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 dischargingThe purpose of a battery is to store and release energy at the desired time and in a controlled manner. This section examines discharges under different C-rates and evaluates the depth to which a battery can safely be depleted. Chapter 5 also observes different discharge signatures and explores how certain patterns can affect battery life. But first, let’s look at charge and discharge rates, also known as C-rate.

 

Depth of Discharge

The end-of-discharge voltage for lead acid is 1.75V/cell; nickel-based system is 1.00V/cell; and most Li-ion is 3.00V/cell. At this level, roughly 95 percent of the energy is spent and the voltage would drop rapidly if the discharge were to continue. To protect the battery from over-discharging, most devises prevent operation beyond the specified end-of-discharge voltage.

When removing the load after discharge, the voltage of a healthy battery gradually recovers and rises towards the nominal voltage. Differences in the metal concentration of the electrodes enable this voltage potential when the battery is empty. An aging battery with elevated self-discharge cannot recover the voltage because of the parasitic load.

A high load current lowers the battery voltage, and the end-of-discharge voltage threshold should be set lower accordingly. Internal cell resistance, wiring, protection circuits and contacts all add up to overall internal resistance. The cut-off voltage should also be lowered when discharging at very cold temperatures; this compensates for the higher-than-normal internal resistance. Table 1 shows typical end-of-discharge voltages of various battery chemistries.
 

End-of-discharge

Li-manganese

Li-phosphate

Lead acid

NiCd/NiMH

Normal load

Heavy load

3.00V/cell

2.70V/cell

2.70V/cell

2.45V/cell

1.75V/cell

1.40V/cell

1.00V/cell

0.90V/cell

Table 1: Recommended end-of-discharge voltage under normal and heavy load
The lower end-of-discharge voltage on a high load compensates for the losses induced by the internal battery resistance.  

Some battery analyzers apply a secondary discharge (recondition) that drains the battery voltage of a nickel-based battery to 0.5V/cell and lower, a cut-off point that is below what manufacturers specify. These analyzers (Cadex) keep the discharge load low to stay within an allowable current while in sub-discharge range. A cell breakdown with a weak cell is possible and reconditioning would cause further deterioration in performance rather than making the battery better. This phenomenon can be compared to the experience of a patient to whom strenuous exercise is harmful.

What Constitutes a Discharge Cycle?

Most understand a discharge/charge cycle as delivering all stored energy, but this is not always the case. Rather than a 100 percent depth of discharge (DoD), manufacturers prefer rating the batteries at 80 percent DoD, meaning that only 80 percent of the available energy is being delivered and 20 percent remains in reserve. A less-than-full discharge increases service life, and manufacturers argue that this is closer to a field representation because batteries are seldom fully discharged before recharge.

There are no standard definitions of what constitutes a discharge cycle. A smart battery that keeps track of cycle count may require a depth of discharge of 70 percent to define a discharge cycle; anything less does not count as a cycle. There are many other applications that discharge the battery less. Starting a car, for example, discharges the battery by less than 5 percent, and the depth of discharge in satellites is 6 to 10 percent before the onboard batteries are being recharged during the satellite day. Furthermore, a hybrid car only uses a fraction of the capacity during acceleration before the battery is being recharged.

Discharge Signature

A classic discharge is a battery that delivers a steady load at, say, 0.2C. A flashlight is such an example. Many applications demand momentary loads at double and triple the battery’s C-rating, and GSM (Global System for Mobile Communications) of a cellular phone is such an example (Figure 2). GSM loads the battery with up to 2A at a pulse rate of 577 micro-seconds (µs). This is a large demand for a small 1,000mAh battery; however, with a high frequency the battery begins to behave like a capacitor and the characteristics change.

gsm1

Figure 2: GSM Pulse of a cellular phone

The 577 microsecond pulses adjust to field strength and can reach 2 amperes.

Courtesy of Cadex

In terms of cycle life, a moderate current at a constant discharge is better than a pulsed or momentary high load. Figure 3 shows the decreasing capacity of a NiMH battery at different load conditions and includes a gentle 0.2C DC discharge, an analog discharge and a pulsed discharge. The cycle life of other battery chemistries is similar under such load conditions.

gsm2

Figure 3:
Cycle life of NiMH under different operating conditions

NiMH performs best with DC and analog loads; digital loads lower the cycle life. Li-ion behaves similarly.

Source: Zhang  (1998)
 

Figure 4 examines the number of full cycles a Li-ion battery with a cobalt cathode can endure when discharged at different C-rates. At a 2C discharge, the battery exhibits higher stress than at 1C, limiting the cycle count to about 450 before the capacity drops to half level.

gsm3

Figure 4:
Cycle life of
Li-ion with cobalt cathode at varying discharge levels

The wear-and-tear of a battery increases with higher loads.

Source: Choi et al (2002)

For a long time, Li-ion had been considered fragile and unsuitable for high loads. This has changed, and today many lithium-based systems are more robust than the older nickel and lead chemistries. Manganese and phosphate-type Li-ion permit a continuous discharge of 30C. This means that a cell rated at 1,500mAh can provide a steady load of 45A, and this is being achieved primarily by lowering the internal resistance through optimizing the surface area between the active cell materials. Low resistance keeps the temperature down, and running at the maximum permissible discharge current, the cells heat up to about 50ºC (122ºF); the maximum temperature is limited to 60°C (140°F).

One of the unique qualities of Li-ion is the ability to deliver continuous high power. This is possible with an electrochemical recovery rate that is far superior to lead acid. The slow electrochemical reaction of lead acid can be compared to a drying felt pen than works for short marking but needs rest to replenish the ink.

Simple Guidelines for Discharging Batteries

  • The battery performance decreases with cold temperature and increases with heat.
     
  • Heat increases battery performance but shortens life by a factor of two for every 10°C increase above 25–30°C (18°F above 77–86°F).
     
  • Although better performing when warm, batteries live longer when kept cool.
     
  • Operating a battery at cold temperatures does not automatically permit charging under these conditions. Only charge at moderate temperatures.
     
  • Some batteries accept charge below freezing but at a much-reduced charge current. Check the manufacturer’s specifications.
     
  • Use heating blankets if batteries need rapid charging at cold temperatures.
     
  • Prevent over-discharging. Cell reversal can cause an electrical short.
     
  • Deploy a larger battery if repetitive deep discharge cycles cause stress.
     
  • A moderate DC discharge is better for a battery than pulse and aggregated loads.
     
  • A battery exhibits capacitor-like characteristics when discharging at high frequency. This allows higher peak currents than is possible with a DC load.
     
  • Lead acid is sluggish and requires a few seconds of recovery between heavy loads.
  • Text taken from  batteryuniversity.com