Blue Sea Systems ML-ACR Automatic Charging Relay - 12V DC 500A

The Blue Sea Systems ML-ACR Automatic Charging Relay automatically manages the charging of two large battery banks on a 12V DC system, combining them while an alternator or shore charger is actively charging and isolating them the rest of the time, so a discharged house bank never drags down a fully charged start battery.

Key features

• 500 Ampere continuous rating to support high-output alternators
• Magnetic Latch (ML) design draws under 40mA to monitor terminal voltage, with only a brief current pulse when changing state
• Dual sensing — senses charge on both battery banks independently
• Start Isolation and Engine Isolation configurable (not simultaneously)
• Includes ML-Series Remote Control Contura Switch, PN 2146

This version operates fully automatically with no manual override — for the same relay with a manual control switch added, see the Manual Control version instead.

Start Isolation temporarily isolates house loads from the engine circuit during cranking, protecting sensitive electronics from voltage sags and spikes; Engine Isolation instead keeps two separate engines' electrical systems apart while both run, protecting each engine's own electronics and maximizing alternator output. The ACR needs a dedicated ground wire run directly to a battery negative terminal — a chassis ground is generally insufficient for reliable operation, and this wire is required, not optional, even though the wiring diagram shows an optional switch in that ground path.

This relay is a strong fit for boats with a large house bank and demanding electronics, where manually remembering to combine batteries for an emergency start — or isolate them afterward — isn't something you want to rely on getting right every single time.