How does a voltage sensitive relay work?

A voltage-sensitive relay (VSR), also known as a split-charging relay, is an electrical device designed to automatically manage the charging and isolation of multiple batteries in a vehicle or marine application.

Its primary function is to ensure that auxiliary or secondary batteries are charged when the primary or starting battery reaches a sufficient voltage level and to isolate them when the vehicle’s engine is not running or when the voltage drops below a certain threshold.

Here’s how a voltage-sensitive relay typically works:

1. Charging: When the vehicle’s engine is running and the alternator is generating electricity, the voltage-sensitive relay monitors the voltage of the primary (starting) battery. Once the voltage of the primary battery reaches a preset threshold (usually around 13.4 to 12.9 volts), the relay engages or closes its contacts. This connection allows the alternator’s output to charge both the primary and auxiliary batteries simultaneously.
2. Isolation: When the engine is turned off and the voltage of the primary battery drops below the preset threshold, the voltage-sensitive relay disengages or opens its contacts. This isolation prevents the auxiliary batteries from draining the primary battery when the engine is not running, ensuring that there is enough power to start the vehicle when needed.

Voltage sensitive relays are commonly used in applications such as recreational vehicles (RVs), boats, campers, UTV’s and other vehicles with multiple batteries. They provide a simple and automatic way to manage the charging and isolation of batteries without the need for manual intervention.