technical article from Blue Sea Systems
Automatic Charging Relays (ACRs) combine and isolate (separate) battery banks in response to changes in voltage. When a charge from a charging source is sensed by the ACR at the engine or house battery bank, the battery banks are combined through the ACR so that the charge is applied to both banks. When there is no charge sensed at either battery bank, the battery banks are isolated (separated) so that both battery banks are not discharged when a load is applied to one.
Engine starting causes voltage sags and spikes in the starting circuit. If engines are started while batteries are combined, the resulting sag or spike may be felt in the house circuit. These transient voltage levels may reset GPS and other navigation equipment, and may damage some sensitive electronics.
Therefore, when starting engine(s), battery banks should be isolated. The new 120 Amp SI (Starting Isolation) ACR, part number 7610, from Blue Sea Systems is designed to do this, as well as provide combining and normal isolation functions. When the starting circuit is engaged, Starting Isolation automatically isolates the house battery bank from the engine battery bank. This isolation protects house circuits from starting transients.
Note: Starting isolation is the opposite of using an ACR as a cross-connect to combine battery banks to boost starting power.
The starting isolation feature is initiated when the start key switch is closed to start the engine. The signal from start key switch to the starter solenoid also is sent to the Starting Isolation input of the 7610 ACR. This signal opens the ACR before the starter is engaged. When installing the 7610 ACR, connect the Starting Isolation input to the starting circuit between the start key switch and starter solenoid. To assure that the reaction time of the Starting Isolation circuit is fast enough to open the ACR before the starter is engaged, the connection should be on the starting switch side of any relays leading to the solenoid.
See Application Brief: Start Lockout for Two Engines for a description of Starting Isolation in two-engine applications.
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Original article from Blue Sea Systems