Lithium coin cells (batteries) are very popular for timekeeping and memory backup applications. Ever wonder why?
The output voltage is 3 V, roughly double that of other common battery types. Designers can use one lithium coin cell instead of two cells of other chemistries.
Three volts is compatible with many of today’s low-power ICs and with SRAM data retention.
Coin cells are small, reliable, don’t easily leak, and have a low self-discharge rate (0.5% per year for the BR series, 1% per year for the CR series).
Here’s how to decipher a coin cell part number.
The first letter, B or C, refers to the battery chemistry:
B indicates poly-carbonmonofluoride lithium, (CF)n, which is used for high-reliability, low-current applications.
C indicates manganese dioxide lithium, MnO2, which is preferred when the load involves high-current pulses.
B has a slightly higher energy density and flatter discharge curve than C, making it a better choice for long-term memory backup and timekeeping since those applications draw a low, relatively constant current.
The next letter indicates the shape (R, for round).
The four digits represent the physical size: the first two are the diameter in millimeters (mm) and the second two are the height in tenths of mm.
So, a BR2032 is a poly-carbonmonofluoride lithium cell, round, 20 mm in diameter and 3.2 mm in height. According to its datasheet, its capacity is 180 milliampere-hours (mAh). There’s one in each 7330 residing in a high retention force holder.
A solder-pin version of the BR2325 (shown at right) is used in the MRC-100 and in the version 1 editions of the 5K, 6K, and 7K. It’s a poly-carbonmonofluoride cell, round, 23 mm in diameter and 2.5 mm in height. Its capacity is 165 mAh.
How long will a coin cell support a memory system? To figure the time in hours, divide the battery capacity in mAh by the current consumption in mA.
For example, the BR2032 in the 7330 controller backs up three ICs: an SRAM, a Real-Time Clock (RTC), and a Temperature-Controlled Crystal Oscillator (TCXO). The total current required for backup is less than 5 μA (0.005 mA).
Dividing 180 mAh by 0.005 mA yields 36000 hours – about 4.1 years. That’s how long the cell will support the memory system in the absence of main power.
How often should the coin cell be replaced? If the 7330 is never powered up (worst case), the cell lasts 4.1 years. If the 7330 is powered up all the time (best case), the cell’s lifetime is its self-discharge rate (shelf life).
Shown at left is a typical set of discharge curves at various loads. The horizontal flatness is evidence that a lithium cell maintains a nearly constant output voltage over most of its lifetime. It also means that a reading of 2.9 V or 3.0 V doesn’t tell you much about a cell’s remaining life.
The 7330 contains a DS1670 System Controller IC that can issue a warning when the battery voltage drops below 2.3 V. You can program a macro to execute upon this warning (see the Battery Backup Monitoring section of the manual). The warning also shows up on the 7330 console. It’s cheap insurance to replace the cell when you first get the warning or if you are unsure of a cell’s age or condition.
Measuring low current
How do you measure the microampere-level current associated with memory backup applications? One way is to place a 1 kΩ resistor in series with the coin cell and measure the voltage across the resistor. For every 1 μA flowing through the resistor, 1 mV is dropped across it. We check every 7330 this way before shipping to make sure that no battery-backed component is defective and drawing excessive current.