Key to proposing a battery design and determining accurate pricing is a good understanding of the technical and programmatic requirements of that battery. At Totex we have put together a worksheet to collect all of this information. Equally important is that it is a concise written record which makes for a handy reference should the project be put on hold for weeks or months at a time. If you are interested in obtaining pricing from us, please click here to access our worksheet.
Below are some explanatory notes for the worksheet. Please refer to this should you have questions while filling out the worksheet.
General Information: Besides contact information, knowing the FDA class (if any), whether or not it’s an Intrinsically Safe application, and what the battery is used for can be critical information.
Project Schedule: Knowing your schedule requirements helps us to determine if what you want is doable for us in the desired timeframe given NRE requirements, compliance testing and engineering resources. We MUST have quantities to quote.
Discharge Information: When thinking about the overall capacity you want, remember to consider beginning of life (BOL) and end of life (EOL) for the cells. As cells age they lose capacity, so that by EOL you may have as little as 50% of the capacity they started with.
Often, the discharge current is not evenly continuous. This may require using an average. If this still does not adequately describe the application, please make use of the comment line along with the fill-in spaces below.
What is the longest required run time between charges that is required? If this is a back-up battery that generally stays in a charged or nearly charged state, please state so in the comments.
The next three boxes for peak current requirements can be tricky if your current profile is irregular. If it is just an in-rush current spike please state so in the how often comment line. The #/cycle box refers to the number of times the peak current will occur between charge cycles. If the peak durations are non-trivial and frequent, they can have an effect on the capacity requirement (not to mention the current capability of components). Please make this clear so it is not left as an open question that needs follow-up. If the current profile is difficult to describe on this worksheet, feel free to provide a screen shot or hand drawn profile.
A common discharge temperature range listed in cell manufacturers’ datasheets for a Li Ion cell is -20°C to +60°C. Note that this only means that the cell will work over this range, NOT that it will maintain the rated capacity, voltage or cycle life if used at the temperature extremes. If you read a cell datasheet or spec closely you will see that the performance is based on very specific charge and discharge conditions. For instance, near -20°C, most Li Ion cell’s performance is dramatically reduced with a sometimes severe drop in voltage. Continuous exposure (even in storage) to temperatures ≥ +60°C will shorten the life of almost any Li Ion battery.
Charging Information: Make sure to include the max charge time allowed. A “typical” Li Ion cell takes 2.5 to 3.5 hours to charge. High rate cells have lower internal impedance and can charge in much shorter times. A common charge temperature range is 0°C to +45°C. A few cells are specially formulated to charge a little outside these limits and some cells can charge at reduced currents a little outside these limits.
Fuel Gauge: Fuel gauge capabilities range from simple state of charge (SOC) determination using voltage measurements to sophisticated (and more accurate) SOC determination along with many other functions including: circuit protection, cell history storage, analog front-end, LED outputs, cell balancing and more.
Mechanical /Environmental Requirements: Note that when determining maximum dimensions, things such as spacing between cells, housing or shrink-film thicknesses, circuit board size and thickness, contact size and tolerances (of aforementioned and cells) need to be taken into account. And, in the case of prismatic and polymer (pouch) cells, swelling over time may need to be taken into account. 6-8% (for thickness) is typical, with a polymer cell growing more than prismatic one. Cells from lower tier cell manufacturers could experience more swelling. Include life in cycles unless this is a back-up application.
Special note on dimensions: The 18650 size is the most common Li Ion cell size in the world. It therefore has the most R&D poured into it and is by far the easiest to replace if you decide to make a capacity change in the future, or if your cell goes EOL. Prismatic and polymer cells come in much thinner sizes than cylindrical cells and are ubiquitous in the tablet/smart phone world. BUT, only a small fraction of the sizes you can find on the internet will have long term future availability.
Compliance testing: UN38.3 is pretty much mandatory for shipping your battery, either from the battery pack assembler to you, or from you to the customer. UL2504 is not mandatory, but not having UL listing (or UL recognized) can be a marketing problem. IEC62133 is the typical test standard used as a basis for supporting a CE mark. TO sell in Europe, you must have a CE mark. CE EMC testing is also required for selling in Europe and everywhere else that requires a CB certificate. IEEE 1625 is for multi-cell mobile computing applications. IEEE 1725 is for cell phones or products with a cellular communication feature. In addition to these basics there are also a myriad of country-specific standards. UL913, ATEX, IECx are IS standards used with environments with flammable or explosive atmospheres.
After completing, please email to firstname.lastname@example.org