Designing a custom battery pack for your application requires figuring out the power specifications. Yet there are several other considerations that dictate the type of battery chemistry to use. In addition to the current, capacity (amp-hours), size requirements, cell configuration, and the number of cells for the battery pack within the application, the voltage must also be determined.
Voltage comprises a unit of current in a battery. This voltage helps to push the right amount of power into the product. When a product is low-voltage, the battery pack will push lower amounts of current out from the battery pack when the product is in use. More high-powered products would require greater amounts of voltage to run at their optimal level.
When you want to figure out the voltage of a battery pack, you first must understand what the nominal voltage of the battery is as well as the voltage of the battery when it is fully charged and discharged.
Voltage by Battery Chemistry
Voltage for batteries does not change. There will be a nominal voltage that could vary within the battery. Also, the voltage differs for battery chemistry. The nominal voltage is the amount of voltage output a cell gives out when charged.
Voltage testing during battery pack manufacturing.
Here are the nominal voltages for the following battery chemistries:
- NiCad: -1.2 volts nominal voltage
- NiMH: -1.4 volts nominal voltage
- Lithium-ion: -3.6 volts nominal voltage
- Lead-acid: 2 volts nominal voltage
Keep in mind that the minimal voltage for each chemistry may not be the actual voltage given. So, if a NiMH battery has 1.2v, it may give out 1.4v for the application when fully charged. Both NiCad and NiMH have a 1.4v fully charged voltage. Lead-acid has a fully charged voltage of 2.1v and lithium-ion has a 4.2v fully charged voltage.
Fully discharged voltage is 1.0v for both NiCad and NiMH, and 1.75v for lead-acid. Lithium-ion batteries should never be fully discharged as this would cause battery damage. A safety cut-off will engage for the lithium-ion battery. The lowest discharge that a lithium-ion battery can go is about 2.8v to 3.0v depending on the safety cut-off switch.
Determining Voltage for an Application
Calculating voltage requires a simple equation of figuring out the resistance of the application and multiplying it to the current measured in amperes (amps). The equation would be:
Voltage (V) = Current (I) x Resistance (R)
To figure out the current draw for the system, you may use a current meter and an adjustable power supply. Use this setup to determine the highest voltage for the system and the lowest voltage for your system. Take note of current measurements. You want to seek a number between the two measurements and make it the lowest power consumption point.
Understanding the voltage range in this manner is important for products and applications that may rely on variable power supplies. Other products and applications will have constant power consumption.
Voltage Ranges
Your application or product will have a set voltage range it can handle before experiencing damage, failure, or shut down. The voltage range for battery packs can be designed for the product depending on its voltage needs as well as available space within the enclosure. If the product needs 14 volts, then 10 battery cells consisting of NiMH chemistries would suffice. However, if the battery enclosure cannot accommodate the typical cylindrical shape of the batteries, a customer may request a different chape to the batteries or a different configuration.
Other times, you may decide on another battery chemistry that offers the same power yet will require fewer battery cells. Lithium-ion batteries offer about 3.7v. Having 4 lithium-ion batteries in the pack would offer 14.8v. This alternative would work if the product can handle the slight increase in voltage.
Cell capacity also plays an important factor in voltage. You want the product to run for a specific amount of time to perform a function before the battery needs to be recharged. The battery will also undergo self-discharge during shipment to a retailer or customer, and when the product is in storage before use. The amount of time the product will be used in one setting and the number of times it will be used on a daily, weekly, or monthly basis should also be taken into consideration to determine the right chemistry.
So, 4 lithium-ion battery cells will give you roughly the same amount of voltage as 10 NiMH cells. However, the discharge rates are higher in NiMH than lithium while the cycle rates are better for lithium-ion. Also, keep in mind that NiMH chemistries are more stable than lithium-ion as there are several travel restrictions for lithium-based batteries. All of these factors may play a part in selection depending on the product.
Shipping Limitations Based on Voltage
Most shipping regulations for high voltage devices focus on lithium-based chemistries due to their instability. This instability could lead to an explosion or fire. These batteries will require UN38.3 transportation certification and will need to be handled and packed in the correct manner for shipment.
According to the IATA, shipping lithium-based batteries may be shipped by themselves or in equipment. If shipped by themselves, they must have a state of charge (SoC) of no greater than 30%. In addition, the number of cells or batteries that can be shipped at one time will be based on the number of grams of lithium metal in the battery and whether they are shipped in a device, by themselves, or shipped with the equipment. Packages may require markings to indicate that a lithium battery is present inside the box.
What to Keep in Mind about Battery Pack Voltage for Product Designs?
You have many battery pack design options and chemistries to select from for your products. When it comes to voltage, figuring out the power needs of the product during the design phases may help you not only decide on the battery chemistry but also the arrangement of the cells and the battery sizes. The battery pack design is crucial when trying to fit the pack into the application, especially for small or lightweight products. You want the battery pack to provide an adequate amount of voltage when in use while also fitting the product or not making the product too heavy.
Like the previous example used, a 10 pack of NiMH battery cells can provide the right amount of voltage for an application requiring 14 volts while keeping costs low. Yet as more battery cells are added, the pack will become bulkier and heavier. More lightweight batteries such as lithium-based batteries with fewer cells in the pack may offer a better solution. Yet keep in mind that the battery packs must undergo transportation certification processes and restrictions, which will increase prices.
Summary
Working with a battery pack manufacturer at the start of the product design process offers several advantages when figuring out the required voltage of the battery pack. At Epec, we can assist you in determining the right chemistry that will provide the necessary power to the application. We also go over designs for the battery pack enclosure for the product, ensuring there will be enough room for the pack. With these designs, you may evaluate the functions of the product and make the necessary modifications.
