Battery Performance and Testing
1. What are the main characteristics of secondary battery?
Its main characteristics include voltage, impedance, capacity, gas pressure, self-discharge rate, cycle life, sealing, safety, storage, and appearance. There still are others such as over-charge, over-discharge, soldering, causticity proof and so on.
2. What is secondary battery self-discharge?
Self-discharge is also named holding electricity capability. It is the capability that a battery holds electricity under open circuit condition. In general, techniques,materials, and storage conditions determine it. Self-discharge is one of the major parameters for estimating a battery performance.
3. What is impedance? How to measure?
Because a cell is equivalent to a resourceful resistance, thus characteristic can be used as the measurement approach. If a cell is connected with a 1000Hz and 50mA constant current while series actions are taken such as sampling its voltage, commutating and filtering wave, its impedance value can be gained accurately.Battery impedance is the resistance when the current flow through an operating cell, in general the internal resistance is included both d.c. and a.c. resistance. For the rechargeable cell resistance is small and electrode is easy to be polarized along with producing a polarization resistance while measure d.c. resistance, the accurate value can be measured. But measuring a.c. resistance can avoid the inflection of polarization, so accurate value will be gained more easily.
4. What is charge efficiency?
It is the ratio between the output and input capacity when the cell is discharged at a certain condition. It can be calculated by the following formula: Charge efficiency=Some of the input energy can transform the active material into charge status; some is consumed to produce oxygen in subsidiary reaction. The charge efficiency is affected by charge speed and ambient temperature. During charging the current must be defined in a certain range, too heavy or too small current can bring low charge efficiency. Because of self-discharge the battery can be fully discharged.
5. What is cell capacity?
It means that the output energy when the cell is discharged to an end voltage at a certain discharge condition. IEC standard specifies that Ni-Cd and Ni-MH cell rating capacity indicates the output energy which the cell is charged at 0.1C for 16hrs, then discharged at 0.2C to 1.0V at temperature of 20±5 ℃. It can be described as C5. As for Li-ion cell, its rating capacity indicates the output energy that the cell is charged at normal temperature, constant current-voltage for 3hrs, discharged at 0.2C to 2.75V, its capacity unit can be indicated in Ah, mAh (1Ah=1000mAh).
6. What are the most usual charge methods?
1) Constant current (ab. CC) charge: it is the most usual method that charge currents are fixed at a certain value during the charge progress.
2) Constant voltage (ab. CV) charge: during the charge progress the voltages between the two terminals of the power supply is maintained at a certain value, while the current in the circuit become smaller and smaller with the battery voltage rises.
3) CC&CV charge: firstly a cell is charged at CC, when the cell voltage rises to a value, its voltage keeps steady, while the current in the circuit drops to the small esttill to zero at last.
7. What is pulse charge? What is the impact on battery performance?
Because Ni-Cd battery is easily polarized, generally at CC or CV charge hydrogen and oxygen can be generated from electrolyte. The oxygen can penetrate toward negative plate under the inner high pressure to react with cadmium plate where CdO is generated; this process will make availability capacity of the plate low. Charge then discharge is the common method used in pulse charge, i.e. charge for 5 seconds, then discharge for 1second, thus the most oxygen generated during charge is reverted to electrolyte at pulse discharge. Not only the gas if cation quantity of inner electrolyte is limited, but also the old batteries which have been seriously polarize can recover to or approach to their primary capacities after using this method for 5-10 times.
8. What are the standards for charging and discharging batteries?
Prior to charging at 0.1C for 16hrs the cell shall have been discharged at 0.2C to1.0V/cell, after that it shall be rested for 1hour, then discharged at 0.2C to 1.0V/cell.That is the charge and discharge standard for batteries.
9. How many items for battery reliability test?
1) Capacity test
2) Impedance test
3) Charge retention test
4) Overcharging endurance test
5) Cycle life test
6) Inner gas pressure test
7) Voltage measurement
8) Drop test
10. What is drop test?
The having been charged batteries are been dropped from 1m high at three different directions twice onto a hard rubber board. There is no damage of the assembly performance and outside packages.
11. What is standard for charge retention test?
Prior to the test, the battery shall be discharged at 0.2C to 1.0C/cell. After the battery is charged at 0.1C for 16h at an ambient temperature of 20±5 ℃ and a relative humidity of 65%±20%, it shall then be stored for 28 days on open circuit.After that the battery is discharged at 0.2C to 1.0V/cell, and the discharge duration of Ni\MH should be not less than 3h15m, while of NiCd it should be more than 3h.
12. What is the high temperature acceleration test?
As the charge retention takes a longer time, generally high temperature acceleration test is suggested for Ni-MH cell. Store the fully charged cell at an ambient temperature of 45? for 3 day, after that rest it for 1h, then discharge at 0.2C to 1.0V and the duration shall be more than 3h.
13. What is high temperature and humidity test?
The cell shall have been discharged at 0.2C to 1.0V and charged at 1C for 75 minutes before it is placed at circumstances of 66 celsius temperature and 85% relative humidity to store for 192h(8 days). Then rest it at normal temperature for 2h. There should be no deformation or leakage, and 80% or more nominal capacity shall be recovered.
14. What is the criterion of overcharge endurance test?
The cell is discharged at 0.2C to 1.0V, and then it is charged 0.1C for 28 days.There shall be no deformation or leakage and the duration shall be more than 5h as discharged at 0.2C to 1.0V.
15. What is IEC standard for lifecycle test?
After the cell is discharged at 0.2C to 1.0V:
1) Charged at 0.1C for 16h, and then discharged at 0.2C for 2h30mins (1st cycle).
2) Charged at 0.25C for 3h10mins, and then discharged at 0.25C for 2h20mins(2-48th cycle)
3) Charged at 0.25C for 3h10mins, and then discharged at 0.25C at 1.0V(49th cycle).
4) Charged at 0.1C for 16h, rested for 1h, then discharged at 0.2C to 1.0C(50th cycle). For Ni-MH cell, repeat the 1-4 step for 400 cycles, among which the 0.2C discharge duration should be more than 3h. While for Ni-Cd cell, repeat the 1-4 steps for 500 cycles, and the 0.2C discharge duration should be more than 3h. Despite this method lifecycle at 1C is adopted by DGT, i.e. after rating discharge, charge at 1C for 80mins, controlling the charge end by delt V=20mV, then discharge at 1C to 1.0V. Repeating as this for 500 cycles, there should be more than 60% of initial capacity.
16. What is 24h self-discharge?
As the standard test on charge retention lasts too longer, the 24h self-discharge becomes the most common way to take fast test. The cell should have been discharged at 0.2C to 1.0V Prior the test. It shall be charged at 1C for 72mins, rested for 15mins, and then discharged at 1C to 1.0V and the cell capacity C1 are gained. Charge the cell at 1C for 72min, and rest for 24h, then test the 1C capacity C2, the (C1-C2)/C1*100% should not less than 15%.
17. What is vibration test?
After the cell is discharged at 0.2C to 1.0V, it is going to be charged at 0.1C for 16hand rested for 24h,then vibrated according to the following conditions: Swing: 4mm Frequency: 1000times vibrating for 30mins in XYZ three directionsAnd the cell voltage change should be between ±0.02V and the impedance change should be within ± 5 om.
18. What is bump test?
After discharged at 0.2C to 1.0V and then charged at 0.1C for 16h at temperature of 20±5 ℃, it will be fixed on the bump platform, then the test is carried out according to the following conditions: The peak acceleration is 98m/S2 (10g), the relevant pulse time (D) is 16m/S and the relevant speed change is 1.00m/S. At the end of 1000 times bump its discharge time that, after rested for 1-4hrs at the temperature of 20±5 ℃, discharged at 0.2C to1.0V should not be less than 5hrs.
19. What is the penetration test?
This is a safety experiment. After a battery is fully charged, penetrate its center with a nail(diameter is 2.5mm-5mm) and make the nail stay in the batter, the battery shall not explode or catch fire.
20. What is the temperature rise test?
Safety test. Put fully charged battery in an oven and raise its temperature at a rate of 5 ℃/min, do that until the temperature reaches 150 ℃, and keep this temperature for 10 minutes, during and after the test the battery shall not explode or catch fire.
21. What is the burn test?
Safety test. Burn the fully charged batteries with blue blaze in a explosion proof cabinet, the battery shall not explode or catch fire and the safety vent shall open after some time.
22. What is short-circuit test?
Safety test. Connect the positive electrode and negative electrode of a battery directly with a lead wire for some times in a explosion proof cabinet, the battery shall not explode or catch fire.
23. What is the temperature cycle test?
A temperature cycle test include 27 cycles, every cycle is composed of next procedures:
1) Put battery in an ambient condition (Temp.: 66±3 ℃,relative humidity:(15±5)%) for 1h from room temperature;
2) Change the ambient condition to 33±3 ℃, (90±5)%) for 1h;
3) Change the ambient condition to -40±3 ℃ for 1h;
4) Rest battery in 25 ℃ for 0.5h.
The 4 steps consist of a cycle. After 27 cycles, battery shall no leakage, swelling, rust or other abnormal phenomena.
24. What is the temperature shock test?
This test needs two constant temperature cabinets. One is set as 66 ℃, and the other is set as -40 ℃. Every cycles is composed of next procedures:
1) Put battery in -40 ℃ for 1h;
2) In 5 seconds transfer battery to 66 ℃ oven for 1h.
This cycle test shall start from low temperature and end in high temperature and the cycle number is 24. After the test the battery shall be capable of charge and discharge, its electrical performance shall not be affected.
25. What is the overcharge test?
After discharge at 0.2C to 1.0V/cell, charge battery at 1C for 3h, measure the deformation and calculate with a formula to get the internal pressure value?The battery shall not leak , explode or catch fire.
26. What is trickle charge?
This method is used to makeup the losing capacity of self-discharge when fully being charged. Commonly pulse charge is adopted. Experience tells that when fully charged, the losing capacity is 5% compared to the rate one. While in theory, continually charging at C/500 can makeup the loss: C*(5/100)*24H*C/500. But for the current is small, in fact the charge rate is extremely low, so that the battery is rarely charge. Pulse charge can be adopted to resolve this matter. Charge at C/10 for 1.2 second, rest for 58.8second. According to the above station, everyday the charge capacity is 5% of rate capacity. In general, pulse charge is applicable to the following situation which will be adopted in field: CC: C/20, CT: 0.1 second to 60 second.
Examples for trickle charge:
Charge (high) Charge (low) Pulse period (S) Daily charge capacity
Current Time Current Time
C/10 1.2s 0C 58.8s 60s 5% of rate capacity
C/20 2.4s 0C 57.6s 60s
C/10 0.6s 0C 29.4s 30s
27. What is the residual discharge capacity of battery?
When discharge a rechargeable battery at high rate current (such as 1C or more), the batteries capacity can be discharged completely when its voltage reaches the cut-off value due to the slow diffuse rate of internal active materials. If discharge the battery at 0.2C to 1.0V/cell after that, the battery can still discharge some capacity,the second discharge capacity is named residual capacity.
28. What are the differences of impedance between fully charged and fully discharged battery?
In general, impedance of fully discharged battery is not stable, and larger than the value when it is fully charged. The impedance of fully charged battery is stable and small. During battery use process, only the charged state impedance has actual meaning. At the end of cycle life, impedance will increase due to the electrolyte dry up and the activity of chemical materials decrease.
29. What is the power output of battery?
A battery’s power output refers to its ability to deliver a specific amount of energy within a fixed period of time. The power output P of a battery is calculated from the product of the discharging current I (measured in amperes) and the discharging voltage U (in volts), thus : P=I x U. The power output is expressed in watts.The smaller a battery’s inner resistance, the higher its possible power output. Its inner resistance must always be smaller than that of the electric device to be operated. Otherwise the battery voltage will break down, i.e. the battery will be unable to operate the device. At a given discharging voltage, a battery’s power output increases with increasing electrode surface and operating temperature, and vice versa. 90.What are the ideas of nominal voltage, OCV, mid-point voltage, cut-off voltage? Nominal voltage means general operating voltage, to secondary Ni-Cd and Ni-MH battery, 1.2V/cell; OCV means open circuit voltage, that is the voltage of a battery without any load; Mid-point voltage is the voltage when a battery has been discharged 50% of capacity, it is used to scale the high rate discharge performance of power tool battery; Cut-off Voltage is the voltage that discharge shall be finished.
30. What shall we pay attention to during use?
1) Read the battery use manual carefully and use the recommended battery;
2) Check the touch parts of electrical appliance and battery and affirm that they are clean; use a wet cloth to wipe up if need, and enclose battery after it is dry;
3) Don let baby replace battery without a guardian, mini type battery (such as AAA) shall be kept in a place that baby can touch them.
4) Don mix to use new and old battery, and don mix to use different types battery;
5) Don try to regenerate a primary battery by heating, charging or other methods;
6) Don short a battery;
7) Don heat battery or throw it in water;
8) Don disassemble battery;
9) Please cut the switch off when stop to use the electrical appliance;
10) Battery shall be taken out if the electrical appliance will not use for a long time;
11) Battery shall be stored in shade, dry and no sunshine beat.
31. What is the explosion of battery?
It is named explosion that the solid-state matter of any parts in battery spurt out momentarily and is pushed to a distance more than 25cm. To judge a battery explode or not, following methods are used: Put a net mantle battery in the center, the distance between battery and every sides of net shall be 25mm; The density of net is 6-7pieces/cm, and reticle is soft aluminum line (diameter is 0.25mm); If no solid-state matter get across the net during experiment, it proves that no explosion occurs.
32. What is over-charge? What are impacts result on cells performance?
The continual charge action after fully charged with certain charge approach is overcharge. For Ni-Cd cell, while overcharged the following reactions are generated:
Positive:4OH- – 4e ~ 2H2O + O2; (1)
Negative:2Cd + O2 ~ 2CdO; (2)
Because the positive capacity is higher than the negative on design, the generated oxygen in positive can composite, through separator, with Cd generated in negative.In general condition the internal pressure will not rise obviously, but if the charge current is so heavy or the charge time is so long, and the generated oxygen can not be used up in time, there will be some quality defect risen such as internal pressure rising, battery distortion, leakage and so on. Meanwhile its performance will decline.