Question to battery experts, connecting ground of AGM and LiFePo4

[LilleVenn]

Hello
Mi question is about connecting the ground of a 600AH AGM bank and a 1200AH LiFePo4 bank together.
At the moment they are separated from each other and each has it's own Victron inverter.
With switches on + and - I can choose which one is charged from 1,5kw solar and which one is feeding 12V to the boat.
From my understanding, it would have necessary to separate only the + side and connect the - from the two banks together, but the professionall electrician in La Rochell wanted to split - as well.
Is there a problem in conntecting them together?
I'm asking because I want to charge the LiFePo4 from the AGM with a B2B charger, but with them, ground has to be the same.
I know that I can connect them with a fused cable on minus to equalize the potential, but are there any other known problems if I connect ground?
(I hope I'm not to confusing with my explanation)

[tkeithlu]

I fear to tread in the world of batteries - there are lots of very knowledgeable battery and electrical people on this forum, and they do not hesitate to throw lightning bolts from the tops of their mountains. But, are not your two battery banks on a common ground already? If it is not true, then each electrical device is negative to one bank and not the other? You have them crossing on the positive side, so how does each get its ground from either if they are not already joined at the negative? That could be a monster problem for bonding/corrosion control.

[YANDINA]

The more you can connect the negative terminals of multiple batteries or banks together, the less chance you have of introducing underwater electrolysis. For electrolytic current to flow there has to be a voltage difference and this is zero if they are connected together.

[acem]

I would expect in every case both banks should be grounded together. Otherwise you can get stray voltages between them and that can cause all kinds of grimelins. Thx-Ace

[mdamours]

We have LiFePO4 with common ground to our starter (AGM) batteries. In our case we have two Victron Cyrix in between the starter batteries (one per starter battery) and the lithium house bank to allow charging from one side to the other but disallow one side depleting the other under 13.0 volts. The (regulated) alternators are wired to the starter batteries so that the AGMs can absorb the hit when the LiFePO4 BMS cuts off.

[roland stockham]

Yes all DC grounds should be common, this includes the ground connection to the engine block unless you have an issolated starter (rare and has 2 heavy batt cable connections). This is not actually a ground it is the DC neg and can be floating i.e. it is only connected to the batt neg not to any external or underwater ground. If you want a true 'floating neg' you need to have the propshaft insulated at the flexible couping otherwise the prop becomes an external ground. Using the prop as a ground can cause a problem with electrolysis. The A/C should definitly have an external ground if you have an inverter or generator onboard and ideally you should have a seperate radio ground. These are both true grounds attached to the equipment casing and ground wires not to the nutral line except at the generator/inverter.

There is one complication. Since you are doing the work in Europe an electrition would normally work to European codes which state that A/C and D/C system grounds must be seperate. The American codes state that they must be connected so if you boat is registered in America you should techically use those codes. I don't like doing it that way as there is a risk that if the connection fails leaving A/C and D/C grounds connected but not grounded externally an A/C fault can mean all you D/C systen neg could be at 120v relative to sea water. Suprisingly nothing would blow up with a floating D/C neg but it presents a shock hazard to the crew.

[CharlieJ]

Post #6 Roland Stockholm:

There is one complication. Since you are doing the work in Europe an electrition would normally work to European codes which state that A/C and D/C system grounds must be seperate.

This very popular misconception is simply not correct. ISO 13297 clearly states:

4.4 The a.c. protective conductor(s) shall be provided with a final (single) connection to the hull of a metallic hull craft, or if the craft has a non-metallic hull, to the main grounding/earthing point of the craft.

[boom23]

According to the Victron document Wiring Unlimited, the ground should be the same, for DC and AC circuits.

https://www.victronenergy.com/upload...limited-EN.pdf

[sailon46]

I think that whenever one of the banks carries a load the imbalance at the cathode would create catholdic Disbondment according to Nigel Calder and you will end up with bottom paint issues and also zincs melting away. Basically you will be creating alkalinity around your boat.i think you should keep the two banks totally separate since they carry different chemistries.
FYI Ernie on the Mary Jane

[CharlieJ]

Post #9
You have conflated electrical requirements and corrosion issues. Your advice is absolutely incorrect.

[john61ct]

Yes, setting aside AC circuit issues

All the various DC circuits - Starter/engine, House, thrusters/windlasses, whatever - absolutely should share a common reference between their negative return circuits.

Unless there is a compelling need for a specific circuit to be fully isolated (very rare, as in "floating ground" equipment), in which case all its chassis common references should be very well insulated and out of reach from all but service pro's.

[LilleVenn]

Thank you all for your contribution, I really appreciate it.
With your support and all I could found out on other resources, I'm going to connect the two bank grounds together tomorrow.

[mbartosch]

Just a different view point: if there actually happens to be a reason to keep separate ground potentials between the banks you might want to look at the Victron Orion-Tr Smart DC-DC Charger Isolated. This device does actually isolate input and output potentials.

[john61ct]

As apparently does the Renogy DCDC

It does cot more money to add that feature, can't for the life of me come up with a scenario where it would be needed.

Some circuits using positive as their common (ground)?