When deciding on the types of power to supply equipment requiring high performance, rechargeable batteries are often chosen for several reasons. With rechargeable batteries, users can obtain longer device performance and consistent power. In addition, rechargeable batteries are ideal for devices that are lightweight, such as mobile applications. These devices may have a battery pack that can be taken from off the device, such as the ones on power tools, and placed into a charger that connects to a wall outlet.
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Yet, other types of batteries cannot be removed. Found in devices such as phones, embedded batteries are placed permanently into the device. The embedded batteries are provided additional protection from the device itself because there is no enclosure lid. Instead, the battery is often buried deep into the device. Due to not being able to take out the batteries to recharge them in an exterior charger, embedded batteries and other rechargeable battery packs will come with embedded battery chargers.
Embedded Battery Chargers Included on Circuit Boards
An embedded battery charger is inside the battery pack; it consists of an integrated circuit (IC) placed within the power path to the battery pack that then leads to the system. In addition to recharging the batteries, embedded chargers also provide system power as they provide dynamic power path management (DPPM) to the device, as well as input-voltage dynamic power management (DPM) for when devices will be charged through other devices that have USB port attachments.
The function of an embedded charger is the same with an external charger. It will provide initialization through a self-test and cell detection. Then it will provide the charge to the rechargeable battery based on the battery's chemistry and charging algorithm.
Embedded charger outlined in battery pack PCBA.
Battery Chemistries with Embedded Chargers
Embedded chargers can be used with nickel-based and lithium-based batteries. Since it is included with the specific type of battery chemistry, the embedded charger ensures that the pack is charged correctly.
For NiCd and NiMH chemistries, the chargers can provide fast charging techniques before terminating the charge when reaching the peak (for NiMH) or right after reaching the peak (for NiCd) when the voltage begins to decline. An optional trickle charge may also be provided for NiCd chemistries.
For lithium-ion rechargeable batteries, a fast-charging procedure can also be provided with a regulated charging voltage as it reaches a maximum rate limit. As the voltage rises, the current will start to taper off to provide a float charge. An optional top-off charge to provide the battery with a power boost to reach its maximum charge level may also be provided for all battery chemistries. An embedded charger for lithium-ion batteries may come as a linear charger, pulse charger, or as a switch-mode charger based on the application.
A linear charger is a simple design usually used for smaller applications or for batteries that require less than 1.5A charge currents, such as hearing aids and smart watches. These chargers don't take up a lot of the circuit board's space as it uses a pass MOSFET transistor to drop the power from the AC adapter to the battery's voltage. One disadvantage to this type of charger is that it does experience high power dissipation.
A switch-mode (or switching) charger uses a step-down converter to bring the AC power down to the voltage of the battery. It is used for larger batteries and when fast charging is desired. It has lower power dissipation yet has a complex design as it requires more components to offer higher efficiency. With the increase in components, the weight will be increased for the battery pack as this can be an issue for some applications that need to be lightweight.
A pulse charger works by combining advantages from both a linear charger and a switch-mode charger. The pass transistor remains on when the battery's voltage is low until reaching its regulated voltage. Then the charger switches to pulses for the input current until coming to the voltage limit. Power dissipation is kept low while the pulse charger comes at a smaller size than the switching charger.
Advantages of Embedded Chargers
Due to the different types of embedded chargers that are available, they offer increased battery life to the cells no matter what the environmental conditions will be for the application. The batteries will not require an external charger, which cuts down on the amount of equipment needed by the end user. An embedded charger can also charge batteries through varying power sources. The device can be plugged into a wall outlet using a wall brick, a car cigarette lighter using an adaptor, or even into a USB port on a computer or laptop that allows for power sharing with the other device.
Another unique advantage is that the same product can use different battery chemistries, as the battery pack will always have the correct charger. A customer could switch from the product using a NiMH rechargeable battery to the same product with a lithium-ion rechargeable battery while knowing the embedded charger in each battery pack will provide the correct current and voltage. The use of an incorrect external charger will never become an issue. Since the charger was custom designed for the specific characteristics and power demands of the battery, the charger can provide a higher level of performance for the battery that is then applied to the application.
Disadvantages to Embedded Chargers
Increased design and manufacturing costs will always be present when selecting an embedded charger versus an off-the-shelf external charger. The costs of the embedded charger will be based on the type of charger that is created, as linear chargers with fewer components will typically cost lower than switching chargers or pulse chargers that require additional parts. Battery pack manufacturers will have different costs based on how they design their battery packs and chargers.
In addition, the type of customized products that will be utilized will also play a factor, as we can use products from companies such as Texas Instruments, Analog Devices, Microchip, Seiko, and many others to create the custom charger. Another factor will be the rechargeable battery's cost per watt hours ($/Wh) as the type of battery chemistry will dictate the costs. For example, a NiMH rechargeable battery will have a cost of $1.5 - $3.0 while a lithium-ion battery will have a cost of $2.5 -$3.5.
Another complicated problem with embedded chargers is the amount of internal heat that is generated near the cells. If there is not enough surface area to allow space for the heat to discharge from the cells, the higher temperatures will cause the chemical reactions in the battery's cells to be increased. This issue causes the battery to age faster.
Lastly, when the battery reaches the end of its life, then it will have to be entirely discarded. So, the embedded charger will be discarded as well even though the circuitry for the charger is still functional.
Deciding on Embedded or External Chargers
When it comes to picking out the charger for your custom battery pack, there are several considerations that will need to be evaluated. The type of battery chemistry, the size of the battery, the number of batteries required and how the application will function will play important roles. In addition, internal and external environmental conditions will also play a factor.
Extreme exterior temperatures may cause too much heat to be absorbed into the battery pack. This issue will be compounded by the amount of heat that will be present from an embedded charger which could cause damage to the circuit board. Another factor is electromagnetic interference (EMI). Certain chargers can create EMI when in operation, such as switch-mode chargers, that can cause interference with other components on the circuit board. The output of a switch-mode charger can also cause radiation that could impact how the nearby circuitry performs.
Figuring out how the charger will impact the battery run times is also vital. Battery manufacturing varies between companies even when the batteries are the same type. Charging these batteries in the same manner using an external, off-the-shelf charger can lead to the batteries being overcharged. The cell charge voltage limit for a lithium-ion battery from one manufacturer may be a 4.1V/cell charge while another manufacturer will develop batteries with a 4.2V/cell charge. However, the end user may decide to place on an external charger with a 4.2V/cell charge, which will cause issues for batteries that need lower charge rates.
On the other hand, undercharging the batteries can cause inefficient run times for the battery. An external charger with a 4.1V/cell charge may be used for a battery requiring a 4.2V cell charge. This will result in a battery pack that will consistently run out of power more frequently as the device will not operate at its optimal level.
Also consider how the battery will be charged in the device and whether the battery pack will be removable, especially with mobile devices used in varying locations. If the product can be charged through other devices using ports or USB connections, then the embedded option will be more suitable for the application.
Summary
Whether you decide on an embedded charger or an external charger, proper matching to the battery pack will be essential. Obtaining both products from the same manufacturer, instead of using different companies, can ensure that the charger's algorithm will be designed to match the power needs of the application and the recharging requirements of the custom battery pack. Then you can consider whether you want the end user to have control over which external charger will be used for the battery, or whether that control will be reserved to the battery pack itself by using an embedded charger design.
Manufacturers are doubling down on their efforts to reduce costs and improve the performance and safety of the battery technology that underpins these vehicles.
While much has been written about the impressive advances in Li-ion battery chemistries, original equipment manufacturers (OEMs) also pay close attention to the selection of the appropriate battery cell format, also referred to as the ‘form factor.’
The importance of the form factor extends beyond mere space and packaging requirements. Indeed, the technical fit alone is usually not the primary consideration. OEMs are increasingly aligning, and adapting, the choice of form factor with their strategic and performance objectives, whether it be high energy density for increased range, or safe and durable extreme fast charging.
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By selecting an appropriate form factor OEMs can optimize functionality to suite their design objectives: For instance a manufacturer may wish to optimize the fast charging capability of its EVs.
StoreDot's proprietary 100inX cell technology is cell form-factor agnostic. Due to the combined properties of its unique electrode design, specialized electrolyte and advanced electrode materials any format can benefit from the fast-charging capabilities of XFC. This empowers OEMs to choose the format that best suits their vehicle design, manufacturing platform capabilities, and market segment without sacrificing charging speed.
Currently, three cell form factors dominate the EV market: prismatic, pouch, and cylindrical formats. Each type brings its own set of advantages and disadvantages, allowing OEMs to customize their EV battery packs to align with their product strategies.
In , the market share of each format across the 6 largest battery manufacturers was: prismatic ~40 percent, pouch ~35 percent, and cylindrical ~15 percent.
Selecting the best of these cell formats for the application is often the key to enabling an OEM to fulfill the functional objectives of its EV strategies.
Battery cell formats play a vital role in enabling OEMs to fulfill their strategies
When deciding on a Li-ion EV battery specification, OEMs have to carefully consider all the interrelated factors, including the strengths and weaknesses of each battery cell form factor in meeting the company’s strategies for the particular model being designed.
The important contribution of the battery-cell form factor in meeting a company’s wider strategy is well illustrated by Tesla’s switch from cylindrical to prismatic cells in all its standard-range Model 3s and Ys. To reduce the cost of its Standard Range lineup Tesla chose to change to lithium iron phosphate (also known as LFP or LiFePO4) battery chemistry, which is also less energy-dense than the nickel cobalt aluminum (NCA) chemistry used in its cylindrical cells. By converting to larger format, higher capacity, prismatic cells – which also allowed the company to embrace cell-to-pack (CTP) design - Tesla was able to partially compensate for the loss in energy density, while retaining the cost benefits.
Tesla’s original choice of the cylindrical cell was primarily driven by availability, safety, and cost. As the oldest form factor, the greatest advantage of the cylindrical cell is its availability and cost-effective manufacturing – thanks largely to economies of scale.
Cylindrical cells, because of their shape (with a beneficial surface area to volume ratio) that allows for cooling pathways between the cells, also have good thermal performance when assembled into packs.
However, there are also drawbacks to the cylindrical format. Due to their shape, battery pack volumetric efficiency is compromised. The gravimetric energy density, at a pack level, is also lower than that of pouch or prismatic formats - thereby affecting applications with strict weight constraints or range requirements.
Additionally, OEMs need to plan for a much higher cell count, compared to prismatic cells, to achieve the same capacity. Even with a with ~25 Ah capacity, as presented by Tesla, the cell count is 4 times that of a 100 Ah prismatic cell to achieve the same pack capacity, which in turn increases the overheads for BMS, TMS, etc.
Due to these limitations, many in the industry believe cylindrical cells have reached their ceiling in terms of performance and optimization, paving the way for pouch and prismatic formats to dominate future strategies.
The pouch cell, although not as widely used in EV applications as the prismatic format, has two characteristics that OEMs can leverage to customize their EV strategies: Weight and flexible packaging.
Because of its construction the pouch cell is flexible, allowing designers to create custom shapes and sizes to fit specific applications. In terms of space optimization, this translates to between 90 to 95 percent packaging efficiency which converts into improved volumetric energy densities. The absence of a rigid casing also contributes to its light weight, making the pouch cell suitable for applications with weight constraints – although this is often offset to some extent by the weight of the battery-pack housing.
One of the critical challenges for OEMs is battery thermal management. Efficient heat dissipation is essential to maintain optimal battery performance, extend battery life, and ensure the safety of the vehicle during fast charging. The downside of pouch cells in EV applications is the poor thermal conductivity of the pouch.
Thus, in applications where thermal management is deemed critical, manufacturers turn to cylindrical cells or increasingly, the prismatic form factor.
Prismatic cells that, thanks to their stacked "layer" configuration, facilitate heat dissipation and temperature regulation also offer excellent space utilization. The stacked design allows OEMs to maximize the available volume within the vehicle, thereby optimizing the energy density which in turn allows the OEM to either increase the EV’s range or reduce the size of the battery for the same performance depending on the OEM’s strategy.
Looking for innovative ways to increase energy density and range, several OEMs are turning to large format prismatic cells, assembled directly into the battery pack, in the CTP format. By foregoing the traditional assembly of cells into modules, weight is reduced and volume utilization optimized – resulting in cost savings and improvements in energy density.
StoreDot’s innovative concept gives OEMs access to a cost-effective extreme fast charging EV strategy
Power density is another critical parameter that OEMs evaluate when selecting a Li-ion battery cell form factor. Higher power density enhances fast charging capabilities and EV dynamic performance, such as acceleration.
Whereas typical cell-to-pack architectures aim to improve energy density to extend range, StoreDot also enables extreme fast charging (XFC) with its I-BEAM XFC™ concept. It overcomes many of the complexities and cost challenges of embedding XFC capability at the vehicle level, allowing OEMs to more easily implement fast charging strategies.
At the core of the I-BEAM XFC™ sits StoreDot's proprietary 100in5 cell technology, which enables charging 100 miles, or 160km of range in just 5 minutes. StoreDot’s 100in5 electrodes are assembled into its new I-BEAM XFC™ prismatic cells, which are designed to be incorporated directly into the battery pack.
Unlike traditional cooling systems, the I-BEAM XFC™’s patented Structural Cooling concept is embedded into the structure of each cell, providing enhanced thermal management. This prevents localized hot spots and maintains uniform temperatures across the battery pack, enabling it to accept the ultra-high currents required for fast charging, with minimal system overhead.
Conclusion
As OEMs seek to expand their model lineups to satisfy the demands of an ever-increasing and diverging range of consumers, the role of the battery cell’s form factor in giving manufacturers the design flexibility to meet unique strategies is likely to intensify.
Currently, there is no unanimous winner in terms of battery cell format, with the industry embracing all three configurations to cater to different needs and strategies. The battle is not only about energy storage but also the physical design that influences critical aspects such as thermal management and overall efficiency – with each cell form factor offering unique solutions.
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