Generating Power: Charging Speeds and the Role of the Battery Management System
Jake Schmalz discusses the importance of a battery management system (BMS) in protecting lithium-ion batteries throughout the charging process to expedite the charging speed and avoid over-heating.
Ensuring that battery charging is efficient and safe is crucial for all battery powered solutions. If the charge time takes too long users will be put off.
However, rushing the charge and forgoing safety could damage the battery. Finding a suitable charge time without compromising on safety is a must to ensure consumer demand and facilitate businesses’ desire to move towards battery powered solutions.
For many businesses wanting to move into the battery world, charging is often an afterthought. Vast amounts of time are spent understanding the power output needs without considering how long or how best to charge the battery to deliver the desired power. Forgoing this crucial step can greatly impact the effectiveness of the power solution for the end user.
When it comes to developing a high-quality and well-designed battery solution, there is a range of factors to consider including: the type of cells, the Battery Management System (BMS), and whether the batteries are interchangeable or fixed.
Figure 1. Briggs & Stratton’s Vanguard® Battery Line-Up. Image used courtesy of Bodo’s Power Systems
Types of Charging Cells
Batteries are made up of a series of charging cells that work together to convert chemical energy to electrical energy due to the build-up of ions in the battery electrolyte. During the charging process the flow of electrons is from the cathode to the anode, via a power source. This enables the conversion of electrical energy back to chemical energy.
The speed at which this process can take place safely depends on the type of cells that the battery consists of. For instance, “Power Cells” are built to handle high charging rates (C-rates) without much degradation to their lifespan. Due to the high demands, there is often trade-off for using these batteries as the increased energy conversion often results in a shortened lifespan from the outset of the battery.
At the other end of the spectrum there are “Life Cells.” Life Cells sacrifice high power output and fast C-rates for longer useful life. This results in a slower charging process; however, the longevity of the battery is greatly increased. Life Cells suffer greatly from charging faster than their recommended limits, so it is important to understand the end users’ requirements before selecting these.
In between these two types are “Mid-Range Cells” which strike a balance between power output and lifespan. For many looking to capitalise on the movement to electrified power, Mid-Range Cells offer an effective solution.
The tendency is to navigate towards increased power output; however, “Power Cells” often have high costs upfront and – in the long-term – the accelerated charging rates may shorten the useful life of the battery. This results in needing to replace the battery sooner, further bolstering the cost to the end user.
Overall, understanding the exact needs from power output to charging speed can help with the structural composition of the battery and the type of cells that it consists of to generate the power required.
Figure 2. Vanguard® 5kWh Commercial Lithium-Ion Battery Pack. Image used courtesy of Bodo’s Power Systems
The role of the BMS
Battery Management Systems (BMS) are ideal for use on lithiumion batteries as they work to safeguard the longevity of the battery.
As current enters the battery pack the BMS intuitively determines where the input is directed throughout the cells. This is carefully done to prevent over-heating and damage to the internal components of each cell. If this were to be left un-monitored by the BMS then there is a risk of too much voltage going to the individual cells, causing damage to the cathode. This can result in the decomposition of the metal structures at the cathode and the production of too many lithium-ions, resulting in an unstable cell structure.
Operating with an effective “Power Map” (such as with Vanguard’s 10kWh Commercial Lithium-Ion Battery Pack), constant information can be gauged from the health of the battery.
From constantly communicating with the charger and monitoring current flow to cell and pack temperature, the BMS can optimise the C-rate helping to speed up charge times without causing battery degradation. This allows for maximum charging power within a safe range, while preserving the life requirements of the pack.
The result is the shortest charge time that will maintain both the safety and long-life expectations for the battery.
Protecting the internal chemistry
During the charging process the BMS controls the transfer rate of lithium-ions within the battery to minimize dendrite growth (a form of lithium plating) on the negative electrode. Current moves from the anode to the cathode during the charging process and uncontrolled build-up can greatly impact the effectiveness of the cathode. This is incredibly important for power output which moves from the cathode to the anode.
Without a BMS – or with an inadequate Power Map – dendrite growth will more quickly limit the useful energy (capacity) of the battery, which may eventually lead to a dangerous shorting out of the battery.
Figure 3. Vanguard® Battery Charger. Image used courtesy of Bodo’s Power Systems
Best practices for boosting battery longevity
To boost battery longevity, it is key to keep the battery away from extremes.
Charging in very low temperatures or from very low states of charge will shorten the life of the battery. This is because the battery is not in the optimum condition for the chemical reactions to take place internally and can result in damage to the anode or the cathode. Another issue can be keeping the battery at full charge for long periods of time.
The BMS plays a vital role in supporting users during non-ideal conditions as it monitors and controls the input and output of the battery to ensure safe operation, reliable performance, and long life. For example, an effective BMS will not allow a battery to begin charging if the temperature is too low.
Figure 4. Vanguard® 3.8kWh Commercial Lithium-Ion Battery Pack. Image used courtesy of Bodo’s Power Systems
Interchangeable or fixed?
Depending on the power requirements, the use of interchangeable or fixed battery solutions varies.
As the list of applications and use-cases for lithium-ion batteries continues to grow, at Briggs & Stratton we work closely with manufacturers and designers to understand their power needs to create an ideal solution bespoke to them. A core understanding of the customer and end user shapes the development of the battery.
Sometimes, large, fixed packs are ideal due to the amount of energy required which makes it unfeasible to easily change the batteries. For others, smaller, interchangeable packs are the best route.
Interchangeable packs certainly offer greater flexibility and essentially infinite run-times (i.e., one charges while one is being discharged); this can be especially valuable to commercial users of compact equipment. However, this is not a solution that can be applied to every power need.
Understanding the needs of the end user is crucial to the development of an efficient and safe battery that generates the specific power output needed without compromising on the safest charging process. From the use of Power Cells to Life Cells, the use of the BMS is fundamental to protecting overall battery health and ensuring the longevity of the battery.
This article originally appeared in Bodo’s Power Systems magazine.