There are up to four phases of battery charging-bulk, absorption, equalization and float. The bulk stage is where the charger current is constant and the battery voltage increases. You can give the battery whatever current it will accept as long as it does not to exceed 20% of the ampere-hour rating and that it will not cause overheating. The absorption phase is the phase where the charger voltage is constant and current decreases until the battery is fully charged. This normally occurs when the charging current drops off to 1% or less of the ampere-hour capacity of the battery. For example, end current for a 50 ampere-hour battery is 0.5 amps (500 milliamps) or less.

The optional equalizing phase is a controlled 5% overcharge to equalize and balance the voltage and specific gravity in each cell by increasing the charge voltage. 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 plates. The frequency recommendation varies by manufacturer from once a month to once a year or is based on a specific gravity test where the difference between cells is .030 (or 30 "points"). To equalize, fully recharge the battery. At this point, increase the charging voltage to the manufacturer's recommendations, or if not available, ADD 5%. Heavy gassing should start occurring. Take specific gravity readings in each cell once per hour. Equalization has occurred once the specific gravity values no longer rise during the gassing stage.

The optional float phase is where the charge voltage is reduced, then held constant and is used to indefinitely maintain a fully charged battery. Please refer to Section 13 for more information about storing batteries and float charging them.

An excellent and easy to understand tutorial on battery charging basics can be found at by drilling down through Charging>Tutorials>Charging Basics. The following are four stage charging algorithms from Deltran (Battery Tender) for three different types of car batteries:

Low Maintenance (Lead-Antimony/Calcium)
[Source: Deltran]

Absorbed Glass Mat (AGM)
[Source: Deltran]

Maintenance Free (Lead-Calcium/Calcium)
[Source: Deltran]

It is important to use the battery manufacturer's charging recommendations whenever possible for optimum performance and life. In addition to the earlier cautions, here are some more words of caution:

9.1. NEVER, NEVER disconnect a battery cable from a car 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 electrical components such as computers, radio, charging system, etc.

9.2. BEFORE recharging check the electrolyte level and insure it covers the plates at all times and that it is not frozen.

9.3. Avoid adding distilled water if the electrolyte is covering the top of the plates because during the recharging process, it will warm and expand. After recharging has been completed and the battery has cooled, RECHECK the levels.

9.4. Insure the vent caps are clean. Reinstall the vent caps BEFORE recharging, recharge ONLY in well-ventilated areas, and wear protective eye ware. Do NOT smoke or cause sparks or flames while the battery is being recharged because batteries give off explosive gasses.

9.5. If your car battery is sealed, avoid recharging with current ABOVE 20% of the RC (or 50% of the ampere hour) rating. For example, 24 amps maximum for a 120 minute RC (48 ampere hour) rated battery.

9.6. 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 occur and this might cause an explosion. Good ventilation or a fan is recommended to disperse the gasses created by the recharging process.

9.7. If a battery becomes hot, over 110 o F (43.3 o C), or violent gassing or spewing of electrolyte occurs, 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.

9.7. Insure that charging the battery in the car with an external MANUAL charger will not damage the car's electrical system with high voltages. If this is even a remote possibility, then disconnect both battery cables from the battery BEFORE connecting the charger.

9.8. If you are recharging gel cell batteries, a manufacturer's charging voltages can be very critical. Sometimes, you might need special recharging equipment. In most cases, standard deep cycle chargers used to recharge wet batteries cannot be used to recharge gel cell and AGM batteries because of their charging profiles; using them will shorten battery life or cause "thermal runaway". Match the charger (or charger's setting) for the battery type you are recharging or floating.

Usually when a car is jump started, it is driven long enough to fully recharge the battery. (Please see Section 7.2.) 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. That is, an alternator is sized by the car 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.

If you have added lights, 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. Ideally the combined load of all the accessories should be less than 75-80% of the charging system's maximum output, so that at least 20-25% is available to recharge the battery.

A better method to recharge batteries is to use an external constant current charger, which is set not to deliver more than 12% of the RC rating of the battery and also monitors the state-of-charge. A timer that will turn off the charger will help prevent overcharging the battery. For fully discharged batteries, the following table lists the recommended battery charging rates and times:

Reserve Capacity (RC) Rating Slow Charge (RECOMMENDED) Fast Charge
80 Minutes or less [32 ampere hours or less] 15 Hours @ 3 amps 5 Hours @ 10 amps
80 to 125 Minutes [32 to 50 ampere hours] 21 Hours @ 4 amps 7.5 Hours @ 10 amps
125 to 170 Minutes [50 to 68 ampere hours] 22 Hours @ 5 amps 10 Hours @ 10 amps
170 to 250 Minutes [68 to 100 ampere hours] 23 Hours @ 6 amps 7.5 Hours @ 20 amps
Above 250 Minutes [over 100 ampere hours] 24 Hours @ 10 amps 6 Hours @ 40 amps

[Source: BCI]

The BEST method is to SLOWLY recharge the battery at 70o F (21.1o C) over a ten-hour period (C/10) using an external constant voltage (or tapered current charger). This technique allows the acid more time to penetrate the plates and there is less mechanical stress on the plates. 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 120-minute RC (48 ampere-hour) battery would have a charging or discharging rate of 4.8 amps for ten hours. A constant voltage or "automatic" charger applies regulated voltage at approximately 14.4 volts with the electrolyte at 70o F (21.1o C). A 5-amp constant voltage charger will cost between $25 and $50 at an auto parts store and is suitable for most simple automotive recharging charging applications. More expensive three-stage microprocessor-controlled chargers are available that will automatically provide bulk, absorption and float charging. A four-stage charger also will provide an equalizing charge in addition to the bulk, absorption and float charging.

To prevent damage to a fully discharged battery, the current should be less than 1% of the CCA rating during the first 30 minutes of charge. With a taper charger, a high current, up to 30 amps, can be applied to non-sealed batteries for a short period up to 30 minutes maximum; the current is then regulated downward until the charge state reaches 100%. An excellent automatic constant voltage battery charger is a 15 volt regulated power supply that has been adjusted to the manufacturer's recommendations or, if not available, to voltages in the table below with the electrolyte at 70 o F (21.1o C):

Battery Type Charging Voltage Float Voltage Equalizing Voltage
Wet Low Maintenance 14.4 13.2 15.1
Wet Maintenance Free 14.8 13.4 15.5
Sealed & VRLA 14.4 13.2 15.1
AGM 14.4 13.6 15.5
Gel Cell 14.1 13.2 N/A
Wet Deep Cycle 14.5 13.2 15.8

To compensate for electrolyte temperature, which has a negative temperature compensation coefficient, adjust the charging voltage .0028 (2.8 millivolts) to .0033 (3.3 millivolts) volts/cell/degree F. For example, if the temperature is 30 o F (-1.1o C), then INCREASE the charging voltage to 15.19 volts for a wet low maintenance battery. If 100 o F (43.3o C), then DECREASE the charging voltage to 13.81 volts.

If left unattended, cheap, unregulated trickle or manual battery chargers can overcharge your battery because they can "decompose" the water out of the electrolyte. Avoid using fast, high rate, or boost chargers on any battery that is sulfated or deeply discharged. The electrolyte should NEVER bubble violently while recharging because high currents only create heat and excess explosive gasses.

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