You don't need a shunt to measure the voltage. Shunts are used to measure current. I like to use water as a metaphor here because it's an easy visual to wrap your head around. Start with some basic concepts:
- Voltage - a bucket full of water. High voltage: lift the bucket to the top of a ladder.
- Current - Water flowing out of the bucket. More current: water flowing out faster.
- Resistance - The size of the a hose or hole in the bucket. More resistance: pinch the hose to restrict the flow.
With the basics in place, you can think of a lithium battery like a wide, shallow baking tray. It can hold the same AMOUNT of water as the bucket, but as the water drains out there might only be a slight change in the level you can "see" if you're standing there. You can easily tell when the bucket is exactly half full. But it's harder to tell that with the baking sheet. (This isn't all precisely true but it's close enough.)
Your indicator is calibrated for the bucket, so it isn't accurate with your new batteries. That much is true. But indicators that are accurate for lithium batteries are cheap, like $15 on Amazon. If that's all you wanted to do, you can just buy one and be done. But bear in mind that although they're really easy to install (you can often just connect them to the same wires as the other display - they measure the exact same thing in the same way) I'm not giving you a link here because if they aren't installed right they aren't accurate. They often need calibration or more steps to make them read properly.
This is because they have to be much more sensitive. Remember the baking pan? Look at the chart below. Lead-acid voltages go down very visibly and linearly as they drain. If you watch a bucket drain, it's very even, right? Well, lithiums are "flatter" so they appear to drain very slowly until they're just about empty, then it seems the water evaporates very fast, almost all at once.
This means while the circuit (and math) to do this right is actually really easy and cheap, it's much more sensitive to all kinds of things: electrical noise, mis-calibration, even different brands and qualities of batteries. Measuring them this way is less reliable.
So what does a shunt do? Well imagine the bucket and hose but now try to answer a different question. Don't ask "how much water is in the bucket." What if you ask "how much water is coming OUT of the bucket RIGHT NOW?" That's what a shunt does. It's like a tiny water wheel and counter at the bottom of the hose.
This is not answering the question you think you want to know. But it can answer a more important one, if you use the data right. Do you really want to know "I am at 60%" and have to check it four times a day to know if you're OK? What if instead, you asked "how much water do I USE?" And something could tell you "You use 5 gallons per shower, 1 gallon per toilet flush, 8 gallons when you wash the dishes, 12 to do your laundry." Now you can say "I want 2 adults to shower every day, flush the toilet 4x, wash the dishes once, and do laundry twice a week. I need a bit under 26 gallons of water per day." Now you know something that's almost more useful than "I have 50 gallons in the tank" and having to verify that constantly. You know that as long as you put at least 26 gallons INTO the tank every day, as long as your usage doesn't go up for some reason, you should be good every day. As long as you can keep measuring what you take out and put in, you don't need to know what's "in there now" except for occasional maintenance-related reasons (do I have a leak?)
This is what shunts add to the equation. They weren't created just for this purpose - some folks who wanted more sophisticated lead-acid-based systems have used them for years. And we can use them with lead-acids too. (I've always installed one in mine.) But they're very useful in lithium-based systems because we have a harder time answering the first question anyway.
Do you need a shunt? Probably not. Would it add value? Yes, maybe, but only if you also install the "counter" device that knows how to take advantage of it to answer this flow question. Should you self install one? Probably not.
But if you really want to, one product I can definitely recommend (without shilling at all - I have no relationship with them) is the rvwhisper.com system. This is a centralized monitoring device with an app for your phone that can talk to sensors around your coach. What makes it unique is they don't make the sensors. The makers decided to standardize on the best sensors already on the market from other vendors, like the Hughes Power Watchdog many of us already use to connect our coaches to campground services, or the Mopeka TankCheck propane level sensors.
Well, one of those is the Thornwave PowerMon. This is a bluetooth-enabled device that you install right next to your battery, and can measure both voltage directly from there as well current flow (via a shunt). Since it's located right next to your battery it eliminates some common cause of trouble like noise or resistance in long wiring runs, and it gives you a ton of data on your battery level AND usage, right on your phone. Paired with an rvwhisper it can do this over the Internet so it can tell you things like your inverter has shut down due to low voltage and your fridge is thus about to thaw and spoil your food (our Milestone's Furrion fridge is electric and requires an inverter.)
