Basics About Discharging
The 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.
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End-of-discharge |
Li-manganese |
Li-phosphate |
Lead acid |
NiCd/NiMH |
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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.
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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.
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Figure 3: 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.
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Figure 4: The wear-and-tear of a battery increases with higher loads. Source: Choi et al (2002)
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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
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The battery performance decreases with cold temperature and increases with heat.
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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).
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Although better performing when warm, batteries live longer when kept cool.
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Operating a battery at cold temperatures does not automatically permit charging under these conditions. Only charge at moderate temperatures.
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Some batteries accept charge below freezing but at a much-reduced charge current. Check the manufacturer’s specifications.
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Use heating blankets if batteries need rapid charging at cold temperatures.
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Prevent over-discharging. Cell reversal can cause an electrical short.
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Deploy a larger battery if repetitive deep discharge cycles cause stress.
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A moderate DC discharge is better for a battery than pulse and aggregated loads.
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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.
Comments
it says discharge methods but nowhere in here does it tell you how to discharge the battery i didnt care how low i can safely discharge or the scientifical pros and cons just smiply how i can discharge the battery problem is the alternator was over charging at around 24.7volts and battery charged to 18.93volts and should be at 12.53-83volts “HOW TO???!!!!” “WHAT TO DO” to achive discharge to get battery back to 12.53 or can it even be discharged that would be better knowllege than whats up there then you can simply say dont over discharge under 1.75volts the exactly how to do this is what most people are looking for not all the other mombogumbo specs. on how low you can discharge that has nothing to do whith the method of doing so or if it can be done like connect a light to it, a speacal charge and discharge charger, let it sit on concrete for ex amount of time scence it draws the charge for some reason
If 12v 150 Ah lead acid bty connected in series, How many max current drain from the bty at a time.
“Does the long or short full charged storing period affects the battery’s life?” - For Lithium based batteries, high charge voltage + high battery temperature = reduced life. Storing the battery at low temperature, but above freezing, is best. Since you do not want the battery to self-discharge to zero, it is best to store the battery with a partial charge. If you don’t know how long it will be stored, your safest bet is to do a full charge before storing. It is not as good as having a partial charge, but an accidental “flatlining” of the battery will render it useless and dangerous to recharge.
lloyd lamb:
On the contrary, these specs were exactly what I was looking for.
As for how to discharge: put a load across it. If it’s at 19V you might start with, say, a 100W 24V automotive bulb, as a 12V one might blow at 19V. Consult a electrician or automotive mechanic for details and help.
Very good resource. I lend a link on the common factors of the discharge of the batteries:
Under charging or neglect of equalizing charge - Description and treatment Of sulphated batteries http://www.amperis.com/en/products/misc/battery-dischargers/
greeting.
My battery charger (for AA, AAA & 9-Volt batteries) has a button that says discharge on it. What is that for? Don’t I just need to charge the drained batteries?
I heard once that you should never place coins near or on a Battery.
Like on a Lithium battery. found a coin laying on top ofone ofmy Lith batterei and am concerned.
Dear Sir/Madam,
We need to test the button cell batteries of lead acid, Li-ion, Li-polymer, Ni-Cad, NiMH, Ultra-Capacitor.
Please help me finding out the maximum charge and discharge in C-rates of each batteries,
Because it helps us to choose the type of battery tester to buy.
We have to do this experiment in 40-50 minutes.
Thanks and regards,
Elango
research associate
Does the long or short full charged storing period affects the battery’s life? For example, if the battery is not used for 3 months, is it better to fully charged or discharged or at what status? Many Thanks!
BR, SG