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A VSR (voltage sensitive relay) is perfect for dual battery systems. They come in a single sense or a dual sense configuration for bi-directional charging.
Single sense VSR : When the engine is started, the starting battery and auxiliary batteries are separated allowing only the starting battery to be charged. When the charge voltage reaches 13.3 volts the voltage sensitive relay (VSR) engages allowing both starting and auxiliary batteries to be charged. When the engine is stopped and the battery voltage falls below 12.8 volts the VSR disengages separating the start and auxiliary batteries. The auxiliary battery continues to power any accessories without risk of draining the starting battery.
Dual Sense VSR : A feature of the model we sell is the "Dual Sensing” VSR . This will operate the same as the single sense but with bi-directional technology, so the voltage of both batteries is looked at rather than just the one. If either battery is receiving a charge, e.g. Solar Panel input, or a mains battery charger is connected to the auxiliary battery, as long as the required "cut in voltage" the VSR will engage and charge both batteries.
This diagram shows how you can use a dual sense VSR for charging both the starter and the house battery.
For more information on our dual sense VSR click here