Adding a second battery
Posted: Thu Apr 02, 2015 8:39 am
Adding a second battery can be as easy as bolting one in and installing a switch to switch between positive battery terminals (negative terminals are tied together to establish a common ground), or can be more complex.
One problem faced with a simple switch is that if the switch is turned to an "off" position while the engine is running, the alternator will no longer be charging a battery and the output voltage will spike up, potentially damaging diodes in the alternator's output rectifier. More complex switches therefore include an alternator field disconnect (AFD) feature which can be configured to cut power to the alternator's field windings when a switch is turned "off," thereby disabling the alternator and saving the diodes from damage.
One such switch from Blue Sea is configured as follows, using the F1 and F2 terminals coupled to the regulator of the alternator to perform the AFD function: Some installations make use of an automatic charging relay (ACR), which couples the batteries together during charging, but leaves the batteries separate when discharging or when starting engines. "Add-a-battery" kits including ACR and a switch may not include AFD functionality in the switch, resulting in potential damage to the alternator diodes if the switch is inadvertently turned off while the engine is running. See, for example, the Bluesea 7650 add-a-battery kit.
One problem faced with a simple switch is that if the switch is turned to an "off" position while the engine is running, the alternator will no longer be charging a battery and the output voltage will spike up, potentially damaging diodes in the alternator's output rectifier. More complex switches therefore include an alternator field disconnect (AFD) feature which can be configured to cut power to the alternator's field windings when a switch is turned "off," thereby disabling the alternator and saving the diodes from damage.
One such switch from Blue Sea is configured as follows, using the F1 and F2 terminals coupled to the regulator of the alternator to perform the AFD function: Some installations make use of an automatic charging relay (ACR), which couples the batteries together during charging, but leaves the batteries separate when discharging or when starting engines. "Add-a-battery" kits including ACR and a switch may not include AFD functionality in the switch, resulting in potential damage to the alternator diodes if the switch is inadvertently turned off while the engine is running. See, for example, the Bluesea 7650 add-a-battery kit.