Auxiliary Battery Charging With Smart Alternator

Author: JRAutofix  Date Posted:31 March 2021 

New Vehicles Technology Can Affect Dual Battery System's How does a smart alternator affect my dual battery system. What equipment should I use to charge my dual battery ...

Dual battery solutionAuxiliary battery are commonly used for camping, various vehicle applications or even used as a back up for the starting battery.

It's always convenient and reassuring to have an extra power storage specially for long trip away.

Now it's important to set it up right that you are able to charge them and keep your main battery alway topped up.

It comes down to a few different things like how often you will be using the battery, how do you want to charge it and which charging solution you would like to use.

But first thing to check is what type of alternator do you have as vehicle manufacturers are having to comply with environmental regulations 
and have develloped new technologies to lower fuel consumption and reduce emissions for this reason new vehicles come with "smart" ECU-controlled alternator.

So for those of us who want to charge a second auxiliary battery whilst driving this presents a problem because traditional Voltage sensitive relay or isolator kit
cannot work automatically with a smart alternator. 


Voltage switch relay


Typically they engage to connect the starter and auxiliary battery together at around 13.7V and disengage to separate them at around 12.8V, 
which in vehicles with traditional alternators coincides with engine start-up and shut-down.  
This is ideal as it makes sure that the auxiliary battery is charging whenever the engine is running. 

However, with a smart alternator, when the output voltage drops to below 12.8V a VSR would disengage, 
meaning that the auxiliary battery would have significant periods where it wouldn't be charging.

How to find out if you have a smart alternator on your car:
- Check the car manual
- Check the voltage your readings are around 14.4V DC then then you most likely have the traditional, non-smart alternator. 
  If your reading is closer to 12.5-13.5V then it's likely that you have a smart alternator.

Smart alternators have their output voltage controlled externally via the Engine Control Unit (ECU) rather than by an internal voltage  regulator as found on traditional alternators. 


The reason why manufacturer have replaced traditional alternator is:
Smart alternators are ECU-controlled it enables manufacturers to vary the voltage output more than can be achieved with an internal regulator. 
This provides the opportunity to bring the voltage below 13.8V during periods when no further charging is required (e.g. when the battery is nearly full) 
meaning that engine loads are reduced, fuel consumption is reduced and emissions are lowered, all helping manufacturers to comply with industry environmental regulations.

So how do I charge my auxiliary battery with a smart Alternator

If you have a smart alternator then you will need to use a Battery-To-Battery charger (also known as a DC-DC charger) to ensure that you can charge 
your auxiliary battery effectively. Battery-To-Battery (or DC to DC) chargers take the highly variable voltage output from a smart alternator 
and boost or reduce it to maintain a stable voltage output according to a multi-stage charging profile (in the same way as a mains charger does) providing a safe, 
controlled and fast charge for your auxiliary battery.

They are easy to install and, just like a VSR, are simply connected between the positive terminals of your starter and auxiliary batteries, 
together with a negative connection to the vehicle chassis or starter battery-ve terminal.

Benefits of a DC to DC charger:

As in a split charge system, the starter and auxiliary batteries are electrically isolated when the engine is not running to ensure that one cannot discharge the main battery.

A solar panel can be added to the charger

Battery-To-Battery charger uses a multi-stage profile to maximise the depth of charge


Tip: Alternatively a VSR (Voltage switch relay) can be used with a smart alternator in a manual mode with a remote switch to control the relay.


Ready for your next adventure? Check out the new 20 Amp DC to DC Charger MB3940 now!

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