Battery Monitors and Testers Information
Battery monitors and testers determine the health and operating capability of batteries. Sometimes known as battery management devices, they can monitor charging levels, voltage and temperature indications, or other conditions as part of more sophisticated battery management systems (BMS). For instance, in a power plant backup batteries are the last line of defense against a shutdown. A battery management system ensures reliability and is a critical component for effective operation.
The goal of battery management devices is to ensure battery functions and maximize lifespan. Several variations exist, from sophisticated smart battery monitors to simple V-meters and Ah-counters that measures capacity consumption.
Battery management devices provide feedback for corrective action if any diminution of battery function is indicated. They also provide data such as battery charge state. These devices extend a battery's lifespan and lead to cost savings.
Sophisticated monitors can adjust for variables such as Peukert efficiency and temperature. These factors have significant effects on battery charging capacity. Battery monitor types include:
Voltmeters: These devices read a battery's voltage level, but don't adjust for factors-such as the effect of charging on battery capacity-that can lead to battery charge measurement inaccuracies. Deep cycle batteries experience a significant reduction in their lifespan when they are repeatedly discharged too much. A voltmeter for battery monitoring can shorten a battery's effective lifespan by providing misreadings.
Ah-counters: Provides a simple measure of the charging hours available in a battery. However, these units do not calculate Peukert efficiency, possibly leading to measurement tolerances.
Multi-function battery monitors: By utilizing Peukert efficiency, temperature and other variables such as charging statistics, these monitors provide precise information about a battery's operating status and capacity.
Passive regulators: These devices aid in balancing batteries or cells by bypassing charging current if the voltage of a cell attains a particular level. Voltage alone does not accurately reflect a battery's operating charge, limiting their usefulness.
Active regulators: These monitors turn a battery load on or off in order to balance a load. Less sophisticated models use cell voltage to accomplish this, while more advanced models evaluate other parameters.
Complete battery management systems use a combination of the above devices along with a display screen or functionality for continuous monitoring and balancing. These systems are of three general types:
Distributed: In these systems one communication cable connects the battery and the controller, with a BMS board in place at each cell. They are the easiest to install and assemble and are typically the most expensive.
Centralized: A centralized system uses a set of wires to connect the battery cells with a single controller. They are the least expensive type of BMS, as well as the most difficult to maintain due to the multitude of wires required.
Modular: Several communication-enabled controllers are used, each monitoring a number of cells. Modular systems are categorized between distributed and centralized systems based on expense, ease of operation, and assembly.
Battery capacity is calculated in Amp-hours (Ah). For instance, if a battery can deliver a current of 10Amps for 10 hours it would be rated at 100Ah (10 x 10 = 100). Battery monitors and testers measure the flow of current into and out of a battery, as well as other values. The device calculates how much energy is taken from or added to a battery. Simply relying on Amp-hours, however, does not provide an accurate picture of a battery's charging capacity. Temperature, the age of the battery, and discharge current all serve a role in battery capacity. As a result, a battery may discharge fully and more rapidly than might be expected due to its Amp-hours rating. This is known as Peukert efficiency. Low temperature can decrease a battery's capacity even further.
Advanced battery monitors and testers display Amp-hours removed from a battery and state of charge (SOC) readings compensating for Peukert efficiency and temperature. The devices report important parameters such as voltage, current, and internal and ambient temperatures. If the monitor detects battery operation is poor, data is transmitted to produce an alarm or disconnect a battery.
Battery monitors track a variety of battery operating conditions including:
Voltage: Total voltage as well as individual cell voltage; the minimum and maximum voltage attributed to a cell or periodic tap voltage.
Temperature: Coolant intake temperature, average temperature, coolant output temperature and individual cell temperature.
State of charge: This parameter measures a battery's charge level.
State of health: Conditions of overall health are evaluated to measure overall battery vitality.
Coolant flow: This measure is used in batteries that are cooled by air or fluid.
Current flow: Measures the current flowing into or out of a battery.
A battery monitor in conjunction with a BMS may also calculate values using the above input, including:
- The maximum discharge current expressed as a discharge current limit
- The maximum charge current expressed in the form of a charge current limit
- The energy (kWh) that has been delivered from the time of the last charge or charge cycle
- A cell's internal impedance (resistance), which reflects open circuit voltage
- Charge (Ah) that has been delivered or stored (also called a Coulomb counter)
- Total time in operation since first use
- Total energy that has been delivered from the time of first use
- Total operating time from the point of first use
- Total number of cycles
The monitor may report the information it gathers or calculates to an external device by means of various communication methods. In conjunction with a BMS, it aids in preventing a battery from working unsafely. These conditions include:
- Excessive current
- Over-voltage during the charging cycle
- Under-voltage during the discharging cycle
- Excessive temperature
- Excessive pressure
- Detecting faults or leakages
If unsafe battery operating conditions are detected, a BMS may take steps to prevent battery operations outside safe parameters by:
- Incorporating a switch that is tripped if unsafe operating levels are detected
- Reducing usage of devices connected to the battery or disconnecting them altogether
- Taking steps to modulate environmental factors such as engaging fans, heaters, air conditioners or liquid cooling systems
A BMS with input from a battery monitor optimizes battery usage by balancing battery cells so that they maintain the same voltage (also known as state of charge). This balancing is accomplished by:
- Draining energy from the cells with the highest charges by hooking them up to a load, often via passive regulators
- Moving energy from high-charged to low-charged cells
- Lowering the charging current so it does not damage cells that are fully charged, while still allowing low-charged cells to charge
- Charging on a modular basis
First, determine if the device needs to be part of a complete battery management system that can take action to both monitor and regulate battery charging and operation. This will establish if a stand-alone battery monitor is appropriate. It also confirms the presence of a pre-existing integrated battery management system. In cases where no integrated system is present, the information provided helps determine if one can be utilized or is required. Stand-alone battery monitors that account for variables that affect operating capacity, such as Peukert efficiency, are especially important.
IEEE 1491 - Guide for selection and use of battery monitoring equipment in stationary applications