Basics about Battery Discharging

The electrochemical battery has the advantage over other energy storage devices in that the energy stays high during most of the cha...

The electrochemical battery has the advantage over other energy storage devices in that the energy stays high during most of the charge and then drops rapidly as the charge depletes. The supercapacitor has a linear discharge, and compressed air is the inverse of the battery by delivering the highest power at the beginning. Figures 1, 2 and 3 illustrate the simulated discharge characteristics of stored energy.

Most rechargeable batteries can be overloaded briefly, but this must be kept short. Battery longevity is directly related to the level and duration of the stress inflicted, which includes charge, discharge and temperature.

Remote control (RC) hobbyists are a special breed of battery users who stretch tolerance of “frail” high-performance batteries to the maximum by discharging them at a C-rate of 30C, 30 times the rated capacity. As thrilling as an RC helicopter, race car and fast boat can be; the life expectancy of the packs will be short. RC buffs are well aware of the compromise and are willing to both pay the price and to encounter added safety risks.

To get maximum energy per weight, drone manufacturers gravitate to cells with a high capacity and choose the Energy Cell. This is in contrast to industries requiring heavy loads and long service life. These applications go for the more robust Power Cell at a reduced capacity.

 

Depth of Discharge

Lead acid discharges to 1.75V/cell; nickel-based system to 1.0V/cell; and most Li-ion to 3.0V/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 devices 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 affinity of metals in the electrodes produce this voltage potential even when the battery is empty. A parasitic load or high self-discharge prevents voltage recovery.

A high load current, as would be the case when drilling through concrete with a power tool, lowers the battery voltage and the end-of-discharge voltage threshold is often set lower to prevent premature cutoff. The cutoff voltage should also be lowered when discharging at very cold temperatures, as the battery voltage drops and the internal battery resistance rises. Table 4 shows typical end-of-discharge voltages of various battery chemistries.

 

End-of-discharge                Nominal	Li-manganese 3.60V/cell	Li-phosphate 3.20V/cell	Lead acid 2.00V/cell	NiCd/NiMH 1.20V/cell
Normal load Heavy load or low temperature	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 4: Nominal and recommended end-of-discharge voltages under normal and heavy load. The lower end-of-discharge voltage on a high load compensates for the greater losses.

Over-charging a lead acid battery can produce hydrogen sulfide, a colorless, poisonous and flammable gas that smells like rotten eggs. Hydrogen sulfide also occurs during the breakdown of organic matter in swamps and sewers and is present in volcanic gases and natural gas. The gas is heavier than air and accumulates at the bottom of poorly ventilated spaces. Strong at first, the sense of smell deadens with time, and the victims are unaware of the presence of the gas.

 

What Constitutes a Discharge Cycle?

A discharge/charge cycle is commonly understood as the full discharge of a charged battery with subsequent recharge, but this is not always the case. Batteries are seldom fully discharged, and manufacturers often use the 80 percent depth-of-discharge (DoD) formula to rate a battery. This means that only 80 percent of the available energy is delivered and 20 percent remains in reserve. Cycling a battery at less than full discharge increases service life, and manufacturers argue that this is closer to a field representation than a full cycle because batteries are commonly recharged with some spare capacity left.

There is no standard definition as to what constitutes a discharge cycle. Some cycle counters add a full count when a battery is charged. A smart battery may require a 15 percent discharge after charge to qualify for a discharge cycle; anything less is not counted as a cycle. A battery in a satellite has a typical DoD of 30–40 percent before the batteries are recharged during the satellite day. A new EV battery may only charge to 80 percent and discharge to 30 percent. This bandwidth gradually widens as the battery fades to provide identical driving distances. Avoiding full charges and discharges reduces battery stress. 

A hybrid car only uses a fraction of the capacity during acceleration before the battery is recharged. Cranking the motor of a vehicle draws less than 5 percent energy from the starter battery, and this is also called a cycle in the automotive industry. Reference to cycle count must be done in context with the respective duty.

Charging and Discharging Lead Acid Batteries

The voltage of a lead acid battery when at rest (not supplying current or being charged) will vary according to how fully charged the battery is. 

The graph shown to the right represents a typical 24 volt lead acid battery which has not been charged or had current drawn from it for a couple of hours.

Battery Discharge Characteristics

A full charged battery will have a voltage of around 25.5 volts. As current is drawn off and the level of charge is reduced, the voltage will fall quite quickly at first (again it would be necessary to stop drawing current for a couple of hours to be able to measure the true voltage of the battery).
With further drawing of current, the rate of voltage drop slows down and will reach around 24.0 volts when the bettery is at half capacity.

As the battery approaches the fully discharged state, the voltage starts to fall more quickly again.

It is important for a battery to never be fully discharged, so your inverter will normally disconnect the supply when the voltage is around 22 volts.

An interesting point to note here is that when an inverter or other power load is drawing a high current from the battery, the voltage will drop. This may mean that the battery needs to be somewhere over 50% charged to avoid the inverter cutting out due to low voltage.

The larger the battery, the smaller this voltage drop will be, and the greater the % of the charge will be useable when drawing high currents. 

Battery Charging

If a voltage is applied to the battery which is greater than the battery's voltage, a current will flow through the battery in the reverse direction to when it is supplying current, and the battery will charge.
The rate of charge or current that will flow will depend on the difference between the battery voltage and the voltage that is applied to it (from solar panels etc). Solar panels intended for a 24 volt system are likely to be capable of producing over 30 volts. This voltage ensures that the panels are capable of charging the battery fully.

While it is beneficial to a battery's performance and life to be fully charged on regular occasions, however once a battery has been charged to it's full capacity, it is important not to continue charging as this will damage the battery. A Charge Controller is necessary to ensure that the battery is not over charged.

Battery Efficiency

The Lead Acid battery is not 100% efficient at storing electricity - you will never get out as much as you put in when charging. Overall, an efficiency level of 85% is often assumed.
The efficiency will depend on a number of factors including the rate of charging or discharging. The higher the rate of charge or discharege, the lower the efficiency.
The state of charge of the battery will also affect charge efficiency. With the battery at half charge or less, the charge efficiency may be over 90%, dropping to nearer 60% when the battery is above 80% charged.
However it has been found that if a battery is only partially charged, efficency may be reduced with each charge. If this situation persists (the batteries never reaching full charge), the life of the battery may be reduced.