Laptop battery life time :: how it works

27 Feb 2010
Posted by Laptop Junction

Laptop battery life time

Laptop battery technology has seen a very steep development curve in just 10-15 years. Nowadays the average laptop, notebook or netbook uses a bunch of complex chips for the sole purpose of charging and monitoring laptop battery cells. Here’s a bit about core technology involved in managing laptop battery life and what you can do to maximize laptop battery life time.

Laptop smart battery design

Most laptop batteries are designed according to Smart Battery specifications and guidelines. Market leaders Texas Instruments and Maxim produce Smart Battery compliant chips of which many found their way in laptop batteries and laptops.

The sole purpose of smart battery chips is to power a laptop from (Lithium-Ion) battery cells as efficient and safe as possible. Efficient laptop batteries have the best battery life time (ie: 1-2 years) and battery run time (ie: 2-4 hours).

Laptop battery gauge chips for a DELL C1295 53WH 4700mAh
A laptop battery gauge chip with at the bottom a row of 5 charge indicator leds.
Right to left : 8-bit Microprocessor (M37516), 8 pin memory chip,
Gauge Chip (4497) , AFE chip (BA3161),

The BQ chip range of Texas Instruments is designed for measuring battery cell capacity and maintaining safety. These chips reside inside the laptop battery casing close to the battery cells. Most laptop batteries based on the Smart Battery specification carry at least 2 chips, a Battery Gauge and a Battery Safety Chip known as the Analog Front End. (AFE)

Laptop battery gauge chip

The laptop battery gauge chip is the “brain” for measuring and calculating battery life time or Full Charge Capacity (FCC). The chip stores measurement and calculation results in flash memory inside the laptop battery casing.

The Full Charge Capacity is used by the laptop as input for the battery charging chip on the motherboard. A lower Full Charge Capacity value results in less energy fed to the laptop battery and usually shorter charge times. To see this effect in action, see the simple laptop battery test.

Updates of the Full Charge Capacity value theoretically can go both ways - up or down. However it’s more likely to be less than the previous FCC value. Only a factory reset of laptop battery memory can restore the FCC value to the designed laptop battery capacity value.

Laptop battery gauge for a DELL C1295 53WH 4700mAh
Laptop battery LED gauge driven by a battery gauge chip

Depending on laptop battery model, a battery gauge chip can be connected to 4 or 5 leds in the laptop battery casing reflecting the actual remaining laptop battery capacity.

The battery gauge chip is a microprocessor running on-chip embedded firmware. It communicates with the laptop though the SMB (System Management Bus) which is a part of the Smart Battery Specification.

The battery gauge chip has a bunch of memory registers which can be read (and set) through the SMB. Full Charge Capacity is one of the memory registers accessible through BIOS functions and with the right software, visible in Windows XP or Vista. Only Windows 7 has native support for reading laptop battery memory registers as specified by the Smart Battery specification.

Laptop battery life time calculation

Based on chip information from Texas Instruments – Full Charge Capacity is a calculated value based on the last full to empty discharge measurement of the laptop battery.

The process is called “Capacity Learning” and involves a calculation with several other internal battery gauge chip register values.

Based on the specification for Texas Instrument battery gauge chips, an update of Full Charge Capacity is the result of a Qualified Discharge

Standalone laptop battery charger
A standalone laptop battery charger - bypassing the internal laptop battery charger and AC power adapter

A Qualified Discharge has the following start / stop condition once the laptop AC power adapter is disconnected:

Start : The laptop battery is at “near-full” capacity
Stop : The laptop battery reaches the “battery low” voltage threshold

The following conditions must be valid during the full “Qualified Discharge”:

1.    The laptop battery is not charged by the AC adapter during measurements
2.    Self-discharge of laptop battery cells is under a preset value
3.    Temperature does not drop below the low temperature threshold
4.    Discharge current does not drop under a minimum preset value
5.    Discharge current does not exceed maximum overload value

The new FCC value can not change more than a maximum decrease or increase preset. For instance the BQ2085 has a maximum decrease preset of 256 mAh and increase of 512 mAh. Based on these values the Capacity Learning process is a gentle laptop battery discharge.

For a new DELL 54Wh laptop battery, the "cost" of one Capacity Learning process cycle is roughly estimated at a maximum of 2% of total laptop battery capacity. (4 hour gentle discharge maxed at 256mAh)

Laptop battery life time conservation

From a user perspective, there are several ways to control or prevent an update of the FCC memory value in laptop batteries based on the Smart Battery specification.

- Connect the laptop AC power adapter
- Removing the laptop battery
- Laptop shutdown or invoke hibernation mode
- Never reach a full or empty laptop battery charge
- Utilizing maximum processing power (CPU / GPU) for a short while (ie: system start-up)

The easiest way is to connect the laptop AC power adapter. However disconnecting it after it's fully charged immediately invokes a Qualified Discharge. This can be stopped by connecting the laptop AC power adapter for a short while once the battery is discharged for about 70 % or so. Doing so disqualifies the discharge for the remaining battery run time and prevents an update of FCC.

If there's no AC mains around, peaking CPU/GPU processing power has the same result. It disrupts a gentle Qualified Discharge needed for the Capacity Learning process.