All you need to know about relays, converters and boosters for campers

Are you thinking of installing a system to charge your auxiliary battery while you are on the move? This solution is very useful to avoid relying solely on the sun and campsite plugs. Especially during the winter, as the days are shorter, the sky is often more cloudy and many campsites are closed… so don’t think twice! Improve the autonomy of your van and install a relay, a converter or a booster to keep your battery charged. If you want to find out which is the best option for you, be sure to read our article.

Comparison of devices for charging the auxiliary battery from the main battery

	Cheaper		Our favorite
Device	Non-automatic or “manual” relay	Automatic relay	Converter	Converter Booster
Price
Installation	Needs connection to the alternator	No alternator connection required	No alternator connection required	No alternator connection required
Charging capacity	Up to 14.7 Ah	Up to 14.7 Ah	10 – 40 Ah	25 – 100 Ah
Operating voltage	12 / 24 V	12 / 24 V	Specific models :12 | 12 V12 | 24 V24 | 12 V24 | 24 V	12 / 24 V same device
Stable output voltage	No	No	Yes	Yes
Step load	No	No	Yes (programmable models)	Yes (programmable models)
Loading in both directions	No	Yes	No	No
Dimensions	Small46 x 46 x 46 x 80 mm approx	Small46 x 46 x 46 x 80 mm approx	Medium130 x 186 x 70 mm approx	Large213 x 120 x 120 x 30 mm approx

	Cheaper
Device	Non-automatic or “manual” relay
Price
Installation	Needs connection to alternator
Charging capacity	Up to 14,7 Ah
Working voltage	12 / 24 V
Stable output voltage	No
Step charge	No
Load in both directions	No
Size	Small46 x 46 x 46 x 80 mm approx

Device	Automatic relay
Price
Installation	No connection to alternator required
Charging capacity	Up to 14,7 Ah
Working voltage	12 / 24 V
Stable output voltage	No
Staged charging	No
Charging in both directions	Yes
Size	Small46 x 46 x 80 mm approx

	Our recommendation
Device	Converter
Price
Installation	Needs connection to the alternator
Charging capacity	10 – 40 Ah
Working voltage	Specific models :12 | 12 V12 | 24 V24 | 12 V24 | 24 V
Stable output voltage	Yes
Staged charging	Yes (programmable models)
Bi-directional charging	No
Size	Medium130 x 186 x 70 mm approx

Device	Booster Converter
Price
Installation	Needs connection to the alternator
Charging capacity	25 – 100 Ah
Working voltage	12 / 24 V same device
Output voltage stable	Yes
Staged charging	Yes (programmable models)
Charging in both directions	No
Size	Large213 x 120 x 30 mm approx

Systems for charging the auxiliary battery on the road from the main battery

As mentioned above, having a way for the van’s secondary battery to recharge while you’re on the road will give you extra range and peace of mind. Imagine you’re in the middle of nowhere, on a dark winter’s day, worrying about whether your battery level will hold out until you find somewhere to plug in your charger…. Being able to turn on the van to charge the battery or have it recharge while you’re moving around can get you out of a pretty big jam…

So, without further ado, these are the devices that will allow you to recharge your auxiliary battery from the main battery or the alternator of your vehicle. With the table you will already have an idea, but we will now tell you the main characteristics, how they are connected and the advantages and disadvantages of each one of them.

How does a non-automatic or “manual” battery separator relay work?

To understand it in a simple way, we could say that a battery separator relay works as a switch that connects or disconnects the two batteries in parallel, depending on the fulfillment of a certain condition. If both batteries were connected in parallel, without a relay or other device to disconnect them, the charging of the main battery and the lifetime of both batteries would be put at risk.

Wiring diagram of a non-automatic or “manual” relay

As can be seen in the diagram, a non-automatic battery separator relay has 4 connection pins. These pins are numbered marked with four numbers: 30, 85, 86 and 87. Each pin has to be connected in the following way:

• Pin 30: To the positive of the main battery of the vehicle.
• Pin 85: To ground or to the chassis of the fugoneta by means of a screw.
• Pin 86: To a source of 12V or to the alternator. It is advisable to insert a switch here.
• Pin 87: To the positive of the auxiliary battery.

The non-automatic relay will be activated, and in turn the batteries will be connected, if current arrives to its pin 86. This pin can be connected either to the alternator or to a device of the vehicle that works when this one is in march, for example to a cigarette lighter socket. When the engine is switched off, the relay will be deactivated, since pin 86 will stop receiving electricity, the circuit will be open and the batteries will be disconnected from each other.

Buying a non-automatic or “manual” battery separator relay on Amazon

What is an automatic battery separator relay?

The main difference between an automatic and a non-automatic battery separator relay lies in the way each is activated and deactivated. While a non-automatic relay needs an external source to activate it, an automatic relay will be activated if it detects a voltage higher than its turn-on voltage which, depending on the relay, is usually between 13 and 13.8V and will be disconnected when the voltage falls below this value.

Wiring diagram of an automatic relay

As can be seen in the diagram above, an automatic relay will generally have 3 connection pins numbered 30, 86 and 87. There are some relay models that, in addition, have a fourth pin marked as start assist or pin 85. This pin allows the relay to be operated manually, by means of a push button or switch, and is used toconnect the batteries in case of emergency, either because the main battery is discharged or because it has stopped working.

• Pin 30: To the positive of the main battery of the vehicle.
• Pin 86: To the negative of the main battery, mass or to the chassis of the fugoneta by means of a screw.
• Pin 87: To the positive of the auxiliary battery.
• Pin 85 or Start assist: To the positive of the auxiliary battery, with an intercalated push button or switch.

Buy an automatic battery separator relay from Amazon

What is a DC-DC converter?

DC converters are devices that receive a DC current at a certain voltage and deliver it at a different voltage. The input voltage may vary within a specific range depending on the converter, while the output voltage is stable.

This is very useful when charging our secondary battery from the vehicle’s alternator since the alternator does not deliver an adequate current and cannot perform an optimal battery charging due to these reasons:

• The current generated by the alternator does not have a stable voltage.
• The voltage it delivers is usually very high, especially for charging gel batteries.
• Obviously, this voltage does not vary adapting to the different phases of load.

That is why the relay, although it is the most used option by the users of campers due to its low cost, is not at all the best option

On the other hand, in the market there are models of intelligent and programmable DC-DC converters that adapt the output voltage according to the charging phase in which the battery is, to get it to charge optimally, extending its life, so that, although a converter is more expensive, in the end it will be worth the investment because you will be taking care of your battery.

For all these reasons, we believe it is the best option to charge the auxiliary battery from the main battery and therefore we decided to install an Orion Tr-Smart 12|12 30A (see on Amazon).

Below is an example of a simple installation diagram:

Buy a DC converter on Amazon

What are the features of a Buck-Boost converter?

After searching and rummaging through the internet, to understand why buck-boost converters are so extremely expensive with respect to “normal” converters, we have come to the following conclusion, if you think we are wrong, please send us a message clarifying 😅:

With a single buck-boost converter you can convert different voltages to different voltages. It’s a bit of a mess to explain but we’ll try: While a normal converter can only convert from 12|12, 12|24, 24|12, etc. depending on the model you buy, a single buck boost device can be configured as needed.

Final opinions and conclusions about battery separator relays, dc converters and buck-boost converters

Before deciding with which type of device to charge the auxiliary battery from the alternator we took stock of the features offered by each type of device, their price and the functionalities we would get with each of them and these were our conclusions about each of them:

Conclusions of the use of battery separator relays

• They are a good option if you have a very tight budget. Always installing a switch to be able to disconnect it manually when the auxiliary battery is full or when turning off the vehicle.

• Never use them to charge gel batteries, since these need a much lower charge voltage.

Conclusions on the use of DC converters

• This was our choice. It seems to us that its price, although higher than that of a relay, if you do not have an excessively big battery does not go up too much. It performs optimal charging, thanks to its phased charging algorithm and adjusts the voltage by selecting the type of your auxiliary battery. Therefore, we believe it is well worth the price as the battery life is extended

• The only drawback we find with this device is that it is much larger than a relay, so it will be one more device that will take up space in the place where you do the installation.

Conclusions of the use of DC converters

• It is not that we believe that this device is not worth the cost, but it is true that, for our use case, we did not think it was worth the investment. If in your case, you have a comfortable budget and you think you can take advantage of the ability to change the voltage settings, it is a very good option.

thank you very much for having made it this far! We hope this article has been useful for you and, as always, if you have any questions or suggestions, do not hesitate to leave us a comment 😉