What Is the SI Post for on an SI-ACR Battery Combiner?
4 December 2022 · 6 min · 25K views
What this video covers
The SI post on a Blue Sea Systems ACR (Automatic Combiner Relay) battery combiner is a starter isolator terminal that disconnects the house and engine batteries from each other the moment the engine cranks, even when the ACR is engaged and the batteries are running in parallel.▶ 3:24 Without it, a combined house-and-engine bank under starting load can drop to 10–10.5 volts, triggering a reboot on chart plotters or sounders wired to the house battery.▶ 5:20 The SI post also lets an owner verify at the dock that the engine battery alone can start the engine — confirming a reserve exists before leaving the slip.▶ 5:38 The broader context is a common rewiring scenario: separating a house battery from a starter battery and installing an ACR so both batteries receive charge from an alternator, solar, or an inverter-charger.▶ 3:05 The ACR parallels the banks whenever charging voltage is sensed on either battery; the SI post interrupts that parallel connection during cranking.▶ 3:24 The feature is rarely implemented because wiring it adds complexity, but the function is straightforward.▶ 4:04
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