Small button- or coin-cell batteries power many of the portable consumer electronics in use today. Efficiently monitoring the health and state-of-charge (SOC) of these batteries without significantly affecting their SOC can be a challenge. This article discusses how some simple, low-power monitoring circuits for small batteries overcome this challenge.
Battery management in portable systems
From a system design point-of-view, the systems engineer must carefully budget the system power requirement. Microcontrollers/microprocessors would be the “brains” managing the system reliably and performing the required functions. As the workhorse of the system, the controller is usually power-hungry, so it does not make sense to let the controller do all the work. In order to prevent system power dissipation, the controller needs to remain, for extended periods, in a sleep state, looking for flags presented in the GPI pins.
Engineers are resorting to using low-power circuits to continually monitor the vital functions of the system. When an event occurs, these circuits flag the micro (usually in the form of interrupts) to perform the required duty. One of their vital functions is to monitor/ control the state of the battery supply. When the battery output voltage is lower than required, this means it is discharged and requires charging. Likewise, when the battery output is higher than required, a flag can be asserted when the battery is completely charged and no longer requires further charging. Monitoring the battery case temperature is also vital as it provides a lot of information about the loading conditions, ambient temperature, or the presence of a fault.
A typical simple solution to monitor battery voltage and temperature can be an analog-to-digital converter (ADC) or comparator with window function. There are also sophisticated battery monitors and fuel gauges, which are specifically designed for this function. But a careful tradeoff must be made, keeping power , speed , accuracy , cost and form factor (space constraint) in mind. Different systems may require different priorities from the aforementioned list. This would dictate the designer’s system design. This article will discuss battery voltage monitoring and temperature monitoring using comparators. But first, below is some basic information about batteries that needs to be considered.
Secondary or rechargeable batteries differ in their chemical composition and structure from one to another. These differences dictate the specific power (maximum current delivered to load), lifespan, and thermal stability of battery cells. Analogous to the real world, they have their tradeoffs. In general, the higher the specific power, the lower the safety rating, life span, cost, and vice versa.
They do wear out and have a charge-discharge lifecycle. Additionally, they also have certain restrictions such as:
How much current they can provide for a specified range of output voltage over a period of time
How much current they can take in (during charging)
To what voltage level they can be charged, or maximum safety voltage
To what voltage level they can be used, or minimum safety voltage
How much heat or cold they can withstand
All of the above affects the lifespan of the battery. If not adhered to, they may wear out sooner or even flare up. The above-mentioned ratings change based on the capacity of the battery, which is directly proportional to the form factor or the size.
Continue to page two on Embedded's sister site, EDN: “Why tiny, ultra-low power comparators are ideal for battery monitoring.”