This article explains how to install a bypass (change-over) relay alongside your DC-DC charger.
We have a separate article that explains the installation of a DC-DC charger here
5-pin “bosch” relay
We’ll be using this 5-pin relay, your relay may differ but the PIN numbers will probably be the same. We’ll get into the detail below but here are what the pins are:
85 & 86 activate the relay (i.e. 12v, in our case from D+)
30 is the common output (in our case it’s output, might be an input for other uses).
87 is connected to 30 when you pass 12v through 85 & 86 (the relay is activated). It’s our input from the leisure battery.
87a is connected to 30 when pins 85 & 86 have no 12v (the relay is not activated). It’s our input from starter battery.
5-pin “bosch” relay
This is what the relay looks like, it’s capable of handling 80a.
You can buy the relay from Amazon here.
Relay plug with wires
We far prefer the relays that come with the plug as shown here.
You can buy the relay from Amazon here.
You will notice that there are three thick (4mm) wires to handle the load (Blue, red and Yellow). There are two thinner wires to activate the relay (yellow and white).
You don’t need to use this plug with wires, using female spade connectors directly on the relay is fine. Just make sure you use quality connectors and it’s a good idea to pinch them closed (just a little bit) with pliers before using them.
We’re using a Victron DC-DC charger in this article but the same applies to any DC-DC charger. We have a separate article explaining the DC-DC charger install, so in this article we’re concentrating on the relay itself.
You’ve probably just bought a Lithium (LiFePO4) battery. Some “experts” tell you to just slap it in and the BMS will protect it. Others have said you must install a DC-DC charger (otherwise known as a battery-to-battery or B2B charger). In this article we assume you’ve decided to install a DC-DC charger (we won’t go into the pros and cons here).
So let’s talk about the relay ….
If you’re active on some forums you may have noticed that opinions vary from “just slap in the DC-DC alongside the split charge system” to “you can’t have two competing systems charging the same battery”. So who do you believe? This is a very contentious subject and we’ll skip the lengthy debate (that’s for another article), but in a nutshell we’ve decided to go with a method to avoid two parallel and potentially conflicting charging systems (the new DC-DC plus original split-charge system).
OK so let’s start with the initial configuration (below) where you have a standard split-charge system as part of your “Electro-block” (which we’ll call the EBL from now on). By Electro-block or EBL we mean the box of electric components with a bunch of wires of different colours (red, black, blue, yellow, bown, etc) coming in and out (some thick-ish and some thin). It probably also has a bunch of blade fuses.
In this next image we install the DC-DC charger.
As you will see we have two parallel and “competing” systems charging the leisure battery. The new DC-DC charger conditions the voltage according to the battery type you have specified but the split charge system simply connects the starter and leisure batteries in parallel.
In this next image we add the Relay as a simple Cut-Off.
It’s a very simple setup in that the relay simply cuts the connection between the starter battery and EBL so there aren’t two competing charging systems. We’ll go into more detail showing the actual wires later - this is a simple schematic to show the basic concept.
In this next (and final) image we’re making the Relay a Change-Over.
We’ll explain in much more detail later, but for now lets just say that the EBL will always have incoming 12v:
From the starter battery when the engine is off
From the leisure (LiFePO4) when the engine is running
Why get this complicated?
Some EBLs don’t mind being totally disconnected so a simple cut-off approach is fine. However, we have dealt with enough motorhomes where a simple cut-off causes problems (usually with the fridge and sometimes automatic retraction of the step). For this reason we take a “Change-Over” approach. Basically the EBL is always powered from both ends (Leisure and Starter batteries), and when the engine is running we power the EBL from the leisure battery.
Let’s get into the wiring details.
We start with the details of the initial split-charge system. There is a wire running from the starter battery to the EBL which is also connected to the leisure battery.
Somewhere built into the EBL (or as a separate item) is a relay, sometimes a standard 4-pin (or 5-pin with pin 87 ignored). Sometimes the relay is built into the logic board and is very difficult to find. The relay simply puts the leisure and starter batteries in parallel - no fancy stuff. It’s as if you connect the two positive posts (terminals) of the batteries using a simple wire.
When we install the relay the picture changes to this:
The relay is essentially installed into the wire running from the starter battery to the EBL. In reality we usually find the incoming +12v wire and disconnect it from the EBL, reconnecting it to pin 87a of the relay. Pin 30 then goes out to the EBL.
Wires on the relay plug
These are the wires in the relay plug, note that the three pins carrying the load are 4mm and the two handling the activation are only 1.5mm
Some more notes on the wiring.
Finding Nemo (aka D+)
In order for the relay to work properly you should find D+ somewhere. Most motorhome EBLs have more than one D+ that you can use. If you’re working on a PVC (panel van conversion) you may have never needed to use D+ for anything (yet).
So what is D+? Basically an alternator needs to have a way to tell the vehicle that it’s generating electricity (i.e. the engine is running), and this is then used for a number of functions in the base vehicle. The way it does this is that it has a second and much thinner wire carrying +12v connected to the alternator, known as the D+. When the engine is not running D+ has nothing, when the alternator starts spinning (ie engine running) it applies +12v to the D+ output.
If you already have D+ close by that’s great. If not, you may have to look for it “under the dash” or you may even need to run a new wire all the way to the alternator.
So how do you find it? There are basically these ways:
Look for it in your documentation. If it’s a motorhome you may find it in the manual - probably the section explaining the electrics. Look at anything to do with the fridge as D+ is pretty essential for it.
Trial and error using a multimeter (or voltmeter). You may need some help here (much easier when there are two of you). If you’re looking in the EBL it will be a thin-ish wire and probably red/white/brown/yellow. Check which wires have +12v when the engine is running and no +12v when it’s not . Actually, start by identifying a few possibilities and check with engine off, and those that are “dead” check again with engine running. Then make sure it’s coming alive when the engine is actually running and not just when the ignition is on.
Go all the way back to the alternator (only do this if the other options don’t work).
Once you have found D+ you will need to splice into it. We find Wago connectors to be the best option. You will need to cut the existing D+ wire that you find and install a 3-way Wago.
What if you have an external Split-Charge Relay?
…then you’re lucky! Perhaps you can simply remove the relay. We find this to be the case with a number of Sargent systems.
Can’t I just pull a fuse out somewhere?
If you have one, yes. Again, with some of the Sargent systems simply pulling a fuse works fine.
Do I need to install fuses?
Yes. It’s like jackets and boots - can never have too many of them. Jokes aside, you’re better off erring on the side of caution and installing too many fuses rather than too few. Fuses should be closer to where the current originates. Make sure they are places where they can be inspected and replaced. Please make sure you use good quality and the correct rating! Fuses are like tyres - don’t buy the cheapest. If the wire melts before your fuse blows you may have a major problem.
COMPONENT & TOOL LIST
Here’s the list of components & tools that we would recommend for a DC to DC Charger installation:
CONCLUSION
Well, that concludes the installation of your DC-DC charger. We hope this article is of help to you, and we’d love to hear from you! If you have any questions or would like to get in touch with us about installing a DC to DC charger in your motorhome, please get in touch.