Miscellaneous/Ask Isidor/Fuel Gauge030811

Fuel Gauge

( August, 03) Q: What is the battery fuel gauge
When the `smart' battery was introduced in the 1990s, one of the main objectives was being able to communicate with battery. This was done by adding a fuel gauge on the battery itself or the equipment it powers. In this paper, we are looking at various fuel gauges, how they work, what advantages they offer and what limitations they pose. Since the system Management Bus (SMBus) is the most widely used `smart' battery, we will focus on this system.

The state-of-charge indicator
Most `smart' batteries are equipped with a charge level indicator. When pressing the `Test' button on a fully charged battery, all signal lights illuminate. On a partially discharged battery, half the lights illuminate, and on an empty battery, all lights remain dark. Figure 4 shows such a fuel gauge.

Figure 4:  State-of-charge readout
of a `smart' battery.
Although the state-of-charge is displayed, the state-of-health and its predicted runtime are unknown.

While SoC information displayed on a battery or computer screen is helpful, the fuel gauge resets to 100% each time the battery is recharged, regardless of the battery's SoH. A serious miscount occurs if an aged battery shows 100% after a full-charge, when in fact the charge acceptance has dropped to say 50% or less. The question remains: 100% of what? A user unfamiliar with this battery has little information about the runtime of the pack.
The reserve capacity can only be established when the SoH is known. Figure 5 illustrates the three imaginary sections of a battery consisting of the empty zone, which can be refilled, available energy and unusable section or `rock content' that can no longer store energy.

Figure 5:     Battery charge capacity.
Three imaginary sections of a battery consisting of available energy, empty zone and rock content. With usage and age, the rock content grows.

A battery fuel gauge should be able to disclose all three sections of the battery. Knowing the battery's SoH can do this. While the SoC is relatively simple to produce, measuring the SoH is more complex. Here is how it works:
At time of manufacture, each SMBus battery is given its specified SoH status, which is 100% by default. This information is permanently programmed into the pack and does not change. With each charge, the battery resets to the full-charge status. During discharge, the energy units (coulombs) are counted and compared against the 100% setting. A perfect battery would indicate 100% on a calibrated fuel gauge. As the battery ages and the charge acceptance drops, the SoH decreases. The discrepancy between the factory-set 100% and the delivered coulombs on a fully discharged battery indicates the SoH.
Knowing the SoC and SoH, a simple linear display can be made. The SoC is indicated with green LEDs; the empty part remains dark; and the unusable part is shown with red LEDs. Figure 6 shows such a tri-state fuel gauge. As an alternative, a numeric display indicating SoH and SoC can be used. The practical location for the tri-state-fuel gauge is on the charger.

Figure 6:  Tri-state fuel gauge.
The Battery Health Gauge reads the `learned' battery information available on the SMBus and displays it on a multi-colored LED bar. The illustration shows a partially discharged battery of 50% SoC with a 20% empty portion and an unusable portion of 30%.

The target capacity selector
For users that simply need a go/no go answer, chargers are available that feature a target capacity selector. Adjustable to 60, 70 or 80%, the target capacity selector acts as a performance check and flags batteries that do not meet the set requirements.
If a battery falls below target, the charger triggers the condition light. The user is prompted to press the condition button to calibrate and condition the battery by applying a charge/discharge/charge cycle. The green `ready' light at the end of the service reveals full charge and assures that the battery meets the required performance level. If the battery does not recover, a fail light indicates that the battery should be replaced. Figure 7 illustrates a two-bay Cadex charger featuring the target capacity selector and discharge circuit. This unit is based on Level 3 and services both SMBus and `dumb' batteries.

Figure 7:  The Cadex SM2+ charger
This Level 3 charger serves as charger, conditioner and quality control system. It reads the battery's true state-of-health and flags those that fall below the set target capacity. Each bay operates independently and charges NiCd, NiMH and Liion chemistries in approximately three hours. `Dumb' batteries can also be charged but no SoH information is available.

By allowing the user to set the desired battery performance level, the question is raised as to what level to select. The answer is governed by the application, reliability and cost.
The nominal target capacity setting is 80%. Decreasing the threshold to 70% will lower the performance standard but pass more batteries. A direct cost saving will result. The 60% level may suit those users who run a low budget operation, have ready access to replacement batteries and can live with shorter, less predictable runtimes. It should be noted that the batteries are always charged to 100%, regardless of the target setting. The target capacity simply reveals the energy, which a fully charged battery can deliver.
`Smart' batteries enabling performance readings are reserved for high-end industrial applications. However, in spite of improvements made over the last ten years, the `smart' battery, the SMBus in particular, has not received the anticipated acceptance. Some engineers go so far as to suggest that the SMBus battery is a `misguided principal'.
Part of the problem is the periodic calibration that is needed to correct the tracking errors that occur between the battery and the digital sensing circuit. Notable errors transpire if a battery is charged and discharged for only brief moments and the load varies widely. Long storage also contributes to errors because the circuit cannot accurately compensate for self-discharge.
Regardless of these limitations, the `smart' battery will continue to serve a critical market. It is conceivable that other methods will be introduced that do not rely on the in and out-flow of energy to establish energy reserve. But the importance of the fuel gauge has been established. There are simply no alternatives for users to whom unexpected downtime is no option.

About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver BC. Mr. Buchmann has a background in radio communications and has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. Award winning author of many articles and books on batteries, Mr. Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers, battery analyzers and PC software. For product information please visit www.cadex.com.