Bulk Stage (Bulk Charge)
The first stage, in which the charger fills the battery quickly by holding the highest current it can deliver constant. The battery voltage rises during this time and the stage ends when it reaches the absorption threshold. In lead-acid, typically 70–80% of the capacity is put back in this stage.
In the bulk stage the limiting element is not the battery but the charging source. A mains charger delivers the amperage on its label, in a solar system as much current flows as the power coming from the panel allows, and on the alternator side the limit of the DC-DC unit applies. Bulk duration is therefore set directly by the power of the source.
A rough calculation is possible: if half of a 100 Ah battery is discharged, the charge to be put back is 50 Ah. With a source delivering 20 A the bulk stage takes about two and a half hours, and then the absorption stage adds its own duration. This is why filling the last 20% of the battery takes longer than filling the first 70%.
The practical consequence in an RV is this: it is hard to fully charge the service battery with short journeys; only the bulk stage is completed along the way. If you want to cover levelling, pump and refrigerator loads for a long time, you need to take the battery to a full charge from time to time with shore power or a solar system of sufficient capacity.
Context in which this term is used: RV automatic levelling guide