For mission-critical applications, micro-chips are embedded into battery packs to provide some level of communication between the battery and user. Similar to a fuel gauge in a vehicle, the intelligent battery keeps track of the amount of current flowing in during charge and deducts the amount of the current flowing out on discharge. The balance is the usable capacity, displayed either on the battery itself or on the equipment it powers.
The unit of electric charge that flows in and out of a battery in a given time is measured in coulombs. Using the coulombic method to obtain state of charge is reasonably accurate, especially if the battery performs well and is in constant use. One drawback is its inability to measure self-discharge. Some intelligent batteries apply a correction factor to reduce the assumed residual capacity according to the duration and temperature of storage.
A simpler method of approximating the state of charge is achieved by reading the battery voltage, both when loaded and in open terminal (no-load) condition. The load/no-load voltage differential can also be measured by applying brief discharge pulses to check the "firmness" of a battery similar to checking the hardness of a basket ball. The obtained voltage levels are then calculated to determine a meaningful capacity reading.
The accuracy of such a voltage method depends heavily on the battery chemistry. On a NiCd with a flat discharge curve, for example, an accurate capacity reading is difficult to predict between 80% and 20%. SLA, coke anode Li-ion and alkaline batteries which provide a more linear voltage drop during discharge obtain a better accuracy.
Another method commonly used to read the charge state of a battery is measuring the battery’s internal impedance. The impedance of a battery changes with the charge state. A fully charged battery indicates a lower impedance than one that is empty. To derive a meaningful reading, however, the internal resistance of the given battery needs to be know in advance. Without this information on hand, an ultra-high capacity battery that is fully charged but has a higher intrinsic impedance may be taken for a standard battery that has a lower intrinsic impedance but is only partially charged. In addition, the impedance readings are not linear to the battery’s charge level. Meaningful measurements can only be obtained towards the end of discharge.
When fully charged, the intelligent battery resets its fuel gauge to 100%. A miscount arises if a battery that has suffered capacity loss shows 100% on the battery even though its ability to hold charge has dropped to 50%. Such a battery would provide only half the run time as compared to a good battery. Periodic capacity analysis with a battery analyzer will still be necessary with most intelligent batteries to establish the battery’s true performance.
Advanced charger/analyzers are being developed that automatically exercise and calibrate the intelligent battery to reflect the actual capacity level. Rather than indicating 100% for all batteries after they have been fully charged, their actual capacities will be reflected according to the battery’s charge acceptance level. These true capacities are obtained by periodically applying a test discharge to a fully charged battery and applying the resulting capacity as reference. On nickel-based battery, such a discharge cycle is required anyhow to control memory.
Memory chips imbedded in some of the more advanced intelligent batteries also keep track of the battery’s service history, age and past performance. Pending maintenance is posted and extra service requested if a NiCd battery does not receive sufficient exercise through its normal use. Such intelligent batteries will remind the operator when maintenance is due in a way similar to a photo copier that advises the user of pending service with a service man symbol flashing on the display panel after a given number of copies have been made.
The intelligent battery brings along some minor disadvantages: the electronic circuit draws a small amount of current, reducing the time the battery can be stored. In addition, a cost premium of about 30% is added over the price of a standard battery. Although most intelligent batteries allow servicing with a regular charger or battery analyzer, those with an external communication bus need to connect briefly to the designated charger to reset its circuit.
Efforts are being made to standardize the intelligent battery into the Smart Battery System (SBS). The goal of the battery manufacturers is to produce a series of global batteries with the SBS capabilities on board. These batteries support a standardized data bus and use common charger and multi-battery selector commands, but may differ in battery size, shape, chemistry, voltage and connector type.
The SBS will also help to develop new software, promote standard batteries and universal chargers and analyzers. Though not all battery manufacturers support SBS, a workable system will emerge over time that will be the accepted norm, similar to the standardization of the audio cassette in the sixties and the VCR in the seventies.
Modern technology has introduced new and improved battery chemistries, each offering distinct advantages over older systems but none providing a fully satisfactory solution to portable power.