A Battery That Won't Tell You How Full It Is
Here's a problem nobody warns you about when you start off-grid.
A battery doesn't have a gas gauge built in. You can measure its voltage, but voltage lies. A nearly-empty lithium battery and a half-full one can read almost the same number. You can't trust it.
So how do you actually know how full your battery is?
You count. Every electron going in. Every electron going out. Add them up. Subtract them. That tells you the truth.
The thing that does the counting is called a shunt.
What A Shunt Actually Is
A shunt is a small bar of metal. That's it. Usually about the size of your finger, made of a special alloy that resists electricity in a very precise, predictable way.
Scientists call it a "low-resistance precision resistor". That's a fancy way of saying it's a piece of metal that lets electricity through, but pushes back a tiny, known amount.
Every electron flowing in or out of your battery has to pass through this little bar of metal.
How It Counts
This is the clever bit.
When electricity flows through the shunt, it creates a tiny voltage drop across it. We're talking small. Like fifty thousandths of a volt small.
A monitor sitting next to the shunt reads that tiny voltage drop. Because the shunt's resistance is known exactly, the monitor can do the math: a voltage drop this big means the current is exactly this many amps.
It's a bit like measuring how fast water is flowing through a pipe by checking the pressure difference between the two ends. You don't have to count drops. You measure something else, and the math tells you the answer.
The monitor does this every second. Then it adds up all the amps coming in (when your panels are charging) and subtracts all the amps going out (when your fridge is running). That running total is what tells you exactly how full the battery is.
→ Why you want a monitoring system in the first place
Common Sizes
Shunts come rated for different amounts of current. You pick one big enough to handle the most current your system will ever push through it.
A 300A shunt handles up to 300 amps. That's plenty for a small cabin or a van.
A 500A shunt is the sweet spot for most house-sized off-grid setups.
A 1000A shunt is for big systems. Whole-home off-grid with a heavy battery bank and a powerful inverter.
Going too small is bad. Pushing 400 amps through a 300A shunt will overheat it. Going bigger than you need is fine, but you're paying for capacity you'll never use.
Real Ones You Can Buy
The most popular one is the Victron SmartShunt. It pairs with your phone over Bluetooth. You stick it next to the battery and read the state of charge on an app.
Renogy makes a similar thing called the Battery Monitor. Slightly cheaper, also works.
Both do the same job. A small bar of metal, a sensor, and an app.
Where The Shunt Goes In Your System
This part matters.
The shunt has to sit between the battery's negative terminal and absolutely everything else. The charge controller, the inverter, every load in your house. All of their negative wires meet at the shunt, then one wire runs from the shunt to the battery.
Why? Because the shunt only counts what passes through it. If you wire something straight to the battery, bypassing the shunt, the monitor won't see that current. Your "battery full" reading will be wrong.
So the rule is: nothing connects to the battery negative directly except the shunt.
→ How home batteries actually work inside
→ Picking the right BMS for your battery
Worth It?
For any battery bigger than a phone charger, yes.
A shunt and its monitor together cost maybe a hundred euros. A battery costs thousands. Knowing exactly how full your battery is means you don't accidentally run it flat (which kills lithium batteries fast) or overcharge it.
It's the smallest, cheapest part of an off-grid setup. And it tells you the one thing you most need to know.
