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What is a VSR and can I use it to charge my battery?

We receive questions almost daily about whether a Voltage Sensitive Relay (VSR) can be used to charge an auxiliary battery or whether a DC to DC charger is required.

To answer this properly, it’s important to outline the 3 general types of alternators used, followed by how a VSR works and then talk about what is required for 100% state of charge when using the alternator as your primary power source, whilst also looking after battery longevity.  

It should be clearly understood that most deep cycle batteries (think AGM or LiFePO4 which covers the large percentage of deep cycle batteries used these days in recreational vehicles) will require a final charge voltage at 100% state of charge of 14.4-14.7V. It must slowly increase the terminal voltage from whatever its starting voltage is, up to the finished voltage to give you the best battery longevity and not just force in full voltage all the way through the charging process.

Regulation of current is also an important part of this charging process. As you will see in the below graph, a typical Constant Current (CC)/Constant Voltage (CV) 3 stage charge profile holds a set current rate (sized correctly for the battery bank) whilst slowly and safely increasing terminal voltage. It doesn’t just ram in fixed voltage like an alternator would.

Types of Alternators

Fixed Voltage Alternators
Typically used in all vehicles made prior to around year 2000. These alternators have a constant voltage output of around 14.0-14.2V regardless of battery state of charge or electrical load on the vehicle. The alternator continues to produce over 14.0V regardless of the electrical load. It will put 14.0V into your battery/s regardless of whether the battery is ready to take that voltage level or not.

Temperature Compensating Alternators
Started being used post 2000 with the introduction of common rail diesel engines (but not exclusively these) and are very common in Toyota vehicles (Hilux, 70 & 200 series cruisers etc). They generally have a MAX output of about 14.0V when cold, but as the engine and hence alternator heat up, they reduce the voltage output and this can drop as low as 13.0-13.2V meaning you will never get your aux battery charged past about 60-70% state of charge.

Variable Voltage or ECU Controlled Alternators
Commonly referred to as “Smart Alternators”, these were introduced around 2010-2011 as a response by some vehicle manufacturers to help meet reduced emissions standards and provide better fuel economy. These alternators are controlled by the ECU to reduce the output from the Alternator once the starting battery voltage is back up to full charge after starting the vehicle. Once that voltage is reached, the output voltage of the alternator is significantly reduced (below 12.7V) and as such is too low to activate a VSR.

Now, what is a VSR and how does it work?

Firstly, what is a relay? A relay is a switching device that is used to either isolate an electrical circuit or change the operating state from active to inactive. Used extensively in automotive applications for things like Dual battery setups, spotlights and light bars, winches etc and many of your factory fitted electrical devices light indicators, headlights etc. The relay is generally activated by an external switch or timer. Using an electromagnet, the relay turns on and off depending on what the switch is doing.

A Voltage sensitive relay uses an internal switching mechanism based on the voltage signal it is receiving from the starting battery. It is installed between the start battery and an auxiliary battery as a way of automatically connecting or isolating the two batteries based on a factory pre-set voltage range on the VSR. For the majority of VSR’s on the market, it will sit in an isolated state until it reads 13.2V from the start battery for more than 5 seconds (this occurs shortly after turning on the engine) then it completes the circuit. This has the same effect of connecting the two batteries in parallel, allowing the flow of volts and amps between the two batteries. The relay will stay in this active state until such time as the relay reads the voltage of the starting battery has dropped below 12.7V for around 10 secs, then isolates the batteries from each other. It does this as it assumes the engine is now off and that no further power is coming from the alternator. This ensures the loads on the auxiliary battery cannot run down the start battery when the engine isn’t running.

VSR’s were developed in the days before DCDC chargers and when most vehicles had fixed output alternators producing over 14V, plus the majority of auxiliary batteries were also flooded lead acid batteries similar to the start battery chemistry and safe to connect these in parallel assuming similar sizes and chemistry of batteries. Most people these days use AGM or LiFePO4 batteries which need to be charged differently to a flooded battery to correctly charge them and to get the best longevity.

With the introduction of Temperature compensating and Variable Voltage Alternators, using a VSR to charge your battery will never reach a full state of charge in your battery with most only ever receiving about 70-80% and in many instances, especially if the battery is not in the engine bay, won’t provide any increase in charge whatsoever to your battery. This is due to the reduced alternator output controlled by the ECU and/or due to excessive voltage drop over distance to the canopy, rear of wagon or even worse for caravans/campers etc. We see many vehicles trying to use this setup only to find they get less than 12.5V coming out the other end which does nothing to charge the battery by more than about 50%.

So do I need one?

These days with most modern vehicles, VSR’s really should only be used for things like load disconnects, providing a power source to a 3 way fridge running on DC power in a towed vehicle (only when the engine is running) or linking two batteries together of the same size/type for winching purposes.

Any circuit that includes an auxiliary battery that needs to be charged from an alternator source needs a DCDC charger and not a VSR. The only real exception is vehicles with fixed output alternators that provide a constant output voltage over 14.2V and with a similar or same battery to the cranking battery. In this instance a DCDC charger is still recommend for best charging performance and longevity, but you can get away with a VSR only as a low cost dual battery setup, but understand this will affect longevity of the battery as it is constantly force fed by the alternator with no voltage or current regulation. If you have a newer vehicle, run AGM or LiFePO4 batteries or any setup that is outside of the engine bay, then a DCDC charger is required instead of a VSR.

It is really important to understand that a VSR is “NOT” a battery charger. It is simply a switch that allows you to connect two batteries in parallel based on a pre-set voltage range. 

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