The best battery chargers, those we refer to in this website, are computerized. These circuits are designed to think as they charge your battery, based on the way you set them up. Basically there are 3 charging modes, no matter the charger. You choose the mode depending on what you are doing.
Battery Charging isn't really rocket science if you just learn a few basic rules of thumb regarding the charging modes. To start, let's understand what each of the charging phases is designed to do;
1. Bulk Charging Phase - 14.4 v (above float) - Charger Max Amperage
This phase of charging literally forces amperage into the battery as quickly as possible because the amperage throttle is opened, and the voltage is above the float voltage. This does not harm a battery that can use the power, or is not full. Think of this stage like filling up a water tank as fast as the hose will allow you to go. The bulk battery charging stage charges a battery from 0% - 90%. To finish the battery charging, the battery charger switches to either Absorption or Float mode, depending on the application.
2. Absorption Phase - 14.4 v (above float) - Amperage Decreases to <.5 amps
This middle, and optional stage of charging fills a battery to it's absolute limit. The phase is designed to charge the battery at a higher voltage than it's ambient, or float voltage, but lowers the amperage as the battery approaches fully charged status.
* Do not charge batteries that are loaded with an absorption phase charge. You cook your batteries.
3. Float Charging Batteries 13.1 -13.4 v - amperage dependent on battery
The final stage of charging and battery readiness is the float mode. Float charging is the same voltage as the battery at full charge. Think of this like a water tank, and a hose at the same pressure, the water doesn't move much. However if a valve is opened, the hose will push water, up to it's amperage limits, and the battery fills the holes.
** For simple battery charging installations, a float voltage charge can be used as shore power. This allows you to install cheaper chargers to get the job done, but means more equipment to properly care for your batteries when you get home anyway.
4. Equalization or Equalizer Battery Charging Mode.
The battery equalizer function on many battery chargers is designed for flooded lead acid batteries only. Basically this mode is a forced overcharge of the battery at roughly 15.5 volts, and should be sustained for several hours. The process literally boils the battery acid inside the battery casing to remix the acids. The acids separate when allowed to sit, and this charging mode remixes them. Many manufacturers recommend a weekly or monthly equalization. (AGM Batteries and GEL batteries are more efficient as the do not require this forced charging click here for more)
* Always use a 1 or 2 stage charger for batteries under load. (Bulk and Float modes only). A problem arises when you have a load hooked up to a 3 stage battery charger (Bulk, Absorption, Float) and a load (loud stereo or motor for instance). As the battery is intermittently used, the battery charger continually charges. After the bulk phase, the battery charger enters Absorption mode, and due to the load is never able to complete it's phase, allowing the amperage to dip below .25 amps. The load tricks the battery charger into continually charging in the Absorption mode instead of switching off to protect the batteries from overcharging. We discuss the math here, but long story short, you cook a battery when you have a load, and a 3 stage battery charging over time.
** Every cell phone tower is designed to run when the power goes out. These are powered by dc systems with lots of batteries. The point of the story is how they charge them, without supervision, for long periods of time, by simply float charging. By charging the battery as fast as they drain the power, via a float charge on one side, and a load on the other, the battery acts as a buffer for a down power grid. This setup does require bi-annual shut down, 3 stage maintenance charging, and inspections to avoid sulphation.