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How to: Install a relay to charge the battery (Split charge)

When your solar panels can’t keep up with your energy usage in a period of bad weather, you can relay on a relay!
A relay is an electromagnetic switch that can close or open circuits in certain conditions, and so with a relay you can connect your car battery/dynamo with your interior battery.
A relay is installed in most campers already, but if you’re making your own van, this article might come in handy!

What do you need?

Why an ON/OFF switch?

In this case we are going to install the relay with a ON/OFF switch. This means that whenever you’re driving you can turn on the relay with the switch.

You can also connect the relay to the ignition, so then the relay will be activated when you start the car.
I myself prefer the switch cause I would like to have control over when the relay gets activated.

Why a switch instead of the ignition?

Because in sunny days the sun gives the battery plenty of charge, and using a relay cost a little bit more fuel because the dynamo has to work harder to produce enough energy. This fuel consumption is not a lot, but if you are traveling a lot it will add up overtime!
Next to this: a switch is also much easier to install!

The relay setup

Below you see the scheme that of how you can install the relay with a simple switch.

Relay numbers 87, 85, 86, 30

As you can see in the scheme there are numbers below the relay, these numbers are also visible on the bottom of the relay itself.
I will try to explain these numbers in the easiest way possible:

Relay number 87 & 30

Wire 87 to the plus of the car battery
Wire 30 to the plus of the interior battery with a fuse in between.

These 2 numbers will be the main circuit from the car battery plus to the interior battery plus IF it’s activated by the switch.
Initially the relay wont let any current trough without this trigger. And this condition is what makes a relay so special! We create this condition/switch with number 85 and 86, but before continuing with these numbers make sure that the minus of both batteries are connected together:

Batteries minus connection

Wire the minus of the car battery with the minus of the interior battery

In the scheme you see that batteries are directly connected to each other on the minus. This might not be necessary if the minus of both batteries are connected to the chassis of the car. If you’re not sure if this is the case, then directly connect the minus together. When this is done you can continue with relay number 85 & 86:

Relay number 85 & 86

Wire 85 to the minus of the car battery
Wire 86 to the plus of the interior battery with the switch in between.

These 2 numbers will create the ground for the relay and is also the port for the switch. Would you connect 85 to the minus and 86 to the plus the relay will be activated! But we of coarse don’t want the relay to be always activated, so we have to add a switch on the wire that goes from 86 to the plus of the interior battery:

 

Activation relay switch

Finding a place for the switch

It’s obvious that you place the switch somewhere in the drivers area. But before drilling first check if your chosen spot is easily accessible from behind the dashboard. Fiddle the wire from the interior battery to the back of the chosen spot. If you managed to get it there you can start drilling!

Wiring the switch

Install the switch on the cable that goes from 86 to the plus of the interior battery

The switch has 2 connection points, it does not matter which connection point you connect to which end of the cables, the switch will always work.
And you’re done! Unless your switch has 3 connection points then the switch has a builded in light:

Switch with builded in light / 3 connection points

Connect the third connection point to the minus of the interior battery

So you got a luxurious switch with a builded in light! This means you have a third connection point.
This extra connection point is ground for the light in the switch, and so you need to connect this connection point with the minus of the interior battery.
If you don’t know which of the 3 connection point is the minus, you can easily test this by putting the switch on its ON position, and then connecting one plus wire end to a connection point, and then touch the other connection points with the other plus wire end. You will hear the relay tick(activating) when you’re touching the right connection point. Put the switch on its OFF position, connect the other plus end and then connect the minus wire to the third connection point.
The relay scheme with a switch with an builded in light:

Connect an external light to the switch.

If you have a simple switch that does not have a light inside you might want to add an external light to show when the relay is activated. Check the scheme below

Connect a wire on the same port of the simple switch that is wired to 86, this wire goes to the plus of the light. Then connect a wire to the minus of the light to the minus of the interior battery.

Which cables do you need?

For the relay it is important to use the right cable thickness for the cable that runs between the plus of the car battery > relay > plus of the interior battery. If it’s to thin it will constantly burn the fuse or even worse burn the cable itself. To find out which cable you need you need to do a little research of your camper and use the table below!

Check Dynamo/Alternator ampere

For the cable thickness you need to do a little research into your own van/camper. You need to know how much ampere your dynamo/alternator produces. You can find this in the mechanical manual of the car, and sometimes on the dynamo itself. Most often it’s not more than 80 ampere, but better be sure and do the research!

The relay that i use in this article is for dynamos of 120 ampere or less, so this will fit any van. But if you are on a tight budget you can take a smaller relay that is the same amperage as your dynamo.

Check how long the cable has to be

Measure the distance between the plus of the car battery to the relay and from the relay to the plus of the interior battery.

Check the table below

Now you know the ampere of your dynamo and the needed length of the cable you can check the table below.
For example: My Citroën C25 has an 50 ampere dynamo and the length of cable i need is 2 meter. Looking at the table I need a cable diameter of 10mm.

Ampere dynamo >

< Cable Length

12
16
20
25
32
50
63
80
105
125
155
1
1
1.5
2.5
4
6
10
16
25
35
50
70
2
2.5
2.5
4
4
6
10
16
25
35
50
70
3
4
4
6
6
10
16
16
25
35
50
70
4
4
6
6
10
10
16
25
25
35
50
70
5
6
6
10
10
16
25
25
35
50
50
70
6
6
10
10
16
16
25
25
35
50
70
70

Which fuse do i need?

This is actually quite simple, the fuse is relative to the thickness of the cable, and so how much current runs trough it. Or in short: your dynamo amperage.
If your dynamo amperage is 80 ampere and the cables running from the relay to the plus of the interior battery aswel then you need a 80 ampere fuse.

And thats it! You are now ready for the cold seasons! Right?

If you’re not planning to drive a lot in your van in the cold seasons, but still living in it, the relay might not be enough to keep your batteries full. In this case we wrote you an article:

Where you can find power sockets in public.

Other schemes you found are different?

The funny thing about relays is that however you connect the cables, the relay will always work! So at other schemes you might have seen that 85 is being used for the switch instead of 86, and yes that is also correct! Installing relays might appear very difficult but is actually really easy!