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 provided that it will not cause over heating.

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 approximately 0.5 amps (500 milliamps) or less.

The optional equalizing phase is a controlled 5% over charge 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 sulphate crystals that might have built up on the plates. The frequency recommendation varies by manufacturer from once a month to once a year, 10 to 100 cycles, 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% to the recommended absorption charging voltage. 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 12 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 http://www.batterytender.com/catalog/chargingbasics.html

It is extremely important to use the battery manufacturer's charging recommendations whenever possible for optimum performance and life. A good rule-of- thumb is not to use a cycling charger (or charging setting) that is greater than 25% of the amp hour capacity (using the 20 hour rate) of the battery or batteries being charged. For float applications, size the charger at least 125% of the worst case electrical load. In addition to the earlier cautions, here are some more words of caution:

Never disconnect a battery cable from a vehicle with the engine running because the battery acts like a filter for the electrical system. Unfiltered (pulsating DC) electricity can damage expensive electronic components, e.g., emissions computer, radio, charging system, etc. Turn off all electrical switches and components, turn off the ignition and then disconnect the battery.

For non-sealed batteries, check the electrolyte level. Make sure it is covering the plates, and it is not frozen before starting to recharge.

Do not add 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 level.

Reinstall the vent caps BEFORE recharging, recharge ONLY in well-ventilated areas, and wear protective eye wear. Do NOT smoke or create sparks or flame while the battery is being recharged because batteries give off explosive gasses.

If your battery is an AGM or a sealed flooded type, do NOT recharge with current ABOVE 12% of the battery's RC rating (or 20% of the ampere-hour rating). Gel cells should be charged over a 20-hour period and never in excess of the manufacturer's recommended level or in excess of 14.1V DC.

Follow the battery and charger manufacturer's procedures for connecting and disconnecting cables and other steps to minimize the possibility of an explosion or incorrectly charging 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 could cause an explosion. Good ventilation or a fan is recommended to disperse the gasses created by the recharging process.

If a battery becomes hot, over 110°F (43.3°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 batteries.

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

If you are recharging gel cell batteries, the 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.

If a battery is over charged and all the electrolyte is "boiled" out some batteries can produce deadly CO (Carbon Monoxide) gas.

Use an external constant current charger, which is set not to deliver more than 12% of the RC rating of the battery and monitor the State-of-Charge. Timers that will cut-off the charger will help prevent overcharging the battery. For 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

Another method is to slowly recharge the battery at 70°F (21.1°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 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 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 70°F (21.1°C). An automatic charger should stop charging when the battery has a full charge. There is less chance of over charging a battery than with a manual charger.

The best chargers are the more expensive four-stage microprocessor-controlled chargers. They will automatically switch between bulk, absorption, float, and equalizing charging. A less expensive three-stage version has bulk, absorption and float charging capability. The microprocessor based chargers can be continuously connected to the battery and will keep it fully charged.

To prevent damage to a fully discharged battery, the current should be less than 10% of the amp hour 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%. 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, assume that the ampere-hour capacity of the battery is 220, then it would take approximately 11 hours to recharge or discharge the battery using a C/20 rate. A good manual 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°F (21.1°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°F (-1.1°C), then increase the charging voltage to 15.19 volts for a wet low maintenance battery. If 100°F (43.3°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 sulphated or deeply discharged. The electrolyte should never bubble violently while recharging because high currents only create heat and excess explosive gasses.

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 deep cycle batteries that are discharged less than 10% of their full capacity, an estimate of time is amp hours to be replaced divided by 90% of the current output of the charger. For example, a 220 amp hour battery with a 10% discharge would require approximately 22 amp hours to be replaced. Using a 25 amp charger it would take approximately one hour (22/(.9x25)) to recharge the battery. A 50 amp charger would take approximately half the time or 30 minutes. For batteries that are deeply discharged, an estimate of time is twice the number of amp hours to be replaced divided by the current output of the charger. For example, a 220 amp hour battery with a 50% discharge would require approximately 110 amp hours to be replaced. Using a 25 amp charger it would take approximately 8.8 hours ((110x2)/25) to recharge the battery. A 50 amp charger would take approximately half the time or 4.4 hours.

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 should not fall below 50% and the battery should be charged once per day. Other experts will argue that opportunity charging significantly lowers the average Depth-of-Discharge and causes multiple, shallow cycles per day, which is better that a higher average Depth-of-Discharge and one deep cycle. The answer to this question probably lies somewhere in the middle. You would need to compare the effects of lower average Depth-of-Discharge and multiple cycles versus greater Depth-of-Discharge and one cycle using the battery manufacturer's data to determine the break even point. Generally opportunity charging is good, especially when the average Depth-of-Discharge is below 50% and you can fully recharge battery at least once during a 24 hour period.