Isolated DC-DC or Common ground

[Snelly]

Thanks for all the advice so far about installing my EPever LiFePo battery in my Elddis 175. I've installed an EPever MPPT solar charger which now seems to be working well. I plan to install a Victron ip22 mains charger - yes I know the original BCA acts as a 12v supply.
My question is about isolated or non isolated DC-DC. I have bought a Renogy 40 DC-DC thinking it was isolated as it had 4 terminals, but now found it is not (didn't check the specs closely enough!). I have checked the negative chassis conductivity and I don't think the negative of the hab battery is bonded to the cab chassis - I'm not getting 0 ohms between to two negatives although there is continuity. There is also 12v between the two. Do I need a different DC 2 DC or will connecting the two provide enough and not cause problems? I intend to put a relay into the hab battery +ve to switch off when the ignition is running via the D+.
Any thoughts before I return the unit?
TIA

 

[Snelly]

Just to add - I am planning to run both -ve and +ve cables - 10AWG about 1.5 m each side of the DC-DC which the Blue Sea circuit wizard seems to indicate is adequate (it calculates 12AWG)

 

[LithiumConvert]

Make sure that leisure battery negative is connected to the negative of the starter battery., independantly of the B2B.

I am confused as to the purpose of the relay.

 

[berni109]

As has been said. Make sure batteries have a common ground - nice thick wire between them.
Take the renogy ground/earth/negative to the leisure battery so it's in the right place when you fit a shunt.
I have seen posts on here that suggests the renogy B2B is not the best but you'll have to do a search to find it. Think it was Lenny HB

 

[AdrianChen]

Should be fine - some of these things are designed to be wired directly to both batteries, rather than via the chassis for the negatives as sometimes there can be chassis continuity issues on vehicles where the manufacturers recommended earth points are not used, and I'm sure I've read of this factor being blamed for failed units. As you've found that's not necessarily the same thing as being isolated, which isn't normally needed on a simple van, more for boats/ambulances etc.

Idealy for your unit a four wire connection to the batteries will work as designed, but I bet if the negs were wired to the chassis it would work just as well - either way for warranty purposes I would follow the manuals instructions, but don't think you need to change it, except that I am more a fan of Victron.

Also confused why you want to isolate the hab battery when the engines running?

P.S. had both an isolated and non isolated Victrons on this van both wired to the neg bussbar connected to the chassis - no problems so far.

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[DBK]

As above I'm confused about the role of the relay.

If you have a shunt to monitor the leisure batteries, or are planning to fit one*, don't connect the B2B directly to the battery negative terminal otherwise the battery monitor won't be able to show what charging current is flowing into the batteries. The B2B should be connected to the shunt terminal opposite the one the battery is connected to.

I don't think your issue of the B2B being isolated or not matters.

What do the instructions say? I would just follow those.

* If you don't have a shunt type battery monitor I would strongly advise getting one. They are invaluable for seeing the state of the leisure battery or batteries.

 

[autorouter]

Vehicle alternators all have their negative terminal permanently connected to the engine metalwork, and from there to the vehicle chassis via a beefy earth strap. The starter battery negative is always also connected to the vehicle chassis. That's how the starter battery gets charged by the alternator.

When adding a leisure battery, the traditional charging method is to use a relay to connect the starter battery positive to the leisure battery positive, so that the alternator charges both batteries while the engine is running, and disconnects the link when the engine stops so that the batteries act separately. For that to work, the leisure battery negative needs to be connected to the chassis somehow, to provide a path for the returning charging amps.

The upshot is that the leisure battery negative should already be connected to the chassis, and no isolated input and output is required. That isolation is more for boats, which can be affected by electrolytic corrosion while floating in salt water. Isolated B2Bs should work fine with the two negatives commoned together, or even separately wired to the chassis if you prefer.

 

[autorouter]

Just to add - I am planning to run both -ve and +ve cables - 10AWG about 1.5 m each side of the DC-DC which the Blue Sea circuit wizard seems to indicate is adequate (it calculates 12AWG)

There's a difference between wiring for safety and wiring for best efficiency. 10AWG (= 6mm²) is fine for safety, but a bit minimalist for efficiency, in that a 1.5m run will have a voltage drop of about 3%. That 3% is usually considered the maximum allowable for the whole wiring run (battery positive to battery positive), so if there are other lengths of wire in that path you should allow for those, maybe increasing it to 8AWG (= 8.5mm²) or even 7AWG (= 10mm²)

Remember that the 40A current flow will not just be an occasional peak, it will work at the full 40A output for the whole of the first charging stage, until the battery gets up to about 80% charged.