Ah) at the 20 hour (C/20) rate, will produce approximately 83 Ah at the eight hour (C/8) rate, 63 Ah in two hours (C/2), and 55 Ah in one hour (C/1). This is due to the Peukert Effect. When you increase the discharge rate, the less power is produced. Good examples of the Peukert Effect on deep cycle battery capacities at various discharge rates can be found at http://www.usbattery.com/specs3.htm. The actual formula is T=C/I^N where N is the Peukert Number used for the specific battery to more accurately calculate the discharge time. The Number generally is in a range of 1.05 to 1.4, with 1.05 the best performing battery due to less internal resistance. A Peukert Number calculator and some specific examples of batteries can be found on Eve's Battery Page at http://www.geocities.com/CapeCanaveral/Lab/8679/battery.html.

Repeatedly discharging a deep cycle battery below 12.0 volts or shallow discharges of less than 10% will significantly reduce the number of life cycles. Please see the graph on average Depth-of-Discharge in Section 7. New batteries often require a precondition or "break-in" period of up to 30 cycles before they will produce their rated ampere hour capacity. The capacity is reduced over time as the active material flakes (sheds) off the plates and some of the pours fill with hard sulfate.

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9.13. How Can I Reduce Recharging Time?

To reduce the amount of time that your charging system is running, only recharge the battery to 90% State-of-Charge level at an amp hour rate not exceeding the number of amp hours that need to be replaced or C/4 (25% of the AH Capacity), whichever is less. For example, if you have consumed 50 amp hours from a 100 amp hour battery, then you do not want to recharge it at rate any greater than 25 amps in one hour. At a 25 amp charging rate, it should take approximately two hours to get to a 90% State-of-Charge. Please note that it will take almost the same amount of time, at a reduced current, to recharge the battery the remaining 10% to bring it to 100% State-of-Charge as it took to recharge it originally from the 50% to the 90% level. If you recharge to the 90% State-of-Charge level, you should recharge to 100% at least every 10^th cycle.

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9.14. How Can I Adjust the Specific Gravity?

Battery manufacturers set the concentration of the sulfuric acid in the electrolyte of a fully charged wet battery to optimize the capacity, service life, water consumption, use in float applications, high discharge rate capability, battery size and self discharge rate. When the Specific Gravity is increased on purpose or by additives, the following occurs: capacity, service life, water consumption, high discharge rate capability, and self discharge rate all increase and use in float applications and battery size decrease. When you decrease the Specific Gravity, the reserves occurs. You might ask why increasing the Specific Gravity on wet starting and motive deep cycle batteries is not a good thing? The answer is that it also accelerates the corrosion of the positive plate grids and connecting straps and you could have a premature battery failure; thus effecting overall service life, but clearly there might be some short termed gains at the expense of increased watering and service life.

Normally, unless there is a spill, battery acid should never be added to a battery. If the temperature compensated Specific Gravity reading needs to be increased in a cell for whatever reason, remove a small amount of some the existing electrolyte and replace it with fresh battery acid with a 1.300 Specific Gravity. Repeat the process until the cell matches the Specific Gravity readings of the rest of the cells or, if the battery is fully charged, the manufacturer's temperature compensated recommended value for a fully charged cell. If the temperature compensated Specific Gravity reading needs to be decreased in a cell for whatever reason, remove a small amount of some the existing electrolyte and replace it with distilled, deionized or demineralized water. Repeat the process until the cell matches the Specific Gravity readings of the rest of the cells or, if the battery is fully charged, the manufacturer's temperature compensated recommended value for a fully charged cell. Some typical Specific Gravity readings at 80° F (26.7° C) for full charged cells are:

• 1.300-1.310 for wet motive deep cycle batteries with tubular positive plates
  
  
• 1.267-1.284 for wet motive Deep Cycle batteries with solid lead positive plates
  
  
• 1.260-1.270 for wet Starting batteries with pasted plates
  
  
• 1.215-1.250 for wet stationary Deep Cycle batteries with solid lead positive plates

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