Blue Sea Systems BatteryLink Automatic Charging Relay 12/24V DC 120A

The Blue Sea Systems BatteryLink Automatic Charging Relay (ACR) automatically shares a single charging source — typically an engine alternator — between a start battery and an auxiliary or house battery, combining them while charging and isolating them once the engine shuts down.

Key features

• 120A continuous, 210A intermittent (5 min)
• 12/24V DC auto-ranging voltage input — one relay covers both system voltages
• Senses charging on both battery banks and combines automatically
• Optional Auxiliary Battery Priority extends auxiliary charging time before combining
• IP67 rated, ignition protected, integrated LED status indicator

Unlike the manually or remotely switched battery disconnects in this category, the BatteryLink ACR needs no operator input at all — it senses voltage on both banks and makes the combine/isolate decision automatically, which is why your workbook's earlier listing of this part as a simple "Solenoid" undersells what it actually does. Blue Sea's current name for it is the BatteryLink Automatic Charging Relay, reflecting that automatic sensing behavior.

This is a common upgrade from a manual battery-combine switch that's easy to forget to turn off, risking a dead start battery. Cable size tops out at 1/0 AWG, and the largest battery bank it supports is a Group 27 — for larger banks or higher continuous current, Blue Sea's ML-ACR family (500A) is the better fit instead.

Because this is a genuine sensing relay rather than a manually operated switch, there's nothing for the operator to remember — the risk of accidentally leaving batteries combined or forgetting to isolate them simply doesn't exist with this design.