The ubiquitous CR2032 battery is a coin-shaped three-volt lithium-ion battery. This class of battery has a diameter of 20 mm and a thickness of 3.1 mm, with some slight variations. Commonly referred to as a CMOS battery or a coin battery, CR2032 battery units are often used in low-power applications, such as powering a computer’s BIOS or a quiescent device’s real-time clock.
What Is a CR2032 Battery Made Of?
CR2032 is an International Electrochemical (IEC) designation. The same unit is called the 5004LC by both the American National Standards Institute (ANSI) and by the National Electronic Distributors Association (NEDA). Specifically, the CR2032 battery is a Lithium-manganese dioxide battery (LiMn02). It is composed of a Mn02 cathode and a lithium anode. The device is specified for a 225 milliamp hours (mAh) and typically operates over a temperature range of –20 ºC to +70 ºC. There is also a Lithium-carbon monofluoride battery, the BR2032/5004LB, with similar parameters.
The construction of a typical CR2032 battery features a stainless steel case to prevent any leakage of the non-corrosive electrolyte. A long storage life is made possible by the typically low self-discharge rate of LiMn02 batteries. These batteries can lay dormant for years and lose very little of their original charge.
Figure 1: Construction of a CR2032 battery button cell. (Source: Varta)
The CR2032 battery is often used to provide power for ultra-low power subsystems such real-time clocks and oscillators, which must stay operational even when system power to the overall device is shut off. Furthermore, when the device is turned on, the unit should be specifically designed to refrain from drawing power from the CR2032 battery, and subsystems like RTCs are powered from the unit’s main power supply for as long as the device is operational. Thus, in practice, the device is usually taken out of service for reasons such as obsolescence long before the CR2032 battery is depleted.
How Long Do CR2032 Batteries Last?
The simplest way to look at battery life is to look at the total charge in milliamp hours, and divide it by current consumption.
Battery Life = 225 milliamp hours/current
In an application drawing a constant 1 microamp or less, this comes out to over 10 years. This might actually be true in a case where the battery is powering computer BIOS, but in most cases, the battery must supply more current continuously or in pulses.
Figure 2: The capacity of a CR2032 battery versus continuous current demand. (Source: Low-Power Design)
Here we see that if the CR2032 battery is tasked with providing 0.5 mA (500 microamps) continuously, its output voltage will not deteriorate to 1.6 volts until it has provided 240 mAh. If on the other hand, it must provide 3.0 mA, it can only provide 155 mAh. So, it can be seen that not only will the battery last only 0.5/3.0, or 17 percent of the time as expected, but there will also be an unexpected 155/240 reduction beyond that, based on the higher drain. Of course, if a circuit can’t function below 2.0 or 2.4 volts, the added drain makes an even starker reduction in useable battery capacity.
If the CR2032 battery must contend with a pattern of pulsed demand for current, that, too will cause a derating of the optimal 225 mAh figure. This can be a significant problem for Bluetooth-enabled devices. This can be a major pitfall, and designers must seriously consider its ramifications.
On the other hand, these CMOS batteries hold up quite well against significant variations in temperature. Panasonic battery CR-2032L/BN is available from Arrow Electronics. The datasheet for this battery reveals that this device weighs a scant 2.9 grams, typical of a CR2032 battery. This unit has an operating temperature range of -30 ºC and +60 ºC.
Figure 3: Temperature characteristics of the Panasonic CR-2032L/BN. (Source: Panasonic)
The chart above shows a continuous draw of 190 microamps. It can be seen that over a wide range of the battery’s operating range, from -10 ºC to +60 ºC, the unit will last almost the same amount of time before the output voltage decreases from 3 V down to 2 V.
Mounting the CR2032 Battery
Soldering a CR2032 battery to a PCB presents some problems because the amount of heat needed is more than the battery can take. It is important to note that any overheated lithium-ion battery poses a very serious explosion hazard. For that reason, manufacturers offer coin cell batteries with solder tabs attached, so heat is not directly applied to the battery.
The alternative is to employ a battery holder, but that, of course, involves extra expense for the OEM. For that reason, unless it is anticipated that the CR2032 battery will need to be replaced during the lifetime of the device it services, manufacturers will avoid this step if it is practical to do so.
Top 5 Considerations When Selecting a CR2032 Battery for Your Next Design
1) Will the finished product need to operate on very low power?2) Is it likely that the battery will need to be replaced during the life of the product?
3) Will your product make continuous, occasional, or pulsed current requirements of the battery?
4) Will you employ a socket for the CR2032 or solder it onto a PCB?
5) Does the unit under design need to have an alert capability or timing system continuously in operation, even when the rest of the unit is quiescent?
What’s New in CR2032 Batteries?
There is probably no field in technology that is attracting more money and research effort than lithium-ion batteries, and the improvements will undoubtedly lead to CR2032 batteries that have greater power capacities.