Alternator and DC-DC charger questions, answered

With a smart alternator, yes, and with a lithium house battery it is the safe default. Victron says controlled charging is indispensable with a smart alternator, and that it protects the alternator in lithium systems, where direct charging can overload it because of lithium's low internal resistance. A DC-DC charger limits the current and charges at the voltage the battery needs; Battle Born also accepts a battery isolator between lead-acid and lithium.

Not when it is set up correctly. Victron and Renogy chargers watch the starter battery and stop when the engine is off: Renogy's 20 A and 40 A models, on a traditional alternator, start at 13.2 V and stop when the starter battery drops to 12.7 V. Victron's Orion-Tr Smart stops charging when its input falls below a shutdown voltage, 13.1 V by default for a smart alternator and 13.5 V for a regular one, which Victron says keeps the starter battery fully charged.

No. The DC-DC charger sits between the starter and house batteries and does the isolator's job: it charges only while the engine runs and stops before the starter battery runs low. Battle Born names a battery isolator or a DC-DC charger as the two ways to link a lithium house bank to a lead-acid battery.

Measure the starter battery with the engine running and everything else switched off, then again after 5 to 10 minutes. Renogy's manual says a reading around 14.4 V points to a traditional alternator, and 12.5 to 13.5 V most likely means a smart alternator. With a smart alternator, Renogy's charger needs its ignition wire connected.

Up to about 512 Wh with a 40 A DC-DC charger and about 256 Wh with a 20 A one, because a 12.8 V battery stores 12.8 Wh per amp-hour. So an hour with a 40 A charger refills up to about 40% of a 100Ah lithium battery. Expect less when the battery is nearly full or the alternator can't keep up.

Match it to your battery and your alternator. Redodo recommends 0.2C of charge current for its 100Ah battery, which is 20 A; at the same rate a 200Ah bank takes 40 A. Renogy asks you to confirm with your vehicle maker that the alternator's output is in the 75 A to 100 A range before installing its 20 A or 40 A charger.