The Fast Charging Process

5 ELECTRIC VEHICLE BATTERY

FAST CHARGING

VRLA and NiMH batteries exhibit low internal resistance and are capable of very high discharge and charge rates. This is exhibited by modern battery modules, which are manufactured using thin electrodes with large active areas, allowing for charge and discharge rates of 10C to 20C. Excessive overcharging can on the contrary become catastrophic on these batteries.

THE FAST CHARGING PROCESS

The fast charging technique for traction batteries account for the battery charge acceptance. The charger adjusts the charge rate continually to match the ability of the battery to accept the charge. Danger from excessive overcharging can be avoided, and the battery modules can arrive at the charge in 20 to 30 minutes. This fast charge also enhances the battery life and provides higher battery efficiency (charge recovery).

Battery modules are sensitive to overcharge. The internal resistance of the battery is generally an obstacle to the fast charge and discharge process. Under room temperature conditions the maximum charge/discharge rate of the battery with low internal resistance may be as high as 15C to 30C at room temperature. The charge rate 1C or one-hour rate is the charge/discharge rate in amperes equal to the capacity of the battery in ampere-hours. Similarly, the 10C charge rate is 10 times higher and also referred to as the six-minute rate.

A high-rate discharge of the battery does not cause a catastrophic failure. The battery voltage collapses, and the battery current to the load ceases. A high-rate charge on the other hand can result in serious battery damage. Thus making it important to know when to stop and how to stop.

A fully discharged battery in a 0% SOC can accept current at the highest, initial charge rate. A fully charged battery at 100% SOC cannot be charged any further. Thus passing additional charge current through

the battery is meaningless as there is no additional energy available at discharge.

As the battery charging progresses towards a fully charged state, there are a reduced number of carriers available for conversion from the discharged to the charged state within the electrode mass. Thus the ability to accept more charge diminishes continually. The charge acceptance curve describes the maximum charge rate, which the battery is capable to accept (i.e., convert into stored electrochemical energy). Anything above this charge rate constitutes an overcharge. Thus the battery pack can be driven into an overcharge at any time, in any state of charge by excessive charge current.

A previous misconception is that the battery must first undergo a charge before it goes into a charge condition. For example, boiling of lead-acid batteries at the end of the conventional charging, often 25 to 35% of excess overcharge energy is delivered, in order to realize full capacity upon discharge. The reason why the last few percent of the battery capacity take the longest time to charge is because the battery’s ability to accept the charge diminishes and tends to zero as the battery pack approaches a full charge.

If the initial charge acceptance ability of the battery pack exceeds the current battery capacity of the charger, 10C as an example, charging will begin at this constant current or “high rate.” This area lies below the charge acceptance curve (i.e., in the undercharge zone of the battery). When the charge acceptance region of the curve is intersected, the charge current gradually decreases to match the battery’s charge acceptance ability. When full charge is reached, the current is turned off. Alternately, until the battery is removed from the charger, it is rested or maintained at the trickle charge current at the rate of 0.02C or 0.05C. This trickle charge is essential to compensate for the battery selfdischarge.

The battery or battery pack being charged using the fast charge process may be used immediately in case the temperature rise during the charge process does not exceed nominal discharge temperature values. In the event the battery or battery pack exhibit a high temperature as a result of the fast charge, it is recommended to allow the battery or the battery pack temperature to stabilize within safe operating values.

The fast charger monitors the battery integrity, battery cell response, high-rate charge, battery controlled charge (battery acceptance and battery matching), finishing charge, and battery charge termination. The fast charger recognizes the boundary of the overcharge zone as shown in Figure 5–1 on the basis of electrochemical potential. The feed-

Figure 5–1 Fast charge profile for traction batteries.

C) Charge Rate ( °

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0 0 10 20 30 40 50 60 70 80 90 100 State of Charge (%) 16 14 12 10 8 6 4 2 0 Charge Current (A)

back mechanism allows continuous adjustment of the charger current to match the battery’s or battery pack’s charge acceptance ability. The fast-charging algorithm also provides battery diagnostics by continuous detection of the electrochemical potential as a function of the battery chemistry and current density. The fast charger provides a high-current density to drive the electrochemical reaction without driving the battery into an overcharge.

Some fast-charging techniques maintain a microprocessor or microcontroller based voltage peak method. The battery pack charge is limited within a predefined charge acceptance curve. This approach is applicable at charge rates up to 1C but cannot be applied successfully at higher battery charge rates. As an example, a 20-minute 3C constant current charge with a conventional battery charger uses voltage peak termination. The pressure rises to 120psig (10 bar), close to the battery cell’s burst limit. This is followed by a sharp rise in the battery temperature and the bursting of safety seals. The heat is produced both during the charging and the discharging process and arises from three main sources. First, as a reversible thermodynamic component, the heat source is the entropy change associated with the temperature dependence of the free energy change of the charge reaction (i.e., with the reversible electrode or cell potentials). The heat produced is -TDS, and its rate of production is linear with respect to the battery charge current. The second component is the heat related to the irreversibility of the reaction, which is