Custom pack-making: designing your IoT batteries to the best standards
Looking into batteries at the very start of your design process is a prerequisite as they have a major impact on your IoT device’s electronics. You can easily avoid power pitfalls by anticipating key parameters. Once you know how many batteries you need to power your device as per your lifetime target, the time comes to consider the actual design of the battery pack. Transportation restrictions and the environment in which the device will be deployed have an impact on the design of the battery pack, thus on its size and cost.
Antoine Derouet is Primary Lithium Battery Development Manager for the Connected Energy Division at Saft. His job? Designing the best battery pack for your complex applications, with the help of his skilled and experienced team of mechanical and electronic engineers, prototypists, technicians and project managers. We’ve invited him to explain what exactly battery assembly is and why you should make sure to submit your design to his team…
What is battery pack-making?
Cells are commonly called batteries, although strictly speaking, only an assemblage of cells should be called a battery or battery pack. To provide the requested voltage and current for your application, we might recommend the use of an assemblage of cells electrically connected in series and/or parallel: a battery.
Connecting two cells in series allows for higher voltage, whereas connecting them in parallel will offer a higher capacity or lifespan.
In series (S), the positive terminal of the cell is connected to the negative terminal of another cell so that the current flows through each cell. You will therefore benefit from the voltage of all connected cells.
In parallel (P), the positive terminals are connected together, as are the negative terminals. The current is divided among the cells which means that the capacity of each of the cell is accumulative. For example, if 1 cell offers 5 years lifetime in the field, two will give you 10 years, 3 will give you 15 years in the field, etc.
After the lifetime calculation performed by our applications engineers, you know how many cells you need to power your device:
- 1S 2P= 1 cell in series, 2 branches in parallel. (1x2)
- 2S 1P = 2 cells in series, 1 branch in parallel. (2x1)
- 2S 2P = 2 cells in series, 2 branches in parallel. (2x2) …
From there, many combinations are possible, depending on your device design: cube, stick or block. 4S1P side by side 2S2P in stick 2S2P in bloc
2S2P in bloc 4S1P side by side 2S2P in stick
If your device requires only one cell, things are pretty straightforward. Once you’ve consulted our Application Engineers to have the best cell recommended to you, just read our article about “How to best integrate your battery into your smart object”, apply our recommendations and you should be all set to go!
However, as soon as you need two or more cells, get in touch with our battery development team: it will save you time, energy and money.
What do you need to consider when planning your battery assembly?
Your project might require a custom designed battery to meet extreme pressure, heat, vibrations, or transport requirements.
Transportation
Transport is one of the most common —and often overlooked— constraints to consider for the battery pack. Under current transportation regulations, Lithium batteries are classed as dangerous goods and as such, they are subject to several safety requirements. Airfreight forwarders will ask you to provide a UN38.3 Test Summary Report to allow the shipping of lithium cells. It consists of a series of 8 safety tests simulating transport conditions such as: pressure, temperature, shock, vibration, impact, altitude, etc.
To pass these tests, batteries must feature a robust design and protections against short-circuits and explosion. Therefore, we may have to add fuses, sheaths, specific connectors or adhesive foams to match transport standards. All these take some space that needs to be accounted for in the electronic design.
Climate and environmental constraints
If your device is deployed in a very humid or warm environment or subject to shocks or vibrations, as is often the case for industrial or ATEX applications, additional insulation may be needed. The goal here is to make sure that your battery can work as planned, even in tough conditions.
Here are some solutions to consider to protect and insulate the battery.
Additional electronic components
We’ve already mentioned the electronic constraints, especially the power and capacity requirements of your device, but depending on how your device has been conceived, specific additional components might be need.
For example, if you don’t want a connector but prefer to solder the battery directly on to the electronic board, a PTC, (positive temperature coefficient or reversible fuse) can be added to protect the device from excessive current or heat generation.
If the battery isn’t the only power source, we might add diodes on the backup battery to protect it against charging. The diode allows the current to pass in one direction, but not in the other.
Also depending on the battery location in your device, the wire length might need to be extended.
Last but not least, we usually offer advice on connector prices and availability to avoid the possibility of you choosing a component that cannot be purchased at an affordable price.
Your project is unique. That’s why Saft’s Battery Development team provide a wide range of design services for Lithium Primary and Lithium-ion batteries, covering most specific needs of your applications. Turning to us to help design your battery pack is the guarantee that your device will meet safety standards, and work as planned once deployed in tough environments, at controlled costs.
Pre-design estimations, lifetime calculations, application criteria and NRE estimates are provided at no cost to you, in all confidentiality. Because your success is our priority. Don’t hesitate to contact us.