Why You Want One
You want to swap a cable on your inverter. The battery is full. Touch the wrong two wires and a big spark jumps.
A battery disconnect switch sits between your battery and everything else. Electricians call this "isolating" the battery. Turn the knob to off and everything after it goes dead: the inverter, the charge controller, the cables to your house. Now you can work on that part, or leave the cabin for winter so nothing drains the battery.
On some switches the knob is removable. Pull the removable knob out and the switch stays off, so nobody can turn the power back on while your hands are in the wires.
What The Switch Can't Do
It does not make the battery itself safe. The battery terminals, and the cable up to the switch, stay live all the time. Drop a metal screwdriver across them and you get sparks, molten metal, maybe a fire.
So near the battery you still want terminal covers, insulated tools, no rings or watches, and a fuse right at the battery.
A plain switch also never trips by itself. That job belongs to a fuse or a DC circuit breaker, which watches the current and cuts it when a short sends way too much, before the cable overheats. Not sure what size you need? Here is how to pick the right fuse, or try the fuse calculator.
That's why many people buy a fusible switch: switch and fuse in one box. A non-fusible switch is just on and off, so you pair it with a separate fuse.
Should A Battery Disconnect Be On The Positive Or Negative?
On the positive. Victron's wiring guide says that if you isolate a battery, you only switch the positive cable.
The fuses go on the positive cable too.
How Do I Wire A Battery Disconnect Switch?
Battery positive, a short cable, the fuse, then the disconnect switch, then everything else. Mount it right next to the battery, on a wall or behind a panel.
Anything before the switch can never be turned off, so every bit of cable between the battery and the switch is "always live". Keep that run as short as you can. Our wiring guide covers the rest.
Then label it: "MAIN BATTERY", "SOLAR INPUT", "INVERTER". In an emergency you don't want to guess.
Sources
- Victron Energy, Battery Switch 275A datasheet
- Victron Energy, Wiring Unlimited
- Victron Energy, Lithium NG battery manual: safety precautions
Hard Words
| Term | What it means |
|---|---|
| Battery disconnect switch | A switch that cuts the battery off from the rest of the system. The battery itself stays live. |
| Short circuit | When current finds a shortcut with almost no resistance and spikes. |
| Fusible switch | A switch with a fuse built in. Both jobs in one box. |
| Non-fusible switch | A plain on/off switch. Pair it with a separate fuse. |
| Always live | The cable between the battery and the switch. It can never be shut off. |
| Trip | When a fuse or breaker cuts the power by itself because it sensed too much current. |
Common Questions
Does a battery disconnect switch work?
Yes, for everything after the switch. Turn it off and the inverter, charge controller and house cables go dead. The battery side stays live.
What is the best battery disconnect switch?
One rated for the full current your system can pull, made for DC, and able to switch under load. Some cannot break DC current under load, so check the specs.
Do I still need a fuse?
Yes. A switch only turns off when you turn it. A fuse or DC breaker cuts the power by itself.
Should I have more than one switch?
Often yes. Bigger systems have separate switches for the solar input, the battery and the inverter, so you can work on one part while the rest runs.