12 volt charger technology has kept pace with the microprocessor revolution, and so current battery charging philosophy uses 3 stage (or 2 or 4 stage) microprocessor regulated charging profiles. These are "smart chargers", and quality units generally are not found in discount stores. The three stages or steps in lead/acid battery charging are bulk, absorption, and float (or complete shut off in some cases). Qualification, or equalization are sometimes considered another stage. A 2 stage unit will have bulk and float stages. It is important to use battery manufacturer's recommendations on charging procedures and voltages, or a quality microprocessor controlled charger to maintain battery capacity and service life.
The older 12 volt charger would feature a fixed charging voltage, high enough to "force" energy (amps) into the battery. The lower the initial battery voltage (state of discharge), the easier this forcing process is, so you may see the amp meter (if so equipped) run up to the charger's maximum output amperage, and stay there a while. As battery resistance rises, as it does as the state of charge increases, the harder it is for the 12 volt charger to force the amps in, so the amp rate decreases. Eventually, the charger reaches a point where it's output voltage can't force any more into the battery, so current almost stops, but depending on where this voltage point is, it may be high enough to overcharge over time, or keep the battery in the gassing stage, drying out a flooded type battery. These chargers should be monitored for this reason, and disconnected when the amp meter drops to the low point.
The "smart chargers" are profiled with contemporary charging philosophy in mind, and also take information from the battery to provide maximum charge benefit with minimum observation. Some gel cell and AGM batteries may require special settings or chargers. True Gel batteries generally require a specific charge profile, and a gel specific or gel selectable or gel suitable charger is called for. The peak charging voltage for Gel batteries is 2.3 to 2.36 volts per cell, and for a 12 volt charger this works out to 13.8 to 14.2 volts, which is lower than a wet or AGM type battery needs for a full charge. Exceeding this voltage in a Gel battery can cause bubbles in the electrolyte gel, and permanent damage, as the bubbles do not dissipate when the over voltage condition ceases.
The BULK stage in a 12 volt charger involves about 80% of the recharge, wherein the charger current is held constant (in a constant current charger), and voltage increases. The properly sized charger will give the battery as much current as it will accept up to charger capacity (25% of battery capacity in amp hours), and not raise a wet battery over 125° F, or an AGM or GEL (valve regulated) battery over 100° F. The target voltage for a 12 volt charger for AGM or some flooded batteries is 2.4 to 2.45 volts per cell, which is 14.4 to 14.7 volts. Some flooded cells will tolerate over 15 volts.
The ABSORPTION stage (the remaining 20%, approximately) in the AGM/flooded 12 volt charger has the charger holding at the absorption voltage (between 14.4 VDC and 14.7 VDC, depending on charger set points) and decreasing the current until the battery is fully charged. If the battery won't hold a charge, or the current does not drop after the expected recharge time, the battery may have some permanent sulphation.
The FLOAT stage is where the charge voltage is reduced to around 2.25 volts per cell, which is around 13.5 VDC and held constant, while the current is reduced to less than 1% of battery capacity. This mode can be used to maintain a fully charged battery indefinitely. Some chargers shut off instead of maintaining a float voltage, and monitor the battery, initiating a charge cycle if necessary.
Recharge time can be approximated by dividing the amp hours to be replaced by 90% of the rated output of the charger. For example, a 100 amp hour battery with a 10 % discharge would need 10 amps replaced. Using a 5 amp 12 volt charger, we have 10 amp hours/(.9x5) amps = 2.22 hour recharge time estimate. A deeply discharged battery deviates from this formula, requiring more time per amp to be replaced.
Recharge frequency recommendations vary from expert to expert. It appears that depth of discharge affects battery life more than frequency of recharge. Basically, lead acid batteries, including the sealed types (AGM and Gel) like to be kept fully charged when possible. For example, recharging when the equipment is not going to be used for a while (meal break or whatever), may keep the average depth of discharge above 50% for a service day. This basically applies to battery applications where the average depth of discharge falls below 50% in a day, and the battery can be fully recharged once during a 24 hour period. This is called "opportunity charging".
Equalization is essentially a controlled over charge. Some charger manufacturers call the peak voltage the charger attains at the end of the BULK mode (absorption voltage) an equalization voltage, but technically it's not. Higher capacity wet (flooded) batteries sometimes benefit from this procedure, particularly the physically tall batteries. The electrolyte in a wet battery can stratify over time, if not cycled occasionally. In equalization, the voltage is brought up above typical peak charging voltage (to 15 to 16 volts in a 12 volt charger) well into the gassing stage, and held for a fixed (but limited) period. This stirs up the chemistry in the entire battery, "equalizing" the strength of the electrolyte, and knocking off any loose sulphation that may be on the plates.
The construction of the sealed batteries (AGM and Gel), all but eliminates any stratification, and most all manufacturers of this type do not recommend it (advising against it). Some manufacturers (notably Concorde) list a procedure, but observing voltage and time specifications are critical to avoid battery damage.
A 12 volt charger can be had from a low milliamp output (100, 200, 500 milliamps), up to 90 amps, that will plug into a 115 volt wall outlet (chargers above about 65 amps usually require a 20 amp circuit, so check). Some of the smaller units are unregulated, and simply have a fixed voltage output, like the chargers of old. These tend to take longer to charge, and should be avoided when possible. Smaller amp capacities are appropriate for smaller batteries, like motorcycle, ATV, etc., or electronic and security type applications in the 1.3 to 12 amp hour range. They also can be used for maintenance on larger batteries. A medium amp output 12 volt charger would be in the range of 20 to 50 amps or so, and be used for many applications using about 100 amp hours of battery and up, or applications with a constant amp load (power supply application). For a power supply type situation, the constant draw should be a low percentage of the charger maximum amp capacity, to keep the charger from going back into the boost or bulk stage, or the charger should have selectability for the power supply or "battery with load" mode. Larger units in the 12 volt charger models are about 55 to 90 amps output. These are used in large amp hour battery banks, or applications desiring faster recharge times (possibly at the expense of maximum battery life). Sometimes the larger units are used where a generator is the AC power source, and generator run time is a consideration.
Most battery manufacturers recommend sizing the charger at about 25% of the battery capacity (ah = amp hour capacity). Thus, a 100 ah 12 volt battery would take about a 25 amp 12 volt charger (or less). Larger chargers may be used to decrease charge time, but may decrease battery life. Smaller chargers are fine for long term floating, e.g. a 1 or 2 amp "smart charger" can be used for battery maintenance between higher amp cycle use, but would be inefficient or burn up if used to bulk charge large capacity, deeply discharged batteries.
For more information, or 12 volt charger application specific recommendations, email us or call the tech line.