When you have an off-grid solar system, you have a battery bank. More and more consumers are opting to add a battery bank to their grid-tied systems as well. Especially if you want to escape the effects of loadshedding or if you want to use your batteries to save even more on your utility bill through a selective time of use application. You might think that being the owner of a battery bank would make you the battery bank manager. Luckily, battery manufacturers had the foresight to take the management part away from owners by introducing a Battery Management System (BMS).
Battery management systems offer powerful tools to “see inside” battery banks and improve lifespan, reliability, safety and performance.
Lithium-ion batteries used as solar storage can be used only in specified conditions, and therefore, a battery management system is necessary to monitor the battery state and ensure the safety of operation. The role of the BMS is to:
- Facilitate the safe charging and discharging of lithium-ion batteries.
- Gather and analyse data to increase operational performance and cell longevity.
- Forecast throughput and capacity attenuation given variable battery conditions.
- Provide better State of Health (SoH) metrics and maintenance information.
State of health (SOH) estimation is used as a qualitative measure of the capability of a lithium-ion battery to store and deliver energy in a system.
How does it work?
A BMS is an electronic system that manages a rechargeable battery (cell or battery pack), keeping the battery from operating outside its safe operating area, monitoring its state, calculating secondary data, reporting that data, controlling its environment, authenticating it and/or balancing it. WIKIPEDIA
A battery pack built together with a battery management system with an external communication data bus is called a smart battery pack. A smart battery pack must be charged by a smart battery charger.
A BMS uses a specialized computer and sensors to make batteries “smart” – and provide real-time information about their performance, along with data collection.
A BMS monitors the state of the battery as represented by various items, such as:
- Voltage: Total voltage, voltages of individual cells, or voltage of periodic taps
- Temperature: Average temperature, coolant intake temperature, coolant output temperature, or temperatures of individual cells. Taken in by temperature sensors sprinkled throughout the pack to get a sense of temperature distribution.
- Current: Current in or out of the battery, is the battery charging or discharging, and at what magnitude.
With these inputs, the BMS is running several algorithms to generate an accurate estimation of the following outputs:
- The SoC (State of Charge). This is typically referred to as the fuel gauge. It lets you know how much charge is left in your battery.
- An SoH (State of Health). This is an estimation of how much capacity the battery pack/bank can store relative to what it was at beginning of life.
In short, a BMS analyses real-time measurements from the chemical battery then adjusts charging/discharging parameters and communicates this information to end-users. These sensors can monitor battery voltage, state of charge (SOC), state of health (SOH), temperature and other critical measurements. They can even display charging time on an easy-to-read “fuel gauge.”
When and where is BMS used?
A BMS offers numerous benefits for many battery chemistries, not only lithium-ion batteries. A BMS is frequently used in off-grid applications and battery backup applications, including generators and power utilities, telecom, hospitals, data centres and more.
But for lithium-ion batteries, a BMS doesn’t just offer benefits; it helps to reduce the likelihood of fires and explosions. That’s because a lithium-ion battery has the highest power density when it comes to batteries, and overcharging lower-capacity cells can cause thermal runaway and combustion.
Key Benefits of BMS for all battery chemistries
- Without a BMS, battery technicians often depend only on routine maintenance to identify upcoming battery problems and determine when to fix or replace batteries.
- These efforts are supplemented by capturing vital operating parameters, cell/unit voltage and current; resistance; cell/unit/ambient temperature; electrolyte levels; and more.
- This data is automatically recorded and can be used for predictive maintenance and more accurate runtime estimates.
Battery monitoring systems offer several safety benefits, including:
- Remote monitoring and alarms.
- Reducing maintenance – which minimizes your contact with high voltage.
- Early warning for system failure as well as dangerous conditions.
- Battery disconnection in case of failure or unsafe operating conditions.
The data logging capabilities of a BMS allow systems to collect trending data and create reports. These reports then allow for estimations, long-term tracking and improved battery use. Remote access and software alarms can reduce maintenance, transit time and costs. This makes a BMS ideal for commercial settings and vacation homes, where maintenance crews or owners may not always be onsite. You won’t have any worries about your holiday home burning down while you are working in your office.
The benefits of a BMS includes:
- Reduced maintenance and replacement costs
Even “maintenance-free” batteries require periodic inspection for optimal performance. And standalone battery management systems can supplement on-site inspections or owners’ maintenance efforts.
A BMS not only optimizes charging/discharging and other variables, but also helps identify maintenance requirements and predict battery failure before it happens. A BMS improves the battery’s lifespan, reducing the frequency and likelihood of replacement. A feature known as a Low-Voltage Shutoff also reduces maintenance and maximizes lifespan, especially in remote applications where routine inspection is more difficult. The ability to do cell voltage monitoring ensures that charge and discharge won’t exceed the manufacturer’s recommendations.
- Protection against extreme temperatures
Keeping extreme temperatures at bay is possible with a BMS, also extending the lifespan of the battery. For every 7°C to 10°C below 26°C, batteries lose 10% capacity – all this while PV production decreases and electrical demand increases. Conversely, every prolonged 7°C above 25°C halves battery lifespan. The temperature of your battery system has an incredible effect on the capacity as well as the lifespan of your batteries. A BMS uses sensors to monitor ambient and battery temperature, allowing for early warning when battery temperatures are outside of optimal ranges. This will improve capacity and prolong the lifespan. With lithium-ion batteries, where temperature readings can influence whether a battery should be charged or discharged (to avoid thermal runaway) these features are especially important. A thermal runaway is a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature.
- Cell balancing for equal voltage
Ideal voltage depends on battery chemistry. But in all cases, using batteries outside this voltage range can drastically diminish cell life. On top of this, each cell has a slightly different voltage window where charging/discharging should occur for long life and proper operation. Cold cells must be charged to a higher voltage. And weak cells can prevent other cells from charging completely. Some BMS systems can ensure equal charging among cells by measuring current and charge rate–and performing either passive or active cell balancing.
Knowledge is power. By understanding and knowing what a BMS is and does, you can optimize battery reliability, safety, maintenance, performance and lifespan.
At first, it may look daunting to decipher all this information, but a battery management system will save you a lot of headaches and a lot of money.
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