Digital Memory Effect on Batteries

Have you ever wished that you had an extra 20 minutes of battery life left in your portable device? How about an hour! A real difference exists between the life of your battery and the displayed battery charge meter on your device? How big of a difference? How often does it occur? Why does it occur? What can be done? In this article we will look at these questions and learn about what you can do to reduce power waste and maximize your battery life.

Help my batteries dead and I can’t power on! The cultural expression of a “dead battery” is the habitual practice that occurs in place of the more technically appropriate reason of declining capacity. Declining capacity is when the amount of charge a battery can hold gradually decreases due to usage, aging, and with some chemistry, lack of maintenance. Declining capacity is inherent in the ultimate design of a battery – due to limitations with technology - you could consider it the natural side effect or wear and tear of the battery (other wear and tear aspects includes increasing internal resistance, elevated self-discharge, and premature voltage cut-off on discharge).

But there is a real problem with declining capacity and that is the capacity that is measured by your device and displayed to you on your battery charge meter is not always correct. Your device could be reading a digital imprint instead of the actual hardware that transmits capacity back to the device. The digital imprint (digital memory effect) causes your device to use the incorrect reading as its base measuring capacity. This action results in forcing a premature voltage cut-off on discharge, which is when a device does not fully utilize the low-end voltage spectrum leaving unused power in the battery. Another fancy word for leaving unused power in your battery is “waste”. Let’s find out what can you can do reduce power waste and maximize battery life by looking at:

What is the Digital Memory Effect?
What Can Be Done To Correct the Digital Memory Effect?

What is the Digital Memory Effect?

The digital memory effect is a failure mode (see my article series on Battery Failure Mode and Effects Analysis) whose effect results in the transmission of improper calibrations of the battery’s fuel gauge to a device.

Now let’s unpack that answer to discover its real meaning.

First we must distinguish between memory effect and digital memory effect. A memory effect is the concept that was derived from cyclic memory. Cyclic memory is the thought that a battery could “remember” how much energy was used up on previous discharges. Cyclic memory only affects nickel-cadmium batteries. Since we are strictly focused on lithium ion and lithium polymer chemistries I don’t want to get into the chemical change that occurs at the molecular level (crystal growth and concealment of active electrolyte material) but simply will state that the memory effect is the common term people use when there is a voltage depression problem with a battery. Voltage depression causes the inaccurate measurement and subsequent unnecessary charging of a battery.

Inaccurate measurement of capacity is the only similarity between memory effect and digital memory effect since digital memory effect has nothing to do with molecular chemical change. Instead digital memory effect is the improper calibration and reading by the device and the battery’s fuel gauge.

More specifically, inside a battery (or more correctly stated smart battery) is specialized hardware that provides calculated on demand current as well as predicted information to and from the device and includes:

the connector
the fuse
the charge and discharge FETs
the cell pack
the sense resistor (RSENSE)
the primary and secondary protection ICs
the fuel-gauge IC
the thermistor
the pc board
the EEPROM or firmware for the fuel-gauge IC.

In addition to the above advanced chip components information flows from these components to the device through the System Management Bus (SMBus) control – a two-wire interface through which simple power-related chips can communicate with rest of the system. The SMBus allows a device to transfer manufacturer information, transfers model or part number to and from the device and battery, save its state for a suspend event, report different types of errors, accept control parameters and return its status.

Now with that back drop of information we can address the digital memory effect. As alluded to above the fuel gauge integrated circuitry calculates remaining battery capacity (power) and transmits that calculation to the device operating system through the SMBus connectors. The fuel gauge also stores present cell capacity characteristics and application parameters within the on-chip EEPROM (electrically erasable programmable read only memory). The calculated capacity registers a conservative estimate of the amount of charge that can be removed given the current temperature, discharge rate, stored charge and application parameters. Capacity estimation is then reported in capacity remaining and percentage of full charge to the device.

But sometimes the reported information is not correct. The incorrect report of capacity remaining and percentage is caused by the fuel gauge not recalibrating its circuitry automatically. The digital memory effect is then a false reading for maximum capacity and thus results in lower battery run time.

What Can Be Done To Correct the Digital Memory Effect?

To correct the digital memory effect and properly recalibrate the fuel gauge circuitry simply do a full cycle discharge/recharge every several dozen charges. There is no real hard number. If you have never done a complete discharge then do so now. By performing a complete discharge you will cause a manual reset of the fuel gauge circuitry and will eliminate the digital memory effect.