The short answer. On a 12V boat, the house positive runs through a main fuse at the battery, a battery switch, then a positive busbar feeding the fuse blocks. The battery negative goes to the shunt’s battery side; every other negative goes to a busbar past it. With a lead-acid start battery, a DC-DC charger moves engine charge into the lithium bank.
You’ll need
- A main fuse rated for your battery (many lithium makers call for class T)
- A battery switch: Egis XD 500A battery switch
- A shunt battery monitor: Victron SmartShunt 500A
- Two busbars, one positive and one negative: Victron Busbar 250A 4-point with cover
- A fused feed and a branch fuse block: Egis ANL fuse block, up to 4/0, 35-750A, Egis RT 12-position fuse block with ground
- For house + start only: Victron Orion-Tr Smart 12/12-30 isolated DC-DC
- Tinned cable sized with the Wire gauge calculator, plus lugs that fit the studs
- Your battery’s manual, open the whole time
Most wiring diagrams online show the battery and the loads and skip the parts in between. The parts in between are where lithium installs go wrong: a fuse three feet from the battery, a charger negative bolted straight to the battery so the monitor never sees it, a switch rated for a bigger cable than the one on it.
You can get every one of those right once you know where each part goes. Here are two diagrams. The first is one lithium battery doing everything. The second is the setup most boat owners end up with: lithium for the house loads, a lead-acid battery kept for the engine.
Fuse sizes and wire sizes aren’t on the drawings on purpose. Those come from your battery’s manual, the length of your runs and ABYC E-11, and no general diagram can pick them for you.
Diagram 1: single lithium bank
Red is positive, black is negative. Not to scale. Each charger lands on the positive busbar on its own fuse.
Work through the numbers in order.
1. Main fuse, at the battery. This fuse protects the main cable if it shorts. Put it on the positive cable as close to the battery as you can fit it, before anything else. ABYC E-11 sets the distance limits, and a surveyor will look for it. Lithium can push a far bigger current into a short than lead-acid, so the fuse has to be able to break that current safely. Many lithium makers name a class T fuse here. Check what yours says.
2. Battery switch. It goes after the fuse, so the fuse also protects the switch cable. Match the switch’s rating to the cable on it, not to the big number on the box. Egis’s own sheet for the XD switch says it handles 500A continuous only with two 4/0 cables. On a single 2/0 cable its continuous rating is 225A, and on 4/0 it’s 300A (Egis XD Series brochure).
3. Positive busbar. One spot where everything positive gathers: the feed to your fuse blocks, the inverter feed, the charger outputs. Victron’s 250A busbar has 5/16 inch (8 mm) studs and a 70V DC limit (S3). Order lugs with a 5/16 inch hole for it.
4. Fused feed. The cable from the busbar to the fuse block gets its own fuse, sized to that cable. An Egis ANL fuse block, up to 4/0, 35-750A holds it.
5. Fuse block. Each circuit (bilge pump, nav lights, electronics) gets its own fuse here. The Egis RT 12-position fuse block with ground has a matching ground block, so the small negative wires have one home too.
6. SmartShunt. The battery negative goes on the side marked BATTERY MINUS, and nothing else goes there. Victron’s manual is blunt about it: anything bolted to that side is left out of the state-of-charge count (S2). Shunts made before 2020 label the other side LOAD MINUS; newer ones say SYSTEM MINUS. Same thing.
The shunt also needs a thin fused red wire from its Vbatt+ terminal to the battery positive. It comes in the box. That wire is how it reads voltage.
7. Negative busbar. Everything negative ends here: the fuse block’s ground block, the charger negatives, the inverter negative, the engine ground. Then one cable runs from the busbar to SYSTEM MINUS on the shunt.
Got a load or charger wired straight to the battery negative right now? Move it to the busbar. That one wire is the usual reason a monitor slowly drifts away from the truth.
Diagram 2: lithium house bank + lead-acid start battery
Yellow boxes are fuses, with one near each source. The start battery also runs the engine and takes the alternator’s charge; the engine’s own start circuit and its switch are left off for clarity.
The house side is the same chain as diagram 1. The only new job is getting the engine’s charge into the lithium bank.
Why a DC-DC charger sits between the two batteries
The alternator only ever sees the start battery. The Victron Orion-Tr Smart 12/12-30 isolated DC-DC draws from the start side and charges the lithium bank at up to 30 amps. That keeps the alternator from running flat out into a battery that will take everything it gives. It also means a flat house bank can’t drain the start battery. Upgrading a boat to lithium: everything around the battery goes into the why.
Wire it in three moves:
- Size both fuses from the charger’s manual.
- Run the start battery positive through a fuse, close to the battery, to the charger’s input.
- Run the charger’s output through a second fuse, close to the + busbar, to the busbar.
Those are the two yellow fuses in the drawing.
Where the start battery’s negative goes
To the negative busbar, on the system side of the shunt. Victron’s manual says the same when you use the SmartShunt’s second input to watch the start battery: that battery’s negative must be on the SYSTEM MINUS side (S2). The engine block ground ends up there too. Wire it to the house battery’s own negative post instead and it sits on the wrong side of the shunt, so the count can’t be trusted.
Want to see the start battery’s voltage in the app too? Run the SmartShunt’s second fused red wire to the start battery positive.
Can I use a battery combiner instead?
Some people do, and some combiners are sold for lithium. A plain combiner doesn’t limit current, though, so the alternator problem comes back. Read your battery maker’s guidance first. Charging lithium from a boat alternator: Orion-Tr vs Cyrix vs combiner compares the options.
Where do chargers and a low-voltage disconnect connect?
Shore chargers and solar controllers land on the positive busbar, each on its own fuse, with their negatives on the negative busbar. Not on the battery posts. That keeps every amp in and out on the shunt’s count.
A Victron Smart BatteryProtect 100A goes between the positive busbar and a group of loads, so it can cut them off before the battery goes too low. It’s built for loads only. Don’t send charging current back through it.
Does this diagram fit your boat?
This applies if: – Your boat runs 12V DC. – You have one lithium house bank, with or without a separate lead-acid start battery. – You’re wiring or checking the DC side yourself, or want to understand what your electrician will build.
This doesn’t apply if: – The boat runs 24V or 48V, or has several lithium batteries in parallel. Follow the battery maker’s wiring guide for those. – You’re installing an inverter-charger, a shore power inlet, a galvanic isolator, or anything else on the AC side. AC work goes to a qualified marine electrician (an ABYC-certified one, ideally), because a mistake there can shock someone in the water or start a fire.
Lugs, torque and the small stuff
Small parts, but a loose lug carrying 100 amps gets hot. Take your time here. Finish every connection the same way:
- Match the lug’s hole to the stud. Victron’s busbar studs are 5/16 inch (8 mm); the SmartShunt’s bolts are M10 (S2, S3). Check the battery’s terminal size in its manual.
- Size each cable for its current and its full length out and back with the Wire gauge calculator, then fuse it to protect that cable.
- Crimp tinned lugs and seal them with adhesive-lined heat shrink: Pacer tinned lug 2/0, 3/8 in stud, 10-pack where the stud is 3/8 inch (check the battery and switch manuals), Pacer heat-shrink ring terminal 8 AWG, 3/8 in stud, 25-pack for smaller runs, also where the stud is 3/8 inch.
- Torque to the maker’s number. Victron lists 110 lbf-in (13 N-m) for the busbar and a 21 N-m maximum for the SmartShunt bolts; Egis lists 140 in-lb for the XD switch studs.
- Cover every positive stud. Victron’s busbar and the Egis switch both come with covers.
Products used in this guide
| Product | Why it’s in the diagram |
|---|---|
| Victron SmartShunt 500A | Counts every amp in and out on the negative side. Phone readout over Bluetooth. |
| Victron Busbar 250A 4-point with cover | One for positive, one for negative. 250A max, 5/16 inch studs. |
| Egis XD 500A battery switch | House bank on/off. Sealed to IP67 and ignition protected to SAE J1171 per Egis. |
| Egis ANL fuse block, up to 4/0, 35-750A | Fused feed from the busbar to the fuse block or an inverter. |
| Egis RT 12-position fuse block with ground | One fuse per circuit, with a ground block for the small negatives. |
Prices show live from the store. Or start from the Boat lithium upgrade kit and leave out what you already own. To size the bank before you wire it, use the Battery bank size calculator.
Bundles for this job: Boat lithium upgrade kit, small, Boat lithium upgrade kit, large, Wiring and fusing kit (12-circuit panel)
Quick answers
Does the main fuse go before or after the battery switch? Before. The fuse sits on the battery positive, as close to the battery as you can fit it, so it protects the cable that runs to the switch too.
Can I connect my charger straight to the battery terminals? It will charge, but the battery monitor won’t count it if the negative is on the battery post. Bring the positive to the positive busbar on its own fuse and the negative to the negative busbar.
Where does the start battery negative go if I have a SmartShunt? To the system side of the shunt, usually through the negative busbar. Victron’s SmartShunt manual says a second battery’s negative must be on the SYSTEM MINUS side.
Is a 500A battery switch good for 500 amps on any cable? Not the Egis XD. Its sheet gives 500A continuous with two 4/0 cables, 300A on one 4/0 and 225A on 2/0.
Do I need a marine electrician for this? Not always for the DC side, if you’re careful and follow your battery’s manual. Call one for anything on the AC side, for 24V or larger banks, or if you can’t tell what an existing wire does. Fusing a lithium bank: Lynx, MRBF and class T and Busbars and fuse blocks: laying out a boat DC panel cover the next steps.
Related guides: Battery switch for a lithium boat, Busbars and fuse blocks: laying out a boat DC panel
Sources
- Victron SmartShunt datasheet: https://www.victronenergy.com/upload/documents/Datasheet-SmartShunt-EN.pdf
- Victron SmartShunt manual, installation: https://www.victronenergy.com/media/pg/SmartShunt/en/installation.html
- Victron Busbar 250A 4P and 6P datasheet: https://www.victronenergy.com/upload/documents/Datasheet-Busbar-250-A-4P-and-6P-with-cover-EN.pdf
- Egis XD Series battery switch brochure (8710-1900). No link yet: Egis’s own site didn’t load on Nov 3 2026.
