Choose either house battery
See how a 1–2–Both switch feeds the same boat circuits from either battery.
Follow the circuit
01
Choose the supply
Position 1 feeds the house panel from battery 1; position 2 uses battery 2. Both connects the positive banks together. Off disconnects the panel.
02
Keep the returns connected
The negative bus stays connected to both batteries. The selector changes the positive supply; it does not switch the return wires.
03
Try each position
Open the example and change the selector in simulation. Use one battery at a time to understand the reserve arrangement; Both does not preserve a separate reserve.
The source and this example
Blue Sea's 9001e instructions show two battery inputs and a common output feeding a starter or other load.
We use the catalog's 9002e main-power selector and a fused house panel. The engine, alternator and field-disconnect contacts are outside this example.
- Blue Sea: selector wiring and operation ↗
Page 2: two inputs and common load output.
- Blue Sea: battery switch operation diagrams ↗
Before you build
- Use compatible batteries before combining banks.
- Protect each battery's outgoing positive cable.
- If adding an alternator, follow its switching requirements; never turn the selector off while it runs.
This is a planning example. Cable sizing uses Crimp’s ABYC-based method; the drawing does not certify an installation.
Cable sizes and connections · 9 conductors
Lengths are one way. Sizes and currents come from the same Crimp model as the drawing. Each drop value covers one drawn conductor; the full circuit includes its supply and return. AWG values are approximate counterparts to the metric areas.
| Connection | Length | Wire | Design current | Fuse | Drop |
|---|---|---|---|---|---|
| C1 · Battery 1 + → 1 / 2 / Both / Off 1 | 70 cm | 4 mm²AWG 11 | 3.7 A | — | 0.09% |
| C2 · Battery 2 + → 1 / 2 / Both / Off 2 | 70 cm | 4 mm²AWG 11 | 3.7 A | — | 0.09% |
| C3 · 1 / 2 / Both / Off C → House circuits IN + | 50 cm | 1.5 mm²AWG 16 | 3.7 A | 15 A | 0.18% |
| C4 · House circuits 1 + → Chartplotter (Garmin GPSMap) + | 2 m | 1.5 mm²AWG 16 | 3.2 A | 7.5 A | 0.62% |
| C5 · House circuits 4 + → Lanterns (LED) + | 3 m | 1.5 mm²AWG 16 | 0.5 A | 3 A | 0.15% |
| C6 · Chartplotter (Garmin GPSMap) − → Negative Bus Bar 3 − | 2 m | 1.5 mm²AWG 16 | 3.2 A | — | 0.62% |
| C7 · Lanterns (LED) − → Negative Bus Bar 2 − | 3 m | 1.5 mm²AWG 16 | 0.5 A | — | 0.15% |
| C8 · Negative Bus Bar − → Battery 1 − | 70 cm | 1.5 mm²AWG 16 | 3.7 A | — | 0.25% |
| C9 · Negative Bus Bar 1 − → Battery 2 − | 70 cm | 1.5 mm²AWG 16 | 3.7 A | — | 0.25% |
A dash means no fuse is assigned to that conductor; upstream protection may apply. Recheck the exact device, holder and placement after edits.
Equipment · 7 components
- Battery 1
12 V · 110 Ah
12 V, 110 Ah AGM Deep Cycle (BAT412101084), rated at C20. Victron prefers charging at 0.2C (22 A); this is a recommendation, not an absolute maximum. Flat terminals have 8 mm through-holes.
- Battery 2
12 V · 110 Ah
12 V, 110 Ah AGM Deep Cycle (BAT412101084), rated at C20. Victron prefers charging at 0.2C (22 A); this is a recommendation, not an absolute maximum. Flat terminals have 8 mm through-holes.
- 1 / 2 / Both / Off
The catalog uses the 9002e selector, with the same main-power switching topology as the source's 9001e. Its alternator-field-disconnect contacts are not used in this house-load example.
- House circuits
Generic 6-circuit switch / distribution panel — one battery+ feed, six individually switched and fused branch circuits. Each branch carries its own ATC blade fuse (rating set per cable). Common helm / cabin pattern (e.g. Blue Sea 8027, 4374).
- Chartplotter (Garmin GPSMap)
3.2 A load allowance
GPSMAP 943xsv 9-inch chartplotter with sonar: 1.37A typical and 3.20A maximum at 12V, with 40.2W maximum at 10V. Accepts 10–32V DC and uses a 6A fast-acting fuse. The dedicated power harness is required even when connected to NMEA 2000.
- Lanterns (LED)
0.5 A load allowance · 6 W
Modern LED navigation lights — port, starboard, stern and masthead. Low draw, suitable for night passages.
- Negative Bus Bar
Negative bus bar — common return with multiple output studs.
Crimp checks · 3 findings to review
These are the current engine findings for the saved example. They remain visible when you open it in Crimp.
- warning: Check fuse suitability: Battery 1 (+1)
- warning: Check current sharing
- warning: No charging source
More boat examples
View all five →A bilge pump that stays ready
Keep the automatic bilge circuit powered when you switch off the rest of the boat.
Solar for a small sailboat
Follow a solar panel through its controller to a house battery, chartplotter and navigation lights.
A separate trolling motor battery
Give the trolling motor its own battery and keep the chartplotter on a separate supply.
Charge two batteries at the dock
Use separate charger outputs to replenish two batteries without joining their positive terminals.
Crimp for AI agents · public beta
Ask your AI. Check the plan in Crimp.
Connect ChatGPT, Codex or Claude Code to look up components, calculate wire and DC-DC charger sizing, or build a small draft you can inspect and edit. Missing facts and assumptions stay visible.
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