Smart Alternator: Split Charge Relay Won’t Work (For Dual Battery)

Today on Off Grid Power Geek we’ll go in-depth into smart alternators and split charge relays for dual battery systems.

We’ll talk about what smart alternators are, why they had to be introduced, their benefits and drawbacks.

We’ll discuss how they work and how they solve some of the issues that traditional alternators have.

Most importantly, we’ll look at how this affects you. Smart alternators affect your ability to charge your auxiliary battery using split charge relays and VSR. And we’ll tell you what the solution for this is.

smart alternator why split charge relay won't work (Dual battery)

Introducing Smart Alternator

Let’s dive right into smart alternators, and get to the bottom of what they are and how they came to be.

What is a smart alternator?

Smart alternators are becoming more and more popular, and in fact are becoming standard in most new vehicles.

That’s been the case for a few years now, so you may already have one.

While they are effective for their purpose of reducing emissions and lowering fuel consumption, they have an unfortunate consequence for people who want to charge their leisure/auxiliary battery while driving. That’s because split charge systems, relays, isolators of any kind will no longer work.

Why were smart alternators introduced?

Quite simply, vehicle manufacturers have to use them to meet emissions regulations. Specifically, the European emissions standards, which cover many vehicle type, including passenger cars and light duty vehicles. The standards involved are Euro 5 and Euro 6. These standards were the ones that place the most severe restrictions on manufacturers to reduce harmful gas emissions.

As a result of increasingly tough regulations, vehicle makers have to invent and implement new technologies to lessen fuel consumption, and so reduce emissions. ECU-controlled smart alternators are one such technology, and they’re becoming increasingly regularly added to new vehicles.

For example, VW T6s, Mercedes Sprinters, Renault Masters.

Generally speaking, this doesn’t apply to boats. Since the Euro 5 and Euro 6 standards don’t apply to boats, there’s no reason that boat manufacturers to use smart alternators.

They can also be called ECU-controlled alternators, or variable voltage alternators.

They operate at a lower voltage compared to a normal/traditional alternator. The voltage then significantly increases to as much as 17 Volts when the driver takes off. This much higher voltage charges the battery more quickly than normal alternators.

What’s the difference between an alternator and a smart alternator?

Normal alternators’ output voltage is controlled by an internal voltage regulator.

Smart alternators, however, have their output voltage controlled by the Engine Control Unit (ECU), that is externally.

If you have a traditional alternator, you can use a split charge relay/ Voltage Sensitive Relay (VSR) to charge your house battery (though be aware this will not fully charge the battery, and it’s slow).

However, if your vehicle has a smart alternator, you can no longer use a VSR / split charge relay to charge the auxiliary battery in a dual battery system. We’ll go into the reasons for this later.

How do smart alternators work?

Let’s first see how any alternator works.

An alternator in operation produces a charging voltage, which generates an electromagnetic field within the alternator. This, in turn, exerts a mechanical load on the engine through the alternator drive belt.

As output voltage increases, the load on the engine increases. Therefore, the engine has to work harder to rotate the alternator, and in order to do so it uses up more fuel.

Normal alternators maintain a voltage of around 13.7 – 14.5V. It’s different for each battery, as it depends on the health of the battery (SoH, or State of Health), the current charge level of the battery (SoC, or State of Charge), the condition of the battery terminals, wiring and various other components.

The problem with normal alternators

Due to this relatively high voltage, which is constant, the alternator is continually exerting a significant load on the engine, and this uses up more fuel than is necessary.

How do smart alternators lower fuel consumption and emissions?

Since smart alternators are ECU-controlled, the voltage output can be varied significantly more than traditional alternators with an internal regulator.

This allows the voltage to be reduced below 13.8V when charging is longer needed, for example, when the battery is already close to full charge.

As a result, the mechanical load on the engine is lowered, and therefore fuel consumption and emissions are reduced.

The benefits of smart alternators is that they’re better for the environment, and they allow vehicle manufacturers to comply with increasingly stringent European standards.

Dual Battery Problems Caused By Smart Alternators

However, smart alternators do create some problems for people who want to charge their leisure battery while they drive. Here’s why:

1. The leisure battery will not be charged for the majority of driving time.

That’s because a lot of the time the alternator output voltage will be below the voltage level needed to charge the leisure battery. And so, during this time, there will be no charging.

So the auxiliary battery will receive much less charge than it would if the system has a normal alternator.

2. The voltage spikes can seriously harm AGM and Gel batteries.

Both these battery types are widely used by campervan/caravan/motorhome owners, for example. That’s because they have several advantages over traditional Flooded/Wet batteries.

But they’re unsuited for smart alternators. The high voltage bursts that are a key parts of the smart alternator arsenal can cause significant damage to AGM and Gel batteries. Voltages above around 14.4V can cause gas bubbles in the gel electrolyte.

And both these battery types are more susceptible to damage by overcharging.

Smart Alternator: Split Charge Relays / VSRs (Voltage Sensitive Relays) Won’t Work

These relays work by turning on and off at specific voltage thresholds. They turn on, connecting the vehicle battery and auxiliary/leisure battery at approximately 13.7-13.8V.

This connection means that the auxiliary battery is being charged.

The split charge relay ensures that the auxiliary battery is only being charged when the engine is turned on and so is producing a sufficiently high charge to charge the auxiliary battery well and fully.

And they turn off, disconnecting the two batteries at approximately 12.8V. The voltage sensitive relay (VSR) ensures that the two batteries are disconnected (thus stopping the charging of the auxiliary battery).

These voltages are perfect for normal alternators, since these are the voltages produced on engine start and engine shut down, in engines with normal alternators. You only want the house battery charging when the engine running, and you want the charging to stop when the engine stops.

However, when a smart alternator is used, the output voltage will often and for long periods drop below 12.8 Volts even when the engine is running.

Therefore, there will be extended periods of time when the auxiliary battery won’t be charging. Therefore, the split charge relay won’t be working effectively to allow you to charge the battery properly. Some VSRs won’t work at all in this scenario.

The frequent spikes and dips in voltage that results from a smart alternator system cause the split charge relay to turn on and off again repeatedly. They weren’t designed for this and so can be damaged by this process.

Ignition relays won’t solve this problem either – the lower voltage of around 12.3-12.5V is far too low to properly charge your leisure battery.

The Solution – DC to DC Chargers

Fortunately, there is a solution. And that is, the DC to DC charger, also knows as a battery to battery charger or B2B charger.

Currently, this is the only solution that allows you to charge an auxiliary battery properly, if you have a smart alternator. It has a few other advantages as well.

A DC to DC charger provides the healthiest, fullest possible charge from a vehicle battery to a leisure battery. Even if you have a normal alternator that’s the case. For this reason, using a battery to battery charger can extend the lifespan of your batteries (here’s how long leisure batteries last).

It provides the same multi-stage charging process that you’d expect from a high-quality mains charger.

In the past, Voltage Sensitive Relays did a good enough job of charging the auxiliary battery. They’d provide a constant high voltage charge that was enough to cram charge into the battery at a decent rate. We now know that this kind of charge will get your battery to around 80% full, but you need a variety of voltages and amperages to get true depth of charge and fill all the edges of the battery. Multi-stage charging provides this, specifically Float and Absorption charges allow for a full charge.

As we discussed earlier, if you have a smart alternator, a split charge relay is not suitable for auxiliary battery charging because of the voltage dips and spikes that results from smart alternators, which are either too high or too low for charging the auxiliary battery.

These highly variable voltages are no problem for DC to DC chargers.

These DC DC chargers can take the smart alternator’s highly fluctuating output voltage and increase or decrease it as necessary for a stable output voltage. These voltages will be exactly the amount required by the auxiliary battery in its current state – this is multi-stage charging.

What are the advantages of this charge profile? The DC DC charger provides a safe, highly controlled charge for the house battery – and best of all, it’s far quicker than could ever be achieved with a Voltage Sensitive Relay / Split Charge Relay.

If you need more info, we have the ultimate guide to DC to DC chargers.

We also have a review of the best DC DC charger, the CTEK D250SE.

Other advantages of B2B chargers?

AGM and Gel batteries are highly sensitive to variations in voltage. High voltage spikes can seriously impair them, while low voltages simply won’t charge them at all. B2B chargers provide exactly the voltage needed for even these battery types. The safest, fastest and most reliable is the CTEK D250SE, which has specific charging profiles for AGM and Gel batteries, with a customised multi-stage charging process specifically for those batteries.

And the same applies to Lithium leisure batteries (here are the best), which also need a unique charging set up. If you have a Lithium battery, you NEED a DC to DC charger.

Do you have a smart alternator?

We’ve answered it in full with all the ways you can find out: “Does My Van Have A Smart Alternator?

Summing Up Smart Alternator: Split Charge Relay Won’t Work

We explored smart alternators and how they are externally controlled by the ECU, instead of an internal regulator as normal alternators are.

We analysed why they had to be introduced, looking at the environmental regulations, the European emission standards, Euro 5 and Euro 6.

We discussed that even though they are better for the environment in terms of being more fuel efficient and so reducing emissions, they have some drawbacks. Specifically, they hinder our ability to use split charge relays and VSRs to charge auxiliary batteries.

We went over why split charge relays won’t work for dual battery systems, when the vehicle has a smart alternator.

And finally, we discussed the solution to this problem, the DC to DC charger.

Hopefully, this information has helped and you can take the action necessary to solve your charging needs.