How To Select The Right Battery For Your Application
The one thing to remember about battery selection is that there is no such thing as a perfect battery that works for every application. Selecting the right battery for your application is about identifying the most important battery metrics and trading these off against others
Energy vs. Power - The runtime of a battery is dictated by the battery capacity expressed in mAh or Ah and is the discharge current that a battery can provide over time.
When comparing batteries of different chemistry, it is useful to look at the energy content. To obtain the energy content of a battery, multiply the battery capacity in Ah by the voltage to obtain energy in Wh.
The open circuit voltage is commonly used in energy calculations (i.e. battery voltage when not connected to a load). However, both the capacity and energy are both heavily dependent on the drain rate. Theoretical capacity is dictated only by active electrode materials (chemistry) and active mass. Yet, practical batteries achieve only a fraction of the theoretical numbers due to the presence of inactive materials and kinetic limitations, which prevent full use of active materials and buildup of discharge products on the electrodes.
Battery manufacturers often specify capacity at a given discharge rate, temperature, and cut-off voltage. The specified capacity will depend on all three factors. When comparing manufacturer
capacity ratings, make sure you look at drain rates in particular. A battery that appears to have a high capacity on a spec sheet may actually perform poorly if the current drain for the application
is higher. For instance, a battery rated at 2 Ah for a 20-hour discharge cannot deliver 2 A for 1 hour, but will only provide a fraction of the capacity.
Batteries with high power provide rapid discharge capability at high drain rates such as in power tools, or automobile starter battery applications. Typically, high power batteries have low energy densities.
A good analogy for power versus energy is to think of a bucket with a spout. A larger bucket can hold more water and is akin to a battery with high energy. The opening or spout size from which the water leaves the bucket is akin to power – the higher the power, the higher the drain rate. To increase energy, you would typically increase the battery size (for a given chemistry), but to increase power you decrease internal resistance. Cell construction plays a huge part in obtaining batteries with high power density.
Temperature range – Battery chemistry dictates the temperature range of the application. For instance, aqueous electrolyte based Zinc-carbon cells cannot be used below 0°C. Alkaline cells also exhibit a sharp decline in capacity at these temperatures, although less than Zinc-carbon. Lithium primary batteries with an organic electrolyte can be operated up to -40°C but with a significant drop in performance.
In rechargeable applications, lithium ion batteries can be charged at maximum rate only within a narrow window of about 20° to 45°C. Beyond this temperature range, lower currents/voltages need to be used, resulting in longer charging times. At temperatures below 5° or 10°C, a trickle charge may be required in order to prevent the dreaded lithium dendritic plating problem, which increases the risk of thermal runaway (we have all heard of exploding Lithium based batteries which could happen as a result of overcharging, low or high temperature charging, or short circuiting from contaminants).
OTHER CONSIDERATIONS INCLUDE:
- Shelf life – This refers to how long a battery will sit in a storeroom or on a shelf before it is used. Primary batteries have much longer shelf lives than secondary. However, shelf life is generally more important for primary batteries because secondary batteries have the ability to be recharged. An exception is when recharging is not practical.
- Chemistry – Many of the properties listed above are dictated by cell chemistry. We will discuss commonly available battery chemistries in the next part of this blog series.
- Physical size and shape – Batteries are typically available in the following size formats: button/coin cells, cylindrical cells, prismatic cells, and pouch cells (most of them in standardized formats).
- Cost – There are times when you may need to pass up a battery with better performance characteristics because the application is very cost sensitive. This is especially true for high volume disposable applications.
- Transportation, disposal regulations – Transportation of lithium based batteries is regulated. Disposal of certain battery chemistries is also regulated. This may be a consideration for high volume applications.
There are many considerations when selecting a battery. Several of these are related to chemistry, while others are related to battery design and construction. This makes it harder and sometimes meaningless to do a battery metric comparison without a more fundamental understanding of the factors that affect that metric, a topic we’ll explore in the second blog of this series.