DC negative connections when adding a new lithium bank to an existing LA bank

[andrzej0nl1ne]

Perhaps a silly question but when adding a new lithium battery bank on a boat with an existing LA battery bank, do you use a common DC negative bus or do you keep them separate?

[ItDepends]

In ALMOST all cases, yes you would use a common negative bus.

The details can depend on exactly what the new battery is being used for, and the complexities of your bonding system (if any).

[andrzej0nl1ne]

Thanks, common is what I thought but hit a senior moment. Not doing anything complex, just adding 200 or 400 ah capacity to support electric galley. Plan to retain existing 400ah firefly bank.

Sent from my SM-N986U using Cruisers Sailing Forum mobile app

[ItDepends]

That's a potentially complex mixed system... You'll be keeping the two banks completely separate as far as charging and discharging?

[andrzej0nl1ne]

I plan to charge the lithium primarily via solar, and via 2 Orion dc-dc chargers from the fireflies when alternator is running. Alt will continue to charge the firefly bank. To charge the fireflies from lithium when engine not running, I will use use my shore charger via an inverter powered by the lithium. Not efficient but simple.

I did notice a drawing on the victron site showing the dc-neg isolated to each battery bank. As I use afloating ground and do not bond, keeping the dc-neg isolated within each bank is not a wiring challenge.

Do you think this would work?

[ItDepends]

Yes, isolated grounds can work. My boat actually was designed that way with isolated negative circuits for the 24V house bank and the 12V engine system.