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Keywords:
analog, digital, comparators, battery monitoring, battery over-voltage, battery under-voltage, Li+ Batteries, Ni-cd batteries, Secondary batteries, re-chargeable
batteries, micro-controller, portable applications, MAX4000, MAX40001, MAX40004, MAX9065
APPLICATION NOTE 6378
HOW TO MONITOR STATE-OF-CHARGE IN SMALL BATTERIES WITH TINY,
ULTRA-LOW-POWER COMPARATORS
By:
Ashwin Badri Narayanan, Member of Technical Staff, Maxim Integrated
Abstract: Many of today’s portable consumer electronic devices are powered by small button- or coin-cell batteries. Users, of course, expect long battery life and reliable
charge-level information. However, it can be quite challenging to efficiently monitor the health and state-of-charge (SOC) of these batteries without significantly affecting
said SOC. In this application note, learn how simple, low-power monitoring circuits for small batteries can address this challenge.
Monitoring Battery Voltage and Temperature
Systems engineers have an important task when it comes to budgeting their design's power requirement. Looking at the system as a whole,
microcontrollers/microprocessors serve as the "brains" managing the system reliably and performing the required functions. The typically power-hungry controller is the
system’s workhorse, so it does not make sense to have the controller do all the work. To prevent system power dissipation, the controller needs to remain in a sleep
state for extended periods as it seeks out flags present in the GPI pins.
For continuous monitoring of the system's vital functions, many engineers are turning to low-power circuits. These circuits flag the micro (usually in the form of interrupts)
to perform the required duty when some event occurs. Monitoring and controlling the state of the battery supply is another of their critical functions. When the battery tput
voltage is lower than required, the battery is discharged and needs to be recharged. Similarly, when the battery output is higher than required, a flag can be asserted
when the battery is completely charged and needs no further charging. It is also essential to monitor the battery case temperature, since this offers valuable insight into
the loading conditions, ambient temperature, or the presence of a fault.
An analog-to-digital converter (ADC) or comparator with window function provides a simple solution to monitor battery voltage and temperature. There are also
sophisticated battery monitors and fuel gauges designed specifically for this function. But a careful trade-off must be made, keeping power, speed, accuracy, cost and
form factor (space constraints) in mind. Different systems can require different priorities from the aforementioned list, affecting the overall system design. Before
examining battery voltage monitoring and temperature monitoring using comparators, this application note first covers some basic, yet important, information about
batteries.
Restrictions of Secondary Batteries
Secondary or rechargeable batteries differ in their chemical composition and structure from one to another. These differences, in turn, dictate the specific power
(maximum current delivered to load), lifespan, and thermal stability of battery cells. There are also trade-offs to consider: the higher the specific power, the lower the
safety rating, life span, cost, and vice versa.
Secondary batteries wear out, possess a charge-discharge lifecycle, and come with certain restrictions, including:
1. The amount of current they can provide for a specified range of output voltage over time
2. The amount of current they can take in (during charging)
3. Maximum safety voltage (the voltage level to which they can be charged)
4. Minimum safety voltage (the voltage level to which they can be used)
5. The amount of heat or cold they can withstand
Each of these restrictions affects the battery's lifespan. Not adhering to these conditions can cause the battery to wear out sooner or even flare up. The above-mentioned
ratings change based on the battery capacity, which is directly proportional to the form factor or the size.
Secondary Batteries for Portable Electronics
Table 1
shows the typical characteristics of the most common secondary/rechargeable single-cell battery types.
Maximum safety operating voltage is the voltage termed as fully charged and ready. Attempting to charge more is possible, but comes with the risk (sometimes
catastrophic) of reducing lifespan.
Minimum cutoff or disconnect voltage is the voltage assuming the cell has drained. Taking the battery voltage below the cutoff point shortens battery lifespan.
Cycle lifetime and lifespan are different. Each time a battery goes through a cycle charge to discharge is considered a cycle lifetime. The more often you
charge/discharge your smartphones, the shorter the lifespan you can expect.
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