isolated vs. non isolated DC to DC charger

[serpa4]

I'm going to install a DC to DC charger between house and start batteries. I have lead start and lithium house. The PO has a ACR which is bi directional and this the lithium is keeping the relay connected.
Are the house and start batteries share a common ground? I have only had my 26 venture for week and have not learned about it yet.
If they are connected by a common ground, then I need a non isolated charger. If they have different grounds, then I'll get e isolated charger .

 

[pcg]

I can't speak to how your specific boat is wired, but I can say that all DC grounds and AC safety ground should be connected together without any ground loops. I would be very surprised if your boat was wired otherwise. You are correct in assuming that you need a DC-DC converter between the FLA start battery and Li house battery and not that ACR, and also that you will need a non-isolated DC-DC converter.

 

[jkidd]

I removed my ACR and install an Isolated DC to DC Charger. This is from Victrons web site.

The non-isolated DC-DC converter there is a common ground connection (negative) between the two voltages

In short, an isolated power converter isolates the input from the output by electrically and physically separating the circuit into two sections preventing direct current flow between input and output, typically achieved by using a transformer. Isolated DC-DC converters are also useful for breaking up ground loops, thus separating parts of a circuit that are sensitive to noise from the sources of that noise. Another feature of an isolated DC-DC converter is a floating output.

The choice between isolated and non-isolated converters depends on many factors. Some applications require isolation for safety reasons, and others may benefit from a floating output by breaking up ground loops or shifting of reference voltages. However, where isolation is not required, a non-isolated converter may provide a decrease in cost, size, and/or an increase in efficiency.

I would download the manual for what ever brand of DC to DC charger you choose and use their wiring diagrams to make your connections.

 

[pcg]

I removed my ACR and install an Isolated DC to DC Charger. This is from Victrons web site.

The non-isolated DC-DC converter there is a common ground connection (negative) between the two voltages

In short, an isolated power converter isolates the input from the output by electrically and physically separating the circuit into two sections preventing direct current flow between input and output, typically achieved by using a transformer. Isolated DC-DC converters are also useful for breaking up ground loops, thus separating parts of a circuit that are sensitive to noise from the sources of that noise. Another feature of an isolated DC-DC converter is a floating output.

The choice between isolated and non-isolated converters depends on many factors. Some applications require isolation for safety reasons, and others may benefit from a floating output by breaking up ground loops or shifting of reference voltages. However, where isolation is not required, a non-isolated converter may provide a decrease in cost, size, and/or an increase in efficiency.

I would download the manual for what ever brand of DC to DC charger you choose and use their wiring diagrams to make your connections.

I'm confused. If you use an isolated DC-DC charger and you then connect all DC grounds, then you no longer have an isolated system. No harm done otherwise. Are you not connecting house battery ground to starter battery ground? No problem with doing it that way and it will certainly eliminate any ground loops that would otherwise be there. If you are careful to not have ground loops though, then, as you state, you can save money and buy a non-isolated converter.

Beware though, if you do not connect the house and start battery grounds and your Li battery should shut down you will not be able to use the start battery as an emergency backup (you would need a battery switch for this) to power house circuits (chart plotter, radar, etc.) because there will be no ground path from DC circuits back to the house battery.

 

[jkidd]

Victron says it is not a problem with common negative from their website:
no problem in using a isolated Orion Tr smart together with common negative. It's a 'must" with isolated grounds and a 'can' with common ground.

At the time all I could find was the Isolated. This is a simplified diagram of the wiring. My start battery and house batteries cannot be combined and I have a jump starter for emergencies.

 

[serpa4]

I guess what I meant to ask is if the C-Dory (venter 26) has the house and start battery to a common ground.
Yes: I'll buy non-isolated DC to DC.
No: I'll buy isolated DC to DC.

Sorry for the confusion.

 

[jkidd]

No better time to go out and trace the wires then you will know for sure.

 

[serpa4]

No better time to go out and trace the wires then you will know for sure.

Not always that easy. :roll:
Need to order parts now so when I'm not doing 14hr days, 6 days in a row, not getting off work till it's already dark and the storage lot is closed.
If someone already knew, then great.

 

[pcg]

Just take an ohmmeter and measure the resistance between the negative terminals of the two batteries. Resistance should be less than one ohm if battery grounds are connected. To make sure you don't damage your voltmeter make sure you measure the voltage between the ground terminals first to make sure it's zero. This is in case you DO have an isolated system and there's something funny going on.

 

[serpa4]

Just take an ohmmeter and measure the resistance between the negative terminals of the two batteries. Resistance should be less than one ohm if battery grounds are connected.

Roger, thanks. Will try it.

 

[pcg]

Roger, thanks. Will try it.

See the note of caution I added above, while you were responding.

 

[Aurelia]

I have had good success with these small waterproof combiners on multiple boats and it looks like they have a lithium version that might be worth looking at for your start to house charging need. I don't see exactly how the lithium version works differently than the standard, but a quick email would likely clear that up. https://yandina.com/products/battery-co ... -batteries

 

[smckean (Tosca)]

One word of warning. I am not 100% confident that I know what I'm talking about here ( :wink: ), but I don't think one is supposed to interconnect the DC system and the AC system grounds. Both systems are totally independent. Note: I'm confused about how inverters and chargers fit into that principal.

 

[pcg]

I don't think one is supposed to interconnect the DC system and the AC system grounds

DC system ground and AC safety ground should always be connected. Here's a hypothetical scenario that illustrates why... Suppose these grounds are not connected and you're running a roof mounted air conditioner off of 120VAC and a hot wire makes contact with the metal housing of a ceiling light. That metal housing is connected to DC ground. Now every piece of metal in your boat that's connected to DC ground is energized with 120VAC waiting to find a path to AC safety ground. And what's connected to AC safety ground?... all the metal frame components of all the AC devices on the boat like the refrigerator, etc. So... what do you want to touch first?
If DC ground and AC safety ground are connected then a breaker will trip as soon as that air conditioner hot wire makes contact with the frame of the ceiling light.

Both systems are totally independent.

Well not really as they're sharing the same ground.

 

[pcg]

I'm confused about how inverters and chargers fit into that principal.

They fit right in with no conflict. The minus output of the charger and the minus input of the inverter both go to DC ground. My Renogy inverter also has an equipment grounding lug which also goes to DC ground. Renogy is confused about this though because they provide a tiny lug for equipment ground on the DC side, which looks like its intended for nothing larger than 12 AWG, even though a fault inside the inverter could cause the full amount of current coming from the battery to return through that equipment ground. So I'm faced with figuring out how to attach AWG 1 cable to that little lug.
On the AC side of the inverter, neutral is bonded internally to AC safety ground (which, in turn, should be tied to DC ground). If you're plugged into shore power the inverter must be "out" of the AC system (via transfer switch) because you do not want neutral tied to ground at more than one location, or you create another safety hazard..

 

[serpa4]

Factory says non-isolated. Both batteries are connected by a common ground.