Voltage sensing split charge relay FAQs

Sometimes we get a few queries on the some components and their features. Below are some of the more popular ones we get on our Voltage sensing split charge relays. 

Is the red light in the top of the relay supposed to be on when the engine is running, or is this a fault light?

The red light indicates that the relay is in it's "engaged state" which means that the relay is currently closed and the current is free to flow from one battery to the other. (think of it like a draw bridge coming down and allowing the traffic to move from one side to the other). This will be because one or both batteries has a high enough voltage to engage the relay. 

What voltage does the relay operate at?

This relay will engage once one of the two batteries reaches approximately 13.8V, a charge voltage, and then it will disengage once the batteries drop below 12.7V approx. (See here for more information on using voltage to read your leisure battery). 

How do I know if it's charging?

As long as you can see the red light on the relay, the relay is closed, meaning that the charge can go from the main battery to the leisure battery. For a more in-depth read on charging, see this article.

My engine is off, but the red light is still on, is this normal?

There are 3 reasons why the relay might be engaged even with the engine off; 

  1. Both batteries are above 12.7V, the relay will stay engaged here until one of the batteries drop below this level. Once one battery is below 12.7V, the relay will open, isolating the batteries.

  2. Your leisure battery is on a mains charge. As the voltage sensing relays are "dual sensing", meaning that they operate the same in both directions, if the leisure battery is on charge from the main, the increased voltage will eventually close the relay, allowing the leisure battery charger to charge the main battery too.

    In this situation, it's very normal to hear some on/off clicking for a short while as the leisure battery chargers often aren't powerful enough to keep the voltage high initially when the relay first engages.

    Eventually the trickle effect into the main battery will lift the voltage high enough for the relay to stay connected, leaving the mains charger to charge both batteries.

  3.  Your leisure battery is being charged from your solar set up. Exactly as for point number 2, but instead of the mains charger lifting the voltage it's a solar charger. In some cases it can be a combination of the two. 

The relay is clicking intermittently whilst my engine is running? 

The relay clicking intermittently can be one of two things. If it's very fast on-off-on maybe a couple of times per second then it's usually caused by the thin black earth wire coming from the relay not having a good earth connection to the chassis of the vehicle. This connection is what the relay uses to read against the positive voltage. So without a good connection, the relay thinks the input voltage is low and switches off. Click here to find out how to make a good earth connection to the chassis. 

The second option would be that the vehicle is fitted with a smart alternator, more information on that here.   With a smart alternator they are typically characterised by a variable voltage charging rate rendering the voltage sensing relays unable to determine whether the engine is running or not. In this case, you will need to replace the relay with a battery to battery charger (DC-DC charger). 

If you have any more questions on the voltage sensing split charge relay, please let us know and we can answer them and add them to this list to help future customers out too! 

Thanks for reading, have a great day!