You want to use your 12V battery to power something, but you have no idea how charged it is. How do you check?
There are a few different ways to check a 12V battery’s remaining capacity (i.e. state of charge). But some methods are much more accurate than others.
Here are the main ways, along with a breakdown of each method’s accuracy.
Pros: Somewhat accurate, easy
Cons: Must disconnect all loads and chargers and let the battery rest awhile
A battery’s remaining capacity is correlated with its open-circuit voltage, and this relationship can be expressed with a voltage chart or discharge curve. Here’s an example voltage chart for 12V LiFePO4 batteries:
To check battery capacity with a multimeter, disconnect all loads and chargers from the battery and let it rest awhile. How long? Battery University recommends at least 4 hours. Doing so allows the battery voltage to settle.
Use a multimeter to measure the open-circuit voltage of the battery. Turn the multimeter to the DC voltage setting, touch the red probe to the positive battery terminal and the black probe to the negative battery terminal.
Compare the voltage reading to the battery’s corresponding voltage chart to get an estimate of the battery’s remaining capacity. Refer to our LiFePO4 voltage charts or lead acid voltage charts for this step. Or there may be one in the battery’s product manual.
This method is relatively accurate and has the benefit of using a multimeter, something most 12V electronics enthusiasts already have lying around. The downside is that it isn’t very precise; you get a rough reading at best.
Still, it’s a very popular way to estimate battery capacity. I use this method most often when I’m checking a battery I haven’t used in a while to get a general sense of its state of charge.
2. Battery Monitor
Pros: Most accurate
Cons: Can be pricey
I love battery monitors. They show your 12V-48V battery’s remaining capacity as a percentage, the same way you can see exactly how much battery your phone has left. They also show other specs like charging or discharging rate. Some premium battery monitors even have features like Bluetooth so you can monitor your battery from your phone.
They’re best for off-grid solar and mobile electrical systems (such as in RVs) where your battery bank is constantly connected to loads and chargers. You’ll have to disconnect everything to install the monitor, but then it constantly tracks your battery’s state of charge.
Battery monitors can be expensive, but they don’t have to be. There are now budget options that do the basic functions quite well. Check out my review of the best battery monitors for some recommendations.
Note: A battery monitor comes with a shunt. There are plenty of cheap voltmeters on Amazon marketing themselves as “battery monitors,” but unless they have a shunt they aren’t real battery monitors.
Voltmeters are cheap but they give you a very rough estimate of battery capacity. They function by measuring the battery’s voltage and using that to estimate its state of charge.
It’s essentially the same process I detailed for using a multimeter, so why is it less accurate? Because voltmeters typically measure voltage while the battery is still connected to loads and chargers. A battery’s voltage raises when it’s being charged and drops when it’s being discharged, so using voltage to measure capacity while the battery is in use results in much less accurate readings.
In practice, this means that a high current charge rate will raise the battery’s voltage, making it appear more charged than it actually is. And a high current discharge rate, such as running an energy-intensive appliance like a fridge or AC unit, will drop the battery’s voltage, making it appear more drained than it actually is.
Unfortunately, most RV and campervan electrical systems come with voltmeters rather than battery monitors. My dad’s fifth-wheel has one. If you’re recreational vehicle came with a battery indicator that you’ve found to be inaccurate, now you know why.
4. Solar Charge Controller
Pros: Convenient in DIY solar systems
Cons: Least accurate
If you’ve got an off-grid solar system, your solar charge controller may provide a rough estimate of battery capacity. How is it doing this? You guessed it — it’s basing it off the battery’s voltage.
This method is incredibly convenient in theory. Just look at your charge controller to get a sense of how much battery you have left! But it suffers from all the shortcomings that voltmeters suffer from, including another one: voltage drop.
The charge controller has no idea what length or size the cables are that connect it to the battery, so it can’t accurately account for voltage drop. Some charge controllers have a compatible battery voltage sensor you can buy that helps the controller get an accurate voltage reading. But that doesn’t solve any of the other problems of this method.