How do you know how full your batteries are? With AGM or flooded batteries, you can get a rough idea using a “state of charge” graph and your system’s current voltage.. but, sometimes you want more details. What if you want an exact percentage, want to know exactly how much current is flowing in/out of your batteries at any given time, or have Lithium batteries that don’t have enough voltage drop to determine state of charge by voltage alone?
I recently upgraded my batteries and saw that Victron has a full-featured battery monitor in the $160 range. Having recently switched to lithium batteries, I needed a solution for monitoring their health, but my wallet was already hurting from the batteries themselves. Like so many other pieces of tech these days, if a company can build it, a Chinese manufacturer can clone it. The battery monitor is no exception. A little bit of research lead me to this, the AiLi Battery Monitor / Voltmeter / Ammeter, which at the time of this article, costs about 70% less than the Victron equivalent.
Functionally, these two are pretty close. There are a couple of things worth your consideration, however!
Backlight turns off after 60 sec.
Available bluetooth module / app
Somewhat attractive design.
Ultimately, the 75% savings of the clone was worth the slight annoyance of the backlight. My refrigerator, propane alarm, hotspot, and a few other items are always drawing power… which means that I rarely see my battery monitor screen off. A sticky-note or clever placement addresses this easily. Bluetooth connectivity? My van is 65 square feet. I can manage.
What to buy.
AiLi Battery Monitor with 100 Amp Shunt (There is a 350 Amp shunt model as well, but I can’t imagine what you’d be running to need it.)
54mm/2 1/8″ Hole Saw
12 ” 4AWG Cable w/ 5/16″ Lugs
16-Gauge Fuse Holder
Standard Blade Fuses (The fuse holders above include 3-amp fuses, but a 1-amp fuse is more appropriate)
You’ll likely want a soldering and/or crimping kit to attach the fuse holder wires to the battery and shunt, see my recommendations for that here.
Note on wire gauge: I have seen people recommend some insanely thick cables — this makes placement of the shunt very difficult as the thicker cable will not flex well. The maximum current for this setup is 100 amps, which at a distance of 1-foot, would be fine with as small as 12-gauge. Again, if your van’s setup has current draw over 100 amps, you’re probably not reading this article on how to setup a battery monitor.
How it works.
The main component of the battery monitor, aside from the display itself, is the shunt. The shunt is essentially a metal bar through which all of the current in/out of the batteries will flow. That metal bar’s resistance is a known value, so the monitor will measure voltage at each side, then use Ohm’s Law to determine the net amps flowing in/out of the battery.
It’s basically a multimeter and a semi-sophisticated calculator with a few buttons and a display. It will tell you the system voltage, total amperage in or out, remaining amp hours, and percentage of amp hours remaining (in relation to the total capacity).
Installing the display.
The display itself will require a 2 1/4″ hole in the wall, and you’ll need to be able to access the rear of the unit to tighten down a wingnut. See below if this isn’t feasible for your intended location! The bracket included is in a plain “C” shape, I added additional bends with pliers (as pictured) because the wall I installed it in was particularly deep and poorly situated.
Once you have made the hole, the battery monitor itself will seat into the hole. It has a small threaded stud that sticks out of the back. From the rear side of the wall, you’ll insert the stud into the metal mounting bracket, then tighten down a wingnut to secure it in place. From here, you’ll route the 4-pin display cable to the negative battery terminal, where it will connect to the shunt.
If you do not have access to the rear side of the wall, or would prefer a surface mounted solution: there is a model available on Thingiverse to 3d print which will allow you to mount the monitor to the surface. I can 3d print this for you for a reasonable price if needed, just reach out via Instagram.
Installing the shunt.
- Move everything that was connected to your battery’s negative post to the “p-” port of the shunt. Exception: if you have two batteries, the “jumper / interconnect” cable will stay on the battery negative post.
- Connect an interconnect cable from the battery negative post to the “b-” post on the shunt. Note: in my photos, you’ll see that I had to use a stud to post adapter on the shunt — my original battery setup was posts and I couldn’t (at the time) cut and reterminate the post clamp with a ring terminal. I also used a flattened copper pipe in place of the interconnect cable.
- Connect a wire from your positive terminal (preferably one with a 1-2 amp fuse) to the green connector nearest the “p-” post.
Configuring the monitor.
This is probably the easiest part. You need to configure the amp hour capacity and tell the unit when your batteries are 100% charged. I created a short video demonstrating this:
I’ve been using this battery monitor for 3 months now and for the price, I can’t recommend it enough. The backlight is my biggest complaint, and a small piece of tape or a 3d-printed cover take care of that well. It will be solid when discharging, or pulsate when charging.
The only hiccup I’ve had was shortly after installation, my percentage screen reset to 0%. I believe what happened was that I set “100%” at a level significantly below 100%, so when the batteries actually charged to 100%, the monitor didn’t know how to react. I reset it by holding the % button, and haven’t had an issue since.