Custom Battery Packs
Purpose of Battery Packs?
Battery Cells come in fixed voltages and capacities. Capacities do vary, but voltages don't. In order to meet your power requirements a battery pack may need to be used. The type of battery, the number of cells, the shape of the pack, and the components of the pack will be determined by the voltage and load current of the device being powered.
Other considerations will be available space, operating temperature, transportation requirements, usage conditions, and charge/discharge specifications.
For more on this subject, see key considerations when specifying a custom battery pack.
At a Glance: Custom Battery Packs
- Custom battery packs combine multiple cells and supporting components to meet specific voltage, capacity, runtime, and environmental requirements.
- Battery pack design involves both mechanical and electrical considerations, including cell matching, protection components, connectors, enclosures, and wiring.
- Proper battery pack configuration helps optimize performance, safety, reliability, and compatibility with the end product.
Battery Pack Assembly
Heat Shrink Tubing
The most common way to hold the pack together is to use heat shrink tubing. Heat shrink tubing is typically made of polyvinyl chloride and varies in thickness based upon battery type and configuration.
Lead Wires
To connect the pack to a device, vinyl clad electrical wire that conforms to UL requirements is typically used. Red for the positive and black for the negative are the standard colors.
Thermal/Thermostat Components
Thermal protectors (thermistors) are typically used to prevent overcharge and overheat. These components are connected in a direct line circuit to the battery.
Connectors
The ends of the lead wires are usually connected to connectors specified by the customer to match their requirement for connection to the device.
Adhesive
There are several standard adhesives that are used to connect the batteries inside the pack that are standard in the industry. Some customers specify which adhesive is to be used that they believe will improve the performance for their specific application.
Nickel Strips
Nickel foil is used to spot weld packs together. Nickel is fairly low resistance, yet has enough resistivity to be spot welded. It is strong, has very good corrosion resistance, and will not oxidize easily.
This table gives examples of the resistance of nickel spot weld strips.
| Cell Size (cm) | Foil Thickness (cm) | Strip Width (cm) | Strip Length (cm) | Resistance (milliOhms) |
|---|---|---|---|---|
| AA | 0.018 | 0.5 | 1.4 | 1.0 |
| AA | 0.025 | 0.5 | 1.4 | 0.76 |
| Sub C | 0.025 | 0.05 | 2.3 | 1.2 |
| Sub C | 0.025 | 1.0 | 2.3 | 0.6 |
| Sub C | 0.018 | 0.5 | 2.3 | 1.7 |
| D | 0.018 | 1.0 | 3.3 | 1.2 |
| D | 0.025 | 1.0 | 3.3 | 0.9 |
| D | 0.025 | 2.0 | 3.3 | 0.4 |
Protective Cases
The most typical type of protective battery cases are injected molded plastic or steel cases. These can be custom designed for every application.
Over the course of life most batteries release hydrogen, and sometimes oxygen. Take this into account if you are designing a closed system, such as waterproof lights, weatherproof installations, etc. Some method of releasing or absorbing the hydrogen, flooding with air or inert gas should be used. In closed cabinets some provision for ventilation is necessary to prevent hydrogen gas from accumulating.
Other various standard materials and components in battery pack manufacturing.
Electrical Considerations
How many amp-hours do I need?
Cell capacity is rated in amp-hours or milliamp hours. The symbol for capacity is C. This is amps times hours. Divide by hours and you get amps, divide by amps and you get hours. For example a 5 amp hour battery is the same as a 5000 milliamp-hour battery. If you want to discharge in 10 hours, you can get a current of 5/10 = 0.5 amps. If you need 100 milliamps current, then you can run for 5000/100 = 50 hours.
Often a discharge or charge rate is given proportional to C. So a discharge rate of C/5 means C/(5 hours), or the constant current to fully discharge the battery in 5 hours.
The calculation of run time versus current is a rough estimate, but is accurate under the right conditions. The faster you discharge, the lower the capacity of a battery. This trade-off depends on the battery chemistry and construction. Usually the capacity of a battery is quoted at a C/20 discharge rate. So an 12 amp hour battery sealed lead acid battery will actually put out a steady 0.6 amps for 20 hours. However, if you discharge the same battery at 12 amps, you would expect to run an hour, but you will only last for 22 minutes. Also, if you wan to run at 10 milliampere you will get less than the expected 1200 days, since self-discharge of the battery will limit your run time.
Voltage Requirements
The first question to answer is "how much voltage do I need?" The second is "how many cells in series do I need?"
The voltage of any cell is a moving target. The following table shows the range of the various chemistries:
Learn more here, Understanding Voltage Requirements of a Custom Battery Pack.
| Chemistry | Type | Nominal Voltage | Fully Charged Voltage | Fully Discharged Voltage | Minimum Charge Voltage |
|---|---|---|---|---|---|
| NiMH | Secondary | 1.2 V | 1.4 V | 1.0 V | 1.55 V |
| NiCad | Secondary | 1.2 V | 1.4 V | 1.0 V | 1.50 V |
| Lead Acid | Secondary | 2.0 V | 2.1 V | 1.75 V | 2.3 - 2.35 V |
So a 10 cell pack of NiMH cells would have 14 Volts when fully charged, and run down to 10 volts when fully discharged. Your system must be able to tolerate this voltage range.
Furthermore, if you want to be able to charge while your system is running, the system must be able to accept the charging voltage, which is always higher than the nominal or the fully charged voltage. Work with the charger manufacturer to make sure that you have this problem solved.
Matching Cells in a Pack
Be careful to match the cells in a battery pack. When a battery pack is near zero volts under load the weaker cells will go into reversal, and suffer damage and perhaps venting.
Frequently Asked Questions
Quick Links
- Why are custom battery packs used instead of individual battery cells?
- What components are typically included in a custom battery pack?
- How is battery pack capacity determined?
- Why is voltage configuration important in battery pack design?
- Why must battery cells be matched within a battery pack?
Why are custom battery packs used instead of individual battery cells?
Individual battery cells are available only in fixed voltages and capacities, which may not match the power requirements of a specific device. By combining multiple cells into a custom battery pack, designers can achieve the voltage, capacity, and runtime needed for the application. Additional factors such as environmental conditions, available space, transportation requirements, and charging specifications can also influence the pack design. A custom battery pack allows these requirements to be addressed within a single integrated power solution.
What components are typically included in a custom battery pack?
A custom battery pack often incorporates more than just the battery cells themselves. Common components include heat shrink tubing or protective enclosures, lead wires, connectors, nickel weld strips, thermal protection devices, and adhesives used for assembly. Depending on the application, additional protection circuitry or monitoring components may also be included. Together, these elements help deliver the required electrical performance while protecting the battery during operation and handling.
How is battery pack capacity determined?
Battery capacity is typically measured in amp-hours (Ah) or milliamp-hours (mAh), representing the amount of energy a battery can deliver over time. Capacity requirements are based on the current demands of the device and the desired runtime between charging or replacement. Actual runtime can vary depending on discharge rate, battery chemistry, operating conditions, and system efficiency. Calculating capacity requirements early in the design process helps ensure the battery pack can support the intended application.
Why is voltage configuration important in battery pack design?
Voltage configuration determines whether the battery pack can properly power the intended device throughout its operating cycle. Because battery voltage changes as cells charge and discharge, designers must consider nominal, fully charged, and fully discharged voltage levels. Multiple cells are often connected in series to achieve the required voltage, while ensuring the device can tolerate the resulting operating range. Voltage requirements also become important when a battery will be charged while powering the system.
Why must battery cells be matched within a battery pack?
Matched cells help ensure balanced performance and reliable operation throughout the battery pack's service life. If cells have different capacities, characteristics, or states of health, weaker cells may become overstressed during charging or discharging. In multi-cell configurations, unmatched cells are more likely to experience voltage reversal or other conditions that can reduce performance and damage the battery. Using properly matched cells helps maximize safety, longevity, and overall pack reliability.
Maximize Your Device's Power Potential
Consult with our engineering team for custom battery pack solutions tailored to your voltage, load, and environmental requirements.
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