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What is a Battery Monitor? Why Do I Need One?
If you're building a DIY off-grid solar system (or already have one), you've probably run into this situation, you've got your lithium or lead-acid batteries set up, your inverter and charge controller wired in, but you have no reliable way to know how much battery capacity you actually have left. That’s where a battery monitor with a shunt comes in.
Most batteries don’t include a built-in monitor, which leaves you guessing based on voltage alone (a method that's not particularly accurate with LiFePO4 batteries) how much capacity you have left. Whether you're using a single 12V battery or a more complex setup, installing a shunt-based monitor gives you real-time insight into your system’s performance and helps you get the most out of your energy storage. There are also other systems worth considering, like hall sensors (similar to a clamp meters), to measure how much current is coming in and out of the battery, but long story short, shunts are much more precise.
In this step-by-step installation guide, I’ll explain in written form how to install any battery monitor with a shunt onto your DIY off-grid solar system. The battery monitor and shunt I’ll be installing in this guide is from REDODO, but all the steps involved and the tools you'll need will apply to installing any battery monitor and shunt (with a few exceptions).
REDODO Battery Monitor + 500A Shunt
https://www.redodopower.de/fr/products/redodo-500a-battery-right?ref=cawwtpkj (EU)
https://shrsl.com/4uyaf (USA)
If you prefer a video installation guide, I’ll leave a link below where you can follow along with my full video tutorial, or stick with this written guide instead. This guide is based on that main video. I'd also like to add that the 500A Battery Monitor from REDODO comes with an excellent installation guide inside the box, which you can also refer to.
So, let’s get started.
Voltage vs Shunt Monitoring: What’s the Difference?
When it comes to monitoring your battery’s state of charge (SOC), there are two main methods you might encounter: voltage-based monitoring and shunt-based monitoring. Both have their place, but understanding the difference can help you decide which method is right for your setup.
Voltage Monitoring
Voltage is the easiest and most straightforward way to get a rough idea of how much charge is left in your battery. As your battery discharges, its voltage drops. With lead-acid batteries, this method works reasonably well because the voltage drop is more predictable. However, for LiFePO4 (lithium iron phosphate) batteries, things get trickier.
With LiFePO4 batteries, the voltage doesn’t change much during the majority of the discharge cycle. For example, your battery might show the same voltage whether it’s at 80% charge or 20%. This can make it hard to tell how much capacity is actually left, especially when the battery is somewhere in between.
That said, if you have a voltage chart handy and understand the general voltage range for your battery, you can still estimate the charge levels, but it’s not nearly as accurate or reliable as other methods.
Shunt Monitoring
This is where a shunt-based battery monitor really shines. Instead of just looking at the voltage, a shunt measures the actual current (amps) flowing in and out of your battery. This gives you a much more accurate reading of your battery’s state of charge because it tracks how much power is being used or added over time.
While voltage monitoring can work in a pinch, using a shunt-based monitor will give you peace of mind knowing you’re working with accurate, real-time data about your battery’s capacity. It’s simple, precise, and makes managing your off-grid power system much easier.
REDODO 500A Battery Monitor Kit Overview
Before we dive into the installation steps, here’s a quick overview of the REDODO 500A battery monitor I’m using for this guide. While this specific model is from REDODO, many shunt-based battery monitors on the market follow a very similar setup, so the steps in this guide will still apply even if you’re using a different brand.
What I like about this REDODO kit is that it comes neatly packaged and includes almost everything you need to get started. Inside the box, you’ll find the display screen, a solid 500A-rated shunt, a 6-meter (20-foot) shielded communication cable, a power wire, mounting screws, and even a small screwdriver for connecting the terminals. The included paper manual is actually useful and clear, which isn’t always the case with this type of gear.
The display shows all the key information you need at a glance, like current, voltage, state of charge, amp hours remaining, and estimated time left. It’s easy to read and simple to operate. The shunt itself is built well, feels sturdy, and is designed to handle high-current systems, which makes it a good fit for most off-grid setups.
One thing to keep in mind is that this battery monitor kit will require M10 cable lugs to connect to the shunt. If your setup doesn’t already include these lugs, fear not! I’ll explain later on in the guide how to make your own custom cables using a crimping kit, or sourcing pre-made cables online, and even a trick where you can keep using your existing cables.
If you're just getting started with DIY solar, this kind of monitor is a great entry point. It gives you all the important data you need without being overly complicated to install or use.
Tools and Parts You’ll Need Before Installation
Before installing your battery monitor, it’s a good idea to make sure you have all the necessary tools and parts on hand. While the REDODO Battery Monitor kit includes most of the core components, there are a few extras you might need depending on your setup, especially if you plan on making your own cables or adapting existing ones.
Here’s a list of recommended tools and parts:
Crimping Kit (includes crimper, wire cutters, and terminals)
Handy if you’re making custom cables.
[https://s.click.aliexpress.com/e/_oDWaXfU]M10 Cable Lugs
These are needed to connect cables securely to the shunt. Make sure the lugs match the cable gauge you're using.
[https://s.click.aliexpress.com/e/_oBQRkVU]Bus Bar (with M8 bolts)
Useful if you're connecting multiple devices (like an inverter, charge controller, and DC fuse box) to the same shunt.
[https://s.click.aliexpress.com/e/_oDIOZF4]Appropriately Rated Cables
You'll need battery cables thick enough to handle the current flowing through your system. If you're using a high-powered inverter, this is especially important.Inverter Cables
If your inverter didn't come with cables, or if you need a different length or lug size, you'll want to have these ready.
[https://s.click.aliexpress.com/e/_oFS5o94]Multimeter or DC Clamp Meter
Helpful for checking voltage or current during setup. I personally use the ANENG ST212.
[https://s.click.aliexpress.com/e/_okX5ZTM]Optional: Heat Shrink Tubing or Electrical Tape
For finishing off your cables if you’re making your own.
Step-by-Step Guide: How to Install a Battery Monitor with Shunt
Installing a battery monitor might seem intimidating at first, but once you understand the basic layout, it’s pretty straightforward. Below is the complete step-by-step guide based on how I installed the REDODO 500A battery monitor. This applies to most other shunt-based battery monitors as well, with a few differences depending on your setup.
Before You Begin
Make sure your system is powered down. Disconnect the battery from your solar charge controller, and make sure no power is flowing through the cables during installation. Safety first, right!
Step 1: Understand the Wiring Layout
Before you touch any cables, it's best to look at a wiring diagram. The REDODO battery monitor includes one in the manual that’s actually very clear. The shunt is installed on the negative side of your battery system, not the positive.
Here’s the basic layout:
The B- (battery minus) terminal on the shunt connects directly to the negative terminal on your battery.
The P- (power minus) side of the shunt connects to all your loads and charging devices — like your inverter, MPPT charge controller, and DC fuse box.
The small power wire for the shunt display connects to the battery’s positive terminal using an O-ring terminal.
If you’re using more than one device (inverter, charge controller, etc.), it’s a good idea to use a bus bar to make clean, organized connections.
Step 2: Plan Your Cable Setup
Depending on your gear, you can either:
Use pre-made cables with M10 lugs (if they’re the right length and gauge), or
Make your own using a crimping kit, which is what I did.
In my case, I reused the thick inverter cables and cut the negative line into three pieces:
One for the battery negative terminal to shunt (terminal B-)
One for shunt (terminal P-) to bus bar terminals
One for bus bar to your loads
Just make sure each cable has the right terminal size — M10 for the shunt terminals, M8 for the bus bar or other standard device terminals.
Step 3: Connect the Shunt to the Battery
Connect your newly crimped cable from the B- terminal of the shunt directly to the negative terminal on your battery. Tighten the bolts snugly but not too tightly, just enough to prevent movement and ensure a solid connection.
Then, connect the small power wire from the shunt to the positive terminal of your battery (its a small O-ring lug). This powers the display.
Step 4: Connect Your Devices to the Shunt
Next, connect the P- terminal of the shunt to:
Your inverters negative line
Your MPPT charge controllers negative line
Then any other devices using the battery’s negative side (like a DC controller box)
If using a bus bar, wire the bus bar to the P- terminal of the shunt, then connect your devices to the bus bar.
Step 5: Connect the Display
Run the provided shielded communication cable from the shunt to the battery monitor display. It’s 6 meters (20 feet) long, so you should have flexibility in placing the screen where you want it. Mount the display wherever it’s most convenient for you to see it.
Step 6: Double-Check Everything
Before powering anything on, make sure:
All connections are secure
All bolts are tight but not over-tightened
No stray wires are touching anything they shouldn’t
Once you’re confident with the setup, it’s time to calibrate the system (explained in the next section).
How to Calibrate Your Battery Monitor (From 0% or 100%)
Once your battery monitor is fully installed, the final step is to calibrate it so that it can accurately track your battery’s state of charge. This step is essential if you want the readings on your display to match your actual battery capacity.
There are two main ways to calibrate a shunt-based monitor: starting from a fully discharged battery (0%) or from a fully charged battery (100%). You can choose whichever method makes more sense for your situation.
Option 1: Calibrate from 0%
To calibrate from 0%, you’ll need to discharge your battery completely. In my case, both of my batteries drop to around 10.8 volts when they’re fully drained. Once the voltage drops to that level, leave the batteries to rest for about 30 minutes to make sure they’ve fully settled at zero.
Then:
Go into the settings menu on the battery monitor display
Set your battery’s full capacity and voltage range
Return to the main screen
Hold the down arrow on the display to calibrate the current state as 0%
Now the monitor will treat this point as an empty battery and start tracking any power that comes in.
This method takes more time, but it’s a good way to get a very accurate calibration if you don’t mind discharging your battery fully.
Option 2: Calibrate from 100%
If you’d rather not run your battery flat, you can calibrate from a full charge instead. Start with a depleted or partially charged battery, and let your solar charge controller or charger bring it up to full.
Here’s how to know when it’s full:
The voltage reaches the fully charged value for your battery (usually around 14.6 volts for LiFePO₄)
The current going into the battery tapers off close to zero
Let it sit at full charge for about 1 to 2 hours with minimal or no current flowing
Once it’s clear the battery is full:
Hold the up arrow on the display to calibrate the state of charge as 100%
This method is faster and easier for most people, especially if you’re just getting started.
Both methods work, and your choice depends on what stage your batteries are in. If you’re starting with brand-new or fully charged batteries, go with the 100% method. If your batteries are nearly empty or you want maximum accuracy, go with the 0% method.
Once calibrated, the battery monitor will begin tracking all current in and out, giving you a reliable state of charge reading from this point onward.
Tips, Common Mistakes, and Beginner Advice
Once you’ve installed and calibrated your battery monitor, there are a few extra things worth keeping in mind. These tips can help avoid common issues and make your install cleaner and more reliable, especially if you're new to DIY solar.
Don’t Overtighten the Bolts
When securing the shunt and cable lugs, make sure the bolts are tight enough to hold everything in place, but not so tight that you risk stripping the threads or damaging the connectors. A snug fit is usually enough to handle thermal expansion and current flow safely.
Match the Lug Sizes Correctly
A common issue is trying to force the wrong size lug onto a terminal. The shunt uses M10 bolts, while many solar components (like bus bars or charge controllers) often use M8. Make sure your lugs match the bolt size of the component they’re connecting to. If you’re making your own cables, double-check the lug sizes before crimping.
Quick Trick: Using M8 Bolts in an M10 Shunt
If you're in a pinch and only have M8 bolts or lugs, you can use them with the shunt. Just keep in mind that this may reduce the current-carrying capacity. The shunt is rated for 500 amps when using proper M10 hardware, but if you downgrade to M8, you should limit your expected load to something more conservative, around 300 amps or less. This trick can be helpful, but it’s best used for smaller systems or if you're not planning to draw full current through the shunt.
Disconnect Power Before Working
Always disconnect your battery from the charge controller and any incoming solar power before wiring anything. It keeps things safe and reduces the chance of sparks or damage to your components.
Keep Your Wiring Neat and Organized
Use a bus bar if you're connecting multiple devices to the shunt. This not only makes the wiring cleaner but also simplifies any future changes or troubleshooting. It’s also a good idea to label cables if your setup is a bit more complex.
Use Proper Cable Thickness
Make sure the cables you’re using are thick enough to handle the current flowing through your system. Undersized cables can overheat, especially if you’re running a powerful inverter or charging multiple batteries. If in doubt, always size up.
You Can Reuse Existing Cables
If you already have cables with the wrong lug size on one end (for example, M8 instead of M10), you don’t necessarily have to throw them out. In my setup, I reused an inverter cable by cutting it into three segments and crimping new lugs on just one side. As long as the cable is thick enough and the connections are solid, it’ll work just fine.
Calibration Isn’t Permanent
If you ever want to recalibrate your monitor (for example, after changing batteries or reconfiguring your system), you can always go back into the settings and run through the same 0% or 100% calibration process again.
These small details can make a big difference in how reliable and safe your system is. Take your time, double-check your connections, and don’t stress if it’s not perfect the first time. You’ll learn a lot just by doing it.
Final Thoughts and Future Feedback Suggestions
Overall, the REDODO 500A battery monitor has proven to be a reliable and easy-to-use tool for managing my off-grid system. The display is clear, the installation guide is well written, and it includes most of what you need right in the box. For anyone getting started with shunt-based monitoring, it’s a great entry point that balances affordability and functionality.
That said, there are a few small things I’d love to see in a future version of this product.
One improvement would be the option for a 300A model that uses M8 bolts instead of M10. Since many DIY solar setups already use M8 hardware as standard, having that flexibility would make installation easier without needing to adapt or upgrade your existing cables and lugs. For smaller systems, 500A is more than what’s needed, so a lower-rated version could make the product even more accessible.
Another feature that would add a lot of convenience is Bluetooth support. Having the ability to view battery data from a phone or tablet would be incredibly useful, especially for those with systems tucked away in less accessible spots. Even a basic companion app with live readouts and settings access would be a welcome addition.
Finally, while the kit comes with nearly everything you need, it would be helpful if it included a few M10 lugs in the box. That small addition would save beginners an extra trip to source parts and make the process feel even more beginner-friendly out of the gate. Since most users will need to either crimp their own cables or adapt existing ones. Having those lugs included would be a helpful nudge in the right direction, especially for anyone who hasn’t done crimping before. Please keep in mind if you'd like to avoid crimping the cables by yourself all together. You can purchase premade cables with M8 lugs on one side and M10 on the other side, but this will cost more compared to making the cables yourself. Again this kind of reinforces the idea of having a battery monitor with a 300A shunt using M8 bolts, for smaller DIY setups and easier installation.
These are by no means dealbreakers, just things I think could take an already solid product to the next level.
If you’ve made it this far, I’d like to mention that I have an exclusive discount code [THEONEGOODROAD] with REDODO for anyone looking to save a bit on a future purchase. I’d also definitely recommend checking out the full video tutorial linked above. It walks through the complete installation process and includes visual context that pairs well with this written guide.
Thanks for reading, and good luck with your DIY solar setup. If you found this helpful, feel free to share it around, and don’t forget to check out my other battery and inverter review videos as well. You’ll find affiliate links throughout this article if you’re looking for any of the tools, products or parts I’ve used.
Maybe I’ll see you in the comments below or in the next DIY solar tutorial.
REDODO Group 31 12v 100Ah Battery
https://www.redodopower.de/products/redodo-lifepo4-12v-100ah-lithium-batterie?ref=cawwtpkj (EU)
https://shrsl.com/4tz81 (USA)
REDODO 12v 100Ah Mini Battery
https://www.redodopower.de/products/redodo-12v-100ah-mini-lifepo4-lithium-batterie?ref=cawwtpkj (EU)
https://shrsl.com/4uy96 (USA)