The first stage in a 3 or 4 stage charging algorithm is the Bulk Stage. Typically the Bulk Stage is a Constant Current (CC) charge but may also be Constant Power, Pulse Current or Taper Charge. In this stage the optimum charge current should be limited to less than or equal to 20 amps per 100 ampere hour (20 hour rate) of battery capacity or .2C. This stage should end when the cell voltage is = to 2.38-2.42V/Cell at 25°C/77°F. The maximum time in hours should = 1.2 times the DOD (in AH) divided by the average charge current in amps. If this time is exceeded, charging should be stopped and the battery and/or charge process should be analyzed. This stage will represent approximately 60% of the total charge time. The battery will be nearing 80%-90% charged at the end of this stage.
The second stage is the Absorption Stage. Typically this stage is a Constant Voltage (CV) stage where the terminal voltage is maintained at 2.38-2.42V/Cell at 25°C/77°F (adjusting for temperature). The charge current is maintained until current acceptance drops by less than .1 ampere over a 1 hour period. This stage should take the battery to 100% charged and should not take longer than 10-12 hours. If this time is exceeded, charging should be stopped and the battery and/or charge process should be analyzed.
The third stage is the Float Stage or maintenance and monitor stage. This step is generally not needed if; no load is present when the batteries device is not in operation; the batteries device is used on a regular basis and does not sit idle for lengthy periods of time. Float voltage should be maintained at 2.25-2.30 V/Cell.
If a Balance Mode is included in the charging algorithm it would typically happen after the Absorption Stage. This would become the third stage and the Float Stage would then become the fourth stage. A balance mode is similar to an Equalize function for flooded batteries but is performed at a lower voltage and/or is tightly controlled against current, voltage and time.
To compensate for battery temperature not at 25°C, subtract 0.005 V/Cell for each 1°C above 25°C, and add 0.005 V/Cell for each 1°C below 25°C.