How a 3-stage battery charger works for lead acid (not lithium) batteries.
Lithium batteries are getting popular and should not use lead-acid charge profiles.
Smart battery chargers try to keep house batteries healthy using a charge profile to manage when to switch between three stages, Bulk, Absorption & Float in that order. Charge voltages are usually based on approximately 70-degree battery temperatures. Different pre-set profiles are used for flooded, AGM & Gel. Often a battery manufacturer will recommend voltage settings. Chargers usually have a custom profile where we get to choose some or all the settings.
Batteries heat up when being charged and to prevent overheating, charge voltage settings are continually reduced as battery temperature increase above 70 degrees. During colder weather charge voltages can be increased as battery temperature lowers. To manage voltages, chargers have a wired temperature sensor attached to battery.
For example, on our custom profile for East Penn Gel batteries we reduce all voltage settings by .02v for each 1-degree Fahrenheit increase and vice versa.
A fully charged 12v lead acid battery will be about 12.6-12.7v. When no-load battery voltage reduces down to about 12.2v battery is considered to be at about 50% capacity. As batteries are charged, internal resistance increases reducing the amps the battery will accept extending the time to get battery back to 100% capacity.
Bulk stage: will normally charge at max amps to about 80% capacity. At this point the profile voltage setting will transition the charger to the Absorption stage.
Absorption stage: charger will hold maximum voltage setting and charge the battery with fewer amps as the battery's internal resistance starts to not accept charger's maximum amps. Once the amps reduce to about 10% of the charger's maximum, it will move to Float stage.
The absorption stage is also time based, if the charger is still in its absorption phase after, for example, 4 hours, the charger will automatically transition to the float stage. This generally happens if the charger is undersized for the battery bank or there are loads running on the system, not allowing the charger to reduce the amps below the transition point.
Float stage: maintain battery at float voltage setting and also support any loads running at the time. If the loads increase past the charger's maximum float amps, then the battery voltage will start to reduce. Once the voltage reaches a "return to bulk" voltage setting of about 12.5v, it will start a new Bulk charge cycle.
Chargers often have an equalization mode that should be turned off for Gel and most AGM batteries.
Since batteries charge faster at higher voltages, the job of the profile is limit battery damages by allowing charging at the highest safe voltage for as long as possible, always a conflict between forces. The better the battery charger is a doing its job, the longer the useful life of the battery. With good battery load management, batteries can last over 10 years with proper care.
We have 9 years of experience with the Progressive Dynamics 9200 series and flooded lead acid batteries. We find we only need water the batteries twice a year.
I've never determined at what voltage the PD decides to go to whoopee, but once it does it's 14.4V for 4 hours. Bulk charge rate.
Then the PD drops to 13.6V for 30 hours to top off the battery. Absorption charge rate.
The the PD switches to 13.2V to float charge the battery supplying just enough EMF to prevent the batteries from discharging.
Every 22 hours the PD bumps the voltage up to 14.4V for 15 minutes to stir the electrolyte.
Using the house batteries to start the generator tricks the PD into thinking the house batteries are at a lower state of charge than they are really.