How to Choose custom lithium battery packs?

Applications may require custom battery packs when off-the-shelf packs cannot meet their requirements. The battery packs may need to use specific materials or have certain power, charge, or safety feature requirements.

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Other times, you must arrange for specific testing and certifications based on the industries that will use the device. There are several key considerations to consider when you are designing a customized battery pack.

Determining Power Requirements

Power requirements for battery packs focus on several key aspects: how much power the battery pack holds, how fast it will give power to the application, and how long will the application run before the batteries need to be recharged.

We determine the power requirements (the speed of drawn-out power by using watts (W) and how much energy is stored in the battery cell by using watt-hours (Wh). Basically, the application will require a certain number of watts to run for a specific number of hours.

So, you have a device that is rated to draw out 50 watts of power per hour. You want this device to run for 4 hours. You would take the number of watts (50) and multiply that by the length of time the battery will operate (4 hours).

The equation would be 50 watts x 4 hours = 200 watt-hours (Wh). Now you know that the battery's capacity should accommodate 200Whs.

Another equation you can do is figure out how long the battery will operate when you are connecting several devices to a single battery pack. If you already know the capacity of the battery pack, you can take the stated capacity of the battery (200Wh) and divide it by the total watts from the connected devices.

For this example, you have one device that has 50W and another that has 10W for a total of 60W. The equation to determine the battery's operation time would be 200Wh battery capacity/60W = 3.33 hours.

Determining the requirements for the battery power and capacity for the application will allow you to figure out the number of batteries that will be required for the pack. It will allow you to narrow down your choices about the type of battery chemistry to use as well as how to design the enclosure.

Calculating Charge Requirements

There are both custom chargers and off-the-shelf chargers for applications. Best practices dictate that if you are obtaining a custom battery pack, you want to supply a charger that is designed for that specific pack. This practice ensures that the charger will provide the right amount of energy without overcharging or undercharging the battery pack to avoid damage and a loss of capacity.

Battery charging is based on voltage (which is the unit of current for every cell in the pack) and how much power it pushes for the required application. Unlike capacity and power, the voltage of the battery is consistent. It never changes based on the chemistry of the cell. Each chemistry has a nominal voltage, such as 3.6 volts for lithium-ion batteries, 1.2 volts for NiCad, and 1.4 volts for NiMH. Keep in mind that battery chemistries can provide higher voltages to the applications than the nominal voltages are for the cells. So, a battery that is 3.6V can offer 4.2V for the device.

When it comes to designing a charger, you want one that can provide the required amount of voltage based on the number of batteries present in the pack. If you have 4 lithium-ion cells in a battery pack that offers 3.6 volts, you will determine this required charger voltage by multiplying 4 cells by 3.6 volts (3.6V x 4 cells = 14.4V). You need a charger that provides 14.4V for the battery pack.

Providing Safety Features

Safety features for battery pack design will depend on the type of battery chemistry that is present. Lithium-ion batteries have more unstable chemistries that could lead to thermal runaway, fires, or explosions. Many national and international regulations have requirements where a battery management system is necessary for lithium-based battery packs.

A battery management system (BMS) can offer temperature monitoring, current and voltage regulation, and thermal management. Additional safety features may be added to the BMS based on the environment where the application will be used, charging or discharging rates, vibrations, venting, and other factors.

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Battery pack being manufactured with BMS and custom enclosure.

Another safety feature can involve cell balancing. Cell balancing ensures that some cells do not get discharged or overcharged more than others by redistributing the charge between cells (active balancing) or dissipating excess charge, so all the cells have the same SoC (passive balancing).

Designing Enclosure & Material Requirements

Battery materials focus on the actual construction of the battery, such as the materials used to create the anodes, cathodes, electrolytes, separators, and connections. The types of materials used will depend on the battery chemistry.

For example, anodes can be made from lithium, graphite, or silicon for lithium-based cells while alkaline batteries will have a zinc-powder paste-like gel. Separators usually consist of foam, and electrolytes may range from potassium hydroxide to lithium hexafluorophosphate. Internal connections may be designed with nickel strips. Care must be taken to ensure that the right materials are used based on the application and any stresses or loads that could be experienced while the battery pack is in operation. There may be certain material requirements based on industry, such as military & defense or medical, that will have to be met. Other times, budgetary restraints may require a close look at whether lower-cost materials can be used while still providing a high-quality battery pack that fits the needs of the application.

The same care must also be taken for the design of the battery enclosure. Battery enclosures may be internally placed into the device, or have a separate compartment attached in another location. Some common designs include metal casings, shrink-wrap, or vacuum-formed plastic. Deciding on the enclosure will depend on how it will protect the batteries, how it will protect the user of the application, environmental hazards, and even appearance.

Certification Requirements for Marketing Purposes

Not every type of battery requires certification, except for lithium-based chemistries, while some manufacturers may need certifications based on industry requirements. Manufacturers may also seek out certification to market their products to the public by using this certification to show the high-quality production standards that are in place.

Battery pack with certification markings on the enclosure.

To determine whether your battery pack requires certification, and which certifications to get, will be based on the standards of the certifying organization. Some of the standard certifications come from UL, IEEC, and IEC testing that may require several battery packs that will undergo testing and testing costs.

Summary

Keep in mind that battery pack testing costs will vary based on the organization and the number of tested packs. Testing times can also vary from 4 weeks up to 12 weeks based on the organization. These factors can impact lead times to market and need to be considered in the overall design timeframe of the battery packs.