A Lithium Ion Battery can be a very complicated product to design but before diving in to the hundreds of variants, it is best to divide and conquer the task by separating into several steps.
Choose the Size
The first step is to specify how long the device is required to operate. A battery that is required to run for hours would be considered to be General Purpose whereas if one hour or less is the expected run time then the battery needs to be designed as a High Rate battery. Even General Purpose batteries can be “pulsey" though, so caution here must be exercised. E.g. A device that has a motor in it, like a printer, tends to fit in to this category. It is essential to make sure that the the device, its power drain and duty cycle is understood before jumping in and taking the wrong design direction.
Once the run time is determined the power must be specified. Power (watts) x Time (Hrs) = Battery Energy (watt-hrs). If there is a pulse application the Power x Time both high rate and low rate parts need to be added together to get the total energy.
A high rate battery must have a low internal resistance (IR). In a pulse application, in which each pulse is less than 1 percent of the runtime, subtracting the IR of the high rate pulse from the average voltage, e.g. 3.7V , and ensuring that it is above the minimum operating voltage of the device, is a good rule. For the sophisticated designer, building algorithms to forecast runtime can be incorporated in to the fuel gauge.
The 3.7 V average voltage supplied by a single LiIon cell in a product may run the device electronics because they are low drain. A motorized unit may require choosing 2 or more cells in series to reduce internal energy loss. The energy loss in the battery is the Current squared, e.g.I x I, multiplied by the pack resistance. By using two cells in series, the current will drop in half for the same power delivered, and so the internal battery energy loss will decrease by a factor of 4. More savings can be gained with 3 or more cells but that will increase cost, battery size and system complexity accordingly. Designing batteries is all about trade-offs whatever the Chemistry.
Choosing cell size is as simple as dividing the Energy (watt-hrs) by the series cell average voltage. For a single cell this is about 3.7 V. This voltage represents the most popular flavor of LiIon for small batteries and is based on Lithium Cobalt Oxide. Different flavors of LiIon cells may have an average voltage slightly lower. LiIon cells with a lower cobalt content can be a little cheaper and more thermally stable. Unless the battery is very large these differences however are small.
So, if 5 hrs. of run time is needed, at 1 watt, and there is only 1 cell in series, then the cell capacity is, (5x1)/3.7, or about 1.35 Ah. Cells come in many standard capacity (Ah) sizes so pick the size that is closest to the device need. If possible, target a popular size because they will always be cheaper than the low volume products.
Choose a charge method
Typically the charger is separated physically from the device to maintain a low weight for the device when it is in use. In this case, the charger weight is not so relevant, and a simple inexpensive linear transforming charger, also known as the brick, can do the job. In the case of making the charger a part of the battery and device, the use of a smaller, lightweight, and more costly digital High Frequency (HF) charger is the right choice.
A charger requires a charge control chip that will limit the charge current at low battery state of charge and then limit the maximum charge voltage at full charge. The degree of sophistication in charge control chips varies widely. The lowest level of sophistication charges a little slower. To charger at faster rates the charger must know a lot about the battery it is charging. To do so they have to communicate with the battery's control chip and so are called smart chargers.
The maximum charge voltage can be set lower (.05 to .1 V per cell) than allowed by the maximum specified cell voltage. Doing so can significantly increase cycle life with a small decrease in cell capacity, if that is a key desire.
The standard way to improve safety is to provide a back-up system. Two safety devices, with very low failure rates, failing at the same time is an extremely rare event. A backup for the charger, for total battery voltage control, and a monitor and control for individual cells in a 2 or more series voltage control, is the Safety (or Battery Monitor) circuit. This device has the key function of not letting the cell voltage exceed the maximum allowable for the chemistry. It also shuts off the charge or discharge if various alarm settings are exceeded. This includes high and low temperature, high current, overvoltage, undervoltage, and short circuit.
If the device does not have its own microprocessor, a fuel gauge can be added that calculates the state of charge of the battery, and that will light a number of LEDs. Even if the device does have its own microprocessor, this function can be offloaded to the fuel gauge chip. Additional functions included with some fuel gauges include authentication, identification, and memory of lifetime parameters, such as maximum temperature, pack resistance, number of cycles, and State of Health (SOH). State of health is an algorithm based on time, temperature, operating rates, pack resistance, etc., that can estimate how much cycle life or calendar life is left in the pack.
Once the cells and your electronics are decided, including temperature sensors, they can be assembled into a pack, or directly into the device. The pack has the advantage of being replaceable. This is great option if the device needs to be in operation at all times and a second pack can be charging while waiting to be used. This option is more expensive ,and is larger and thicker, due to the additional pack plastic.
Ready for operation
Altogether, the pack now has enough power and run time to do the job. It can charge as quickly as needed and not exceed safe temperatures or state of charge limits. The battery can be allowed to run until the job is done and it is never over discharged The pack will announce its state of charge and even its state of health.