Voltage Sensitive Relays (VSR)

A switch that watches voltage: it joins the second battery while the alternator is charging and separates them when it stops.

Hardkorr 50A 12V Dual Battery Voltage Sensitive Relay

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Voltage Sensitive Relays (VSR)

Overview

Voltage Sensitive Relays (VSR) Overview

About Voltage Sensitive Relays (VSR)

A voltage sensitive relay is a switch that watches voltage.

That is all it does, and for a lot of setups that is enough. When the starting battery rises above a set voltage, which in practice means the alternator is charging it, the relay closes and the auxiliary battery joins the circuit and takes a charge. When the engine stops and the voltage drops, the relay opens and the two batteries are separate again. The fridge runs the auxiliary flat and the starter is untouched.

It is the simplest way to run a second battery, and it is the cheapest. No charge profile, no display, no settings. It still needs cable sized to the current, protection at both battery ends and an installation that follows the relay's own instructions.

The limit is that it passes on whatever the alternator gives. On an older vehicle with a fixed-output alternator feeding an AGM, that is usually fine. On a modern vehicle with a smart alternator that drops its voltage to save fuel, the auxiliary never sees a full charge. And on a lithium battery, which wants a specific charge profile and a controlled current, a relay is the wrong tool.

For those, a DC to DC charger is the answer. For a straightforward AGM second battery on a vehicle that charges properly, a relay does the job for a fraction of the cost.

FAQs

Voltage Sensitive Relays (VSR) FAQs

VSR vs DC-DC charger: Which is better for a dual battery system?

A VSR is a simple and cost effective way to charge an auxiliary battery by linking it to the starter battery once the engine is running. While reliable for older vehicles, a VSR does not manage the charge profile or boost voltage, which means it often fails to reach a true 100 percent state of charge in deep cycle batteries.

A DC-DC charger is the superior choice for modern touring because it uses multi stage charging to maximize battery health and lifespan. It also manages the different voltage requirements of modern alternators and varied battery chemistries. While a VSR is a practical entry point for simple setups, travelers seeking maximum performance and battery longevity usually prefer the smart charging capabilities of a DC-DC unit.

Explore our selection of DCDC Chargers for a more advanced setup, or see our complete Dual Battery Systems to find a match for your vehicle.

Can I use a VSR with a smart alternator?

Standard Voltage Sensitive Relays are generally not compatible with modern smart alternators. These alternators are designed to lower their voltage output once the starter battery is full to save fuel and reduce engine load. This drop in voltage often falls below the cut in threshold of the relay, causing the VSR to disconnect your auxiliary battery even while the engine is running.

If the relay stays open, your secondary battery will not receive a charge during your drive, which can leave you without power for your fridge or lights at camp. To solve this issue, most travelers with newer vehicles switch to a charger that can detect an ignition signal or handle lower input voltages. This manages the charging process correctly and makes certain your storage is restored regardless of the alternator behavior.

Check out our selection of DCDC Chargers which are specifically designed to work with smart alternators and provide a full charge to your second battery.

Can you use a Voltage Sensitive Relay with a Lithium (LiFePO4) battery?

Standard VSR units are not suitable for lithium batteries. These batteries require a specific charging profile and precise voltage control that a basic mechanical relay cannot provide.

Lithium cells maintain a higher resting voltage compared to traditional lead acid batteries. This often prevents a VSR from cutting out properly, which can lead to your auxiliary battery draining your starting battery while the vehicle is parked. Using a dedicated charger is a much better way to limit current flow and protect your alternator while providing the correct voltage levels to maintain battery health over thousands of cycles.

Explore our range of Lithium Batteries to find a match for your rig, or see our DCDC Chargers for a compatible charging solution.

Do I need a fuse on both sides of a Voltage Sensitive Relay?

Yes, you must install a fuse on both sides of a Voltage Sensitive Relay. This is a critical safety step because the relay connects two separate power sources, your starter battery and your auxiliary battery.

If the cable connecting these two batteries gets damaged or shorts out against the vehicle chassis, both batteries will immediately dump their full current into the fault. Without a fuse located as close as possible to each battery terminal, this can lead to an electrical fire. Using the correct fuse rating for your cable size protects your wiring and prevents a minor short from becoming a total system failure.

Explore our range of electrical accessories to find the right safety gear for your project.