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|Frequently Asked Questions|
1-Parameters for Battery Performance
Voltage: Normal voltage during discharge, maximum and minimum permissible voltages, discharge curve profile
Duty cycle: Conditions the battery experiences during use. Type of discharge and current drain, e.g., continuous, intermittent, continuous with pulses, etc.
Temperature: In storage and in use. Temperatures that are too high or too low can greatly reduce battery capacity.
Shelf life: How rapidly the cell loses potential while unused.
Service life: Defined either in calendar time or, for secondary cells, possible number of discharge/charge cycles, depending on the battery application. Service life depends on battery design and operational conditions, i.e., the stress put on a battery. For stationary and motive power application, the end of service life is defined as the point at which a battery's capacity drops to 80% of its original capacity. Exceptions would include car batteries where the service life ends when the capacity falls below 60%.
Physical restrictions: These include dimensions, weight, terminals, etc.
Maintenance and resupply: Ease of battery acquisition, replacement, charging facilities, disposal.
Safety and reliability: Failure rates, freedom from outgassing or leakage; use of toxic components; operation under hazardous conditions; environmentally safe
Cost: Initial cost, operating cost, use of expensive materials
Internal resistance: Batteries capable of a high-rate discharge must have a low internal resistance.
Specific energy: As discussed in the definition section, this is a measurement of possible stored energy per kilogram of mass. This number is purely theoretical as it does not take into account the mass of inactive materials, nor the variation in chemical reactions.
Specific power: Also defined in the definitions section, a P=E/t, so the specific power is discussed at a specific discharge rate. It is possible for batteries with a high specific energy to have a low power density if they experience large voltage drops at high discharge rates.
Unusual requirements: Very long-term or extreme-temperature storage; very low failure rate; no voltage delay, etc.
2- Battery Storage
NiMH (Nickel Metal Hydride) Battery Storage
Ni-MH batteries can give years of safe and reliable service if they are used in accordance with recommended procedures and are not abused. Batteries should be kept clean and dry both during use and storage. They can be stored for many months in a charged or discharged state without any detrimental effects as long as they are not exposed to extreme temperatures for any long period of time. After periods during which the battery has not been used, the battery should be charged before being placed in service. Extended overcharging or overheating of the battery should always be avoided.
Storage temperatures between -20℃ and +35 ℃ are recommended, relative humidity approximately 50%. In case of long term storage cells must be recharged once a year.
If a battery is stored for a prolonged time connected to a load, electrolyte fluid will leak, the battery will begin to deteriorate, and capacity will be impaired after storage. During long time storage battery deactivation may tend to occur, and for this reason charging may stop early during recharging after storage. This problem can be solved by charging and discharging the battery several times.
1. The inside of the cell is a hydrogen atmosphere at low pressure, which gradually reduces the active materials at the
positive electrode, resulting in a drop of cell capacity. Accompanied by this, the negative electrode which is
thermodynamically unstable in its charged state gradually gives off hydrogen gas, thus reducing cell capacity.
2. The active materials at the positive electrode in its charged state self-decompose, causing the cell capacity to decrease. 3. Impurities within the cell, especially nitric ions, are reduced at the negative electrode and diffuse to the positive electrode
where they are oxidized. This results in a lowered cell capacity.
The factors (2) and (3) also apply to Nickel-Cadmium Batteries. As discussed above, the self-discharge of Nickel-Metal-Hydride Batteries during battery storage causes a loss in stored energy. However, once recharged, this lost portion of the capacity will be almost completely restored. The self-discharge characteristics of Nickel Metal Hydride batteries is affected by storage temperature. If the battery is stored at high temperatures, the self-discharge will be accelerated. Also, the longer the storage period, the more the cell capacity decreases. As mentioned above, since the capacity of Nickel Metal Hydride batteries lost by self-discharge can be restored by recharging, there are virtually no noticeable adverse effects of battery storage. However, prolonged storage at high temperatures may deteriorate or deform the gasket or the separator, and should be avoided. Either fully charged or discharged, Nickel Metal Hydride ( NiMH ) batteries may be stored indefinitely. In either cased (charged or discharged) the capacity is recovered within two or three charge / discharge cycles.
Nickel Cadmium Battery Storage
The sealed Nickel-Cadmium cell can be stored in the charged or discharged state without damage. It can be restored for service by recharging (one or two charge/discharge cycles).
Store NiCd batteries in a dry location with low humidity, no corrosive gasses, and at temperature range of -20°C to +45°C. Because long term storage can accelerate battery self-discharge, and lead to the deactivation of reactants, locations where the temperature ranges between +10°C and +30°C are suitable for long term storage.
When storing batteries for more than one year, charge at least once a year to prevent leakage and deterioration of performance due to self-discharging. When using a rapid voltage detection charger carry out charge and discharge at least once ever 6 months.
Lithium Ion, Lithium-Ion polymer Battery Storage:
Storage temperature range is -20°C to 60°C. Recommended storage voltage range is 4.1 to 2.0 volts per cell. For prolonged storage periods, store discharged [i.e. 2.0 to 3.0 volts per cell] and at -20°C to 25°C.
The batteries should be stored at room temperature, charged to about 30% to 50% of capacity. We recommend that the batteries be charged about once per year to prevent overdischarge.
Alkaline Battery Storage
A refrigerator, with a temperature range from 0°C to 10°C [32°F to 50°F], is a good place for storing batteries, especially primary batteries. The refrigerator may, of course, also be used to store secondary batteries, but since they are rechargeable, their loss of capacity during storage may be better compensated by recharging, particularly as they can take up substantial space in the refrigerator (e.g. automotive batteries).
What impact may a "special" environment have on primary batteries? When storing primary batteries over several years in a refrigerator, it is important to remember that a refrigerator exhibits a rather low relative humidity. This phenomenon is familiar from uncovered food which is stored for a couple of days or longer: The food (e.g. cheese, meat) will loose moisture and dry out. This also happens - even if only slowly - to unpacked batteries if stored over an extended period of time (years). The water-vapor permeability of the batteries' plastic seal determines how quickly they dry out. The rate at which the water vapor permeates the plastic seal depends on its cross-section and surface and on the relative humidity of the battery's hydrous electrolyte.
Generally this rate is very, very low. Nevertheless it cannot be ignored over extended periods of time, leading finally to a noticeable increase in the battery's internal resistance, while reducing its load capability. Thus, if anyone has to store primary batteries for a longer period of time in a refrigerator, they should be stored in a vapor-proof packaging, such as plastic-laminated aluminum foil. This precautionary measure is only necessary where batteries are stored for several years in a refrigerator or an extremely dry environment. Before use, primary batteries should be removed from the refrigerator soon enough to allow them to adapt to the ambient temperature.
Another tip: During the summer months, the glove compartment of an automobile is a quite unsuitable place to keep a flashlight. If the sun shines down on the car, temperatures may rise up to and even exceed 60°C. Consequence No. 1: The internal resistance of the batteries increases and the batteries dry out. Consequence No. 2 : When it is needed the flashlight may provide only a dim flicker. The same applies to a battery-operated emergency light in the car's trunk. This too should be checked regularly to ensure that it is functioning properly, and if necessary, the batteries should be replaced in good time.
3-NiMH Battery Charging
The coulometric charging efficiency of Nickel metal hydride batteries is typically 66%, meaning that you must put 1.5 amp hours into the battery for every 1 amp hours you get out. The faster you charge the worse this gets.
The minus delta V bump that is indicative of end-of-charge is much less pronounced in NiMH than NiCd, and it is very temperature dependent. To make matters worse, new NiMH batteries can exhibit bumps in the curve early in the cycle, particularly when cold. Also, NiMH are sensitive to damage on overcharge when the charge rate is over C/10. Since the delta V bump is not always easy to see, slight overcharge is probable.
As the battery reaches end-of-charge oxygen starts to form at the electrodes, and be recombined at the catalyst. This new chemical reaction creates heat, which can be easily measured with a thermistor. This is the safest way to detect end-of-charge during a fast charge.
The cheapest way to charge a Nickel metal hydride battery is to charge at C/10 or below (10% of the rated capacity per hour). So a 2000 mAh battery would be charged at 200 mA for 15 hours. This method does not require an end-of-charge sensor and ensures a full charge. Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10. The minimum voltage you need to get a full charge varies with temperature--at least 1.41 volts per cell at 20°C. Even though continued charging at C/10 does not cause venting, it does warm the battery slightly. To preserve battery life the best practice is to use a timer to prevent overcharging to continue past 13 to 15 hours.
Using a timer it is possible to charge at C/3.33 for 5 hours. This is a little risky, since the battery should be fully discharged before charging. If the battery still has 90% of its capacity when the timer starts you would have a good chance of venting the battery. One way to ensure this doesn't happen is to have the charger automatically discharge the battery to 1 volt per cell, then turn the charger on for 5 hours. The advantage of this method is to eliminate any chance of battery memory.
If a temperature monitor is used NiMH batteries can be charged at rates up to 2C (in other words 100% of the battery capacity in amp-hours for 40 minutes).
This board also has the ability to sense voltage and current for more sophisticated algorithms.
When terminating on temperature rise the dT/dt value should be set at 1 to 2 degrees C per minute.
In a standby mode you might want to keep a Nickel metal hydride battery topped up without damaging the battery. This can be done safely at a current of between 0.03 C and 0.05 C. The voltage required for this is dependent on temperature, so be sure to regulate the current in the charger.
dT/dt versus -dV/dt
These two termination methods work well for NiCds, and are both applied to NiMH as well. dT/dt measures the temperature rise at the end of charge. After the battery is fully charged it starts new chemical reactions in order to absorb the unneeded current. In Nickel hydroxide style batteries this consists in generating and recombining oxygen. This process heats the battery. The sudden increase in temperature rise can be used to terminate the charge.
Another effect of the oxygen generation/recombination cycle is to depress the voltage of the battery slightly. If you can detect this voltage depression you can use this signal to terminate the charge. Of course, -dV/dt is the easiest because it doesn't require a temperature sensor. The best method for NiMH is the dT/dt method. There are two main reasons. With the NiMH battery the voltage depression is smaller, and harder to detect than with the NiCd battery. This almost always ensures an overcharge, which will limit the total number of charge/discharge cycles before battery failure. Second, a new NiMH battery has false peaks early in the charge cycle, and so the charger will terminate too soon.
There are new algorithms that use microprocessor control to use the -dV signal to detect the end of charge. These can work very well and several of our chargers use this technique, which involves pulsing the charger on and off to do the voltage measurements. This technique seems to be sensitive to imbalance in the capacity of the cells. The dT/dt is still more reliable, especially for large packs, but in cases when only two wires are available solutions are now available.