Marine battery chargers: sizing, chemistry, split banks

Marine shore-power battery chargers (Victron Skylla, Sterling ProNautic, ProMariner ProNauticP, Charles Marine) come in single-bank and multi-bank versions. Sizing rule: charge amps should be 20-30 percent of house-bank Ah for AGM, 40-50 percent for lithium (lithium accepts fast charge without heat problems). Example: 400 Ah AGM = 80-120A charger; 400 Ah lithium = 160-200A. Multi-bank chargers charge separate banks (house + start + genset start) from one shore power input via internal isolators or DC-DC converters. Rule: never use a simple diode battery combiner between chemistries, a shared charger with independent outputs is the safer path. Chemistry profile setting is critical: an AGM-only charger cooking a lithium bank overcharges to 15V+ (lithium wants 14.4V max, BMS disconnects), a lithium-profile charger undercharging AGM only reaches 13.8V (AGM never gets to 100 percent, sulfates over time). Modern smart chargers have selectable profiles for wet, AGM, gel, lithium, calcium, set it correctly at install and label the charger. Common failures: (1) Charger stuck in bulk forever = defective battery (won't accept charge, voltage never rises) or configured for wrong chemistry. (2) Buzzing sound = loose DC output connection, tighten every terminal to spec (typically 10-15 Nm on M8 studs). (3) Overheat shutdown = ventilation blocked, most chargers need 3-4 inches clearance and thermal air flow. (4) Ripple on the DC output visible on an oscilloscope = failing filter caps, replace the charger, cheap fix relative to the electronics it will fry.