Charging from the alternator

Assumptions

How much from the alternator: the Buck-Boost converter manual recommends setting the charger’s maximum current to no more than 60% of the vehicle alternator current — the alternator also powers the vehicle itself. We apply this rule to the current the charger draws from the alternator. When the voltage changes (for example 12 V → 24 V), we calculate input current from power: charge current × charge voltage (14.4 / 28.8 / 57.6 V) ÷ (alternator voltage 14 / 28 V × 90% efficiency).

How much the battery accepts: typically up to about 0.5C for LiFePO4 and about 0.3C for lead-acid batteries (Victron Blue Smart charger manual). The datasheet for the specific battery takes priority.

Models: output currents from the Orion XS datasheets (12/12-50A, 12/12-70A, XS 1400), Orion-Tr Smart (isolated and non-isolated) and Buck-Boost 25/50/100 A for the given voltage pair. Orion XS and Orion-Tr Smart can be connected in parallel — the calculator suggests this when several smaller units will provide more current than one. Orion XS and Orion-Tr Smart also let you set a lower current in VictronConnect.

Charging time: as in the charging time calculator — bulk phase to about 95% (LiFePO4) or 80% (lead-acid) = missing Ah ÷ charge current. At idle the alternator may deliver only about 50% of its power (Orion XS support page), so charging can be slower when stationary.

What the calculator does not check: cables and fuses (tables for each model are on the Orion XS, Orion-Tr Smart and Buck-Boost datasheets; you can also calculate cable cross-section in the cable calculator), engine running detection or battery temperature. With a LiFePO4 battery, Buck-Boost requires a CAN-bus Temp. Sensor sensor.

Sources