Testing and Calibrating Smart Batteries

When Gaston Planté invented the rechargeable battery in 1859, a new system of store energy emerged. The digital world has been intruding to make the electrochemical battery smart by adding a see-through window to removing its opaqueness and reveal state-of-function.

The smart battery was hailed as an engineering marvel when introduced in 1994 by Intel and Duracell. The heart is the system management bus, or SMBus, that tracks state-of-charge (SoC) and captures performance data. SMBus also incorporates the battery management system (BMS) to assure safe operation of Li-ion batteries by limiting over-voltage and preventing high current draws.

Unlike a regular battery in which the charger is in command, the smart battery becomes the host that controls charge functions in a Level 2 charger. Being the master enables charging future battery chemistries for which no charge algorithm currently exists. Level 3 is a hybrid charger that accommodates batteries with SMBus as well as regular batteries. This is the preferred system as the charger takes control should SMBus communication fail. Level 1 chargers only supports single chemistry and has been discontinued.

To maintain SoC accuracy, a smart battery requires periodic calibration. If calibration is not available, the device manufacturer advises to occasionally apply a full discharge in the device. This resets the discharge flag, followed by the charge flag when full charge as illustrated in Figure 1. Calibration thus establishes a linear line between full and empty to measure SoC.

In time the line blurs again and a recalibration is needed. Device manufacturers advise to calibrate smart batteries every three months or after 40 partial discharges. Calibration error is recorded by the Max Error metric. A number 1 reflects a well-calibrated battery; higher figures indicate the need for service.

A charge-discharge-charge calibration cycle as shown in Figure 1 does not correct loss of capacity. Even though the SoC gauge shows 100%, a fully charged battery with a usable capacity of 50% will only deliver half the specified runtime. As the battery fades, the energy storage capability shrinks that Figure 2 simulates by adding rocks.

A battery must also have low internal resistance (Ri) to deliver power. Although capacity-loss and rising Ri do not correlate, the anticipated runtime can only be delivered if Ri is within specifications. Capacity is the leading health indicator, a value that in most cases governs the end-of-life when dropping below 80%. An elevated rise in Ri as part of cycling and aging is less common.

Impedance Tracking

The modern smart battery also reveals the usable capacity shown in Full Charge Capacity (FCC). When new, a smart battery’s FCC is equal to the design capacity of 100%. However, as the battery fades the percentage of usable capacity decreases. FCC can be read with a Smart Bus Reader reflecting the battery’s “digital capacity.”

How well does the usable capacity track with FCC? Cadex labs discovered an accuracy discrepancy of greater than 5% on one-third of random smart batteries tested. This explains why users experience sudden blackouts when moments before the battery showed 20% SoC. In spite of these anomalies, the smart battery provides valuable information; frequent calibration upholds accuracies.

The usable capacity on a modern smart battery is made readable with Impedance Tracking. Batteries with Impedance Tracking count in-and-outflowing coulombs* during charge and discharge. An analogy is a glass holding a liquid content of 20% that is filled to 100% while measuring the inflowing energy. Residual capacity plus added charge discloses the usable capacity as demonstrated in Figure 3.

Capacity estimation by Impedance Tracking requires assessing the remaining charge (old fill) before charge. The smart battery does this by measuring the open circuit voltage (OCV), a value that is compared against a reference curve matching the battery chemistry.

Because of agitation after a charge and discharge, rest periods are needed to reach voltage equilibrium to enable SoC estimations. As an example, after-charge needs a minimal rest of two hours; after-discharge requires a five-hour rest. The system also adds temperature compensation as cold and heat affect the cell voltage.

Despite these precautions, FCC loses accuracy. Calibration of a smart battery with Impedance Tracking needs rest periods, a service that is best done with a battery analyzer. This so-called formal calibration also resets the Max Error, a function that a full cycle alone will not provide. Testing batteries on an analyzer also displays the real usable capacity with Ri to verify SoH.

All batteries should be serviced with a battery analyzer before replacement. Some smart batteries fail due to digital defect that the analyzer may correct. For best result, calibration should be repeated as certain type of smart batteries only correct the reading by a limited percentage point.

* One coulomb is equal to the amount of charge delivered by 1A of current in one second

Calibrating an EV Battery

The BMS in an electric vehicle (EV) works similarly to a smart battery, but here the driver is relieved of calibration. We ask: “Why does my smart battery need calibration while the EV goes free?” The answer lies in self-calibration that applies to both EV and smart batteries featuring Impedance Tracking.

Self-calibration sets SoC Orientation Points (SoC-OP) as shown in Figure 4. This occurs when a battery reaches equilibrium after a charge or discharge. Adding or subtracting coulombs between these points enables assessing the energy storage capacity and making adjustments as the battery fades as part of self-calibration. Best results are achieved when the SoC-OPs are spaced far apart. Each BMS has its own mechanism that is not disclosed.

A low SoC-OP typically occurs at the end of the day or after a deliberate full discharge. Adding a delay before charge provides the required rest period to solidify the low SoC-OP; a rest after a full charge sets the high SoC-OP to complete self-calibration. User patterns that occur naturally during normal use can be manipulated to improve self-calibration by a thoughtful battery user.

EV batteries use a similar principle, a method that can also be improved with clever timing between use and charge by the vehicle owner. Because of the flat discharge curve of a Li-ion battery in mid-SoC range, the best SoC-OP locations are below 30% and above 70% SoC. The LiFePO (LFP) in the lithium battery family has a very flat midrange curve, but the more popular NMC has a measurable mid-charge tilt. Knowing these characteristics, an EV battery can be calibrated without tools by following this procedure:

1. Apply a deep discharge by driving the extra mile. Be mindful when at low charge state as the vehicle’s indicated range can be off by as much as 30%. Extreme low SoC is noticed when acceleration becomes sluggish. Do not drive further as the battery enters a high-stress mode. A driver can also get stranded.
2. At low SoC, allow the battery to rest for 4 to 6 hours before beginning a charge. Ensure that the car is in ‘deep-sleep mode’ by disabling all auxiliary loads.
3. After the allotted time, charge the battery to between 80% and 100%. Avoid ultra-fast charging as this causes added stress. Level 1 and 2 EV chargers work best. See also Charging an Electric Vehicle.
4. After charge, allow a 2-to 4-hour rest with no load on the battery. All Li-ion chargers apply a topping charge that will agitate the rest. A deep-sleep rest must have zero current for two hours.

Calibration can improve range prediction by up to 80km (50 miles). To get full benefit, the service may need to be repeated. Some service centers provide calibration for given EVs but this is expensive and time-consuming. Battery calibration is recommended once or twice a year and when buying a used EV.

Calibrating Energy Storage Systems (ESS)

Batteries in Energy Storage Systems (ESS) share similarities with the EV battery in that the battery system contains modules of serial and parallel-connected cells managed by a BMS. Most ESS’s are monitored by observing cell voltage, load current and temperature. Voltage and current measurements enable SoC and Ri readings, but capacity assessment to determine the end-of-life on capacity is unattainable. Some ESS include Artificial Neural Networking as described in Advancements in Battery Testing by “massaging” big data to assess SoH. Self-calibration with Impedance Tracking can also be used for ESS applications.

CAN Bus

SMBus is not the only communications for a smart battery. The Controller Area Network (CAN Bus) is a vehicle bus standard that allows the battery to communicate with a host system. Developed by Robert Bosch in 1983, the CAN Bus is primarily used in hybrid vehicles, including e-bikes, drones and robots.

Cell Balancing

With thousands of cells connected in series and parallel, a cell imbalance can occur in time. The best cell balancing happens at the battery assembly plant by using quality cells that are tightly matched in capacity. Cell balancing is not as effective as calibration because weak cells remain weak, even after being fully charged. Cell balancing does not correct a battery pack in the same way as calibration does.

Conclusion

The modern smart battery self-calibrates when given the opportunity during charge or undisturbed discharge. Sufficient rest time must be given to establish equilibrium, an event that forms the SoC-Ops in a battery with Impedance Tracking. The best results are achieved when applying formal calibration with specified rest periods on a battery analyzer. Periodic calibration is also recommended for the EV.

The smart battery is indeed smart, but left unattended, the reading can get off by as much as 30%. Unless regularly calibrated, SoC and FCC data of portable batteries should be taken as reference readings only.