Why you need a battery monitor
One of the most basic functions of a battery monitor is to display the remaining charge of your battery system. In lead-acid batteries, the battery voltage drops significantly as you use them. This voltage drop typically gives some indication that your batteries are running low.
What is a battery monitor?
A battery monitor measures battery voltage and current flow into & out of a battery bank and uses these measurements to accurately track the amount of charge left in the battery bank. In other words, a battery monitor is to your battery bank what a personal finance program is for your bank account!
Shunt based battery monitoring systems are more reliable than simple voltmeters that are traditionally used as a gauge for tracking the state of charge (SoC) of a battery bank.
What Does a Battery Monitor Do?
Most battery monitors display (among other things):
Percent of battery capacity remaining (it’s state of charge)
Net amperage going in/out of the battery
Cheaper battery monitors may just show voltage and net amperage.
There’s a display screen to put in the van’s living area for quick glances at battery status. This data is provided to the screen by a shunt near your electrical components. The shunt placed between two pieces of the electrical system.
Limitations of Battery Monitors
The thing is, battery monitoring is not an exact science. In short, their figures are inaccurate due to variability in how batteries function. But, battery monitors are pretty close and are (way) better than nothing.
A battery monitor is often compared to the gas gauge in your car, but that is not a good analogy. The battery monitor works by using voltage and current entering and leaving the battery. Some problems though:
Batteries lose capacity over time which the battery monitor cannot accurately account for
A battery’s voltage varies significantly due to temperature, size, discharge rate, age, how recently it was discharged, etc.
It’s nearly impossible to total the number of Amps being absorbed when you don’t know how efficiently they are being absorbed
A battery monitor’s state of charge computation is like a gas gauge – but imagine the level of gas in the tank (battery voltage) is always fluctuating. Battery voltage can go up or down just because it got colder or hotter. Battery voltage is a moving target.
Battery monitors also display usage statistics (Amp-hours consumed). This is like your car trying to tell you miles-per-gallon without knowing how fast you were driving.
Battery Monitor Vs. Battery Management System (BMS)
Lithium batteries have an integrated battery management system (BMS) that helps optimize their performance and protect them from operating outside of safe conditions. The BMS is the control center for individual batteries in a system, not the system as a whole.
The main function of the BMS is to prevent overcharging and over-discharging, which can damage a battery and shorten its life. The BMS also calculates the remaining charge, watches the battery’s temperature, keeps an eye on the battery’s health and safety by checking for loose connections and internal shorts, and balances the charge across all of the cells in the battery.
If unsafe conditions are detected, the BMS shuts the battery down to protect the lithium-ion cells and the user.
A BMS collects a lot of the same information as a battery monitor. However, instead of displaying the information to the user, the BMS uses it to optimize the performance and health of each battery.