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 Chargers |
What should we ckeck when purchasing a charger:
• To 100% charge the rechargeable batteries (fast charge followed by completion and maintainance)
• To detect and notify the charging end
• To display the charging stages: Not connected, Loading/Charging, Charged, Wrong cell/Fault
• To detect and notify if there are problems with the connected elements: dead rechargeable, wrong polarity, primary battery, overheating
• To work according to an algorithm adapted to the galvanic element connected to the charger (NiCad or Ni-MH); to constantly monitor the charging rate and to notify about the charging steps or the possible faults
• To charge and control each cell separately - when multiple cells are connected simultanously in the charger
• To avoid overcharge and its consequences, the charger must have a preset "timeout" period after which the charging must be stopped
• The temperature of each cell must be monitored and the charging process must be adjusted to the cell's temperature. Also, the charging process must be terminated in case of overheating.
Features:
1. Standard charging/ fast charging
2. Detecting charging end
3. Outage threshold
4. Lifetime for rechargeables
5. Ni-MH, NiCad, Li-Ion rechargeables
1. The rechargeable batteries must be charged according to the manufacturer's indications.
1.1. Usually the manufacturer recommends a standard charging (constant charging current at a 0.1 value of the nominal capacity for 16 hours)
Advantages:
• longer lifetime for the rechargeables if the recommended indications are followed
• require a slow charger
Limitations:
• slow charge
• require manual disconnection after 16 hours
1.2. For some types of rechargeables fast charging is possible
Advantages:
• shorter charging period
Limitations:
• a more complex charger is required, able to detect end of charging (see pt 2)
• shorter lifetime than in standard charging but this can be countered by using a more performant charger. Fast charging is followed by a "completion" charging and then a "maintainance" charging to compensate the self-discharge of the rechargeable, therefore it will be completely charged when removed from the charger. The completion and maintainance charging methods can be continuous or discontinuous (pulse)
Recommendation: purchase a good charger. A good charger has a micro-controller. Also be careful what energy source you use for charging. There are chargers which can be connected to the mains (220V; 50Hz) or to a 12V DC power source (car-lighter) or the USB port of the PC or even a solar pane charging source.
2. Detecting charging end
2.1. The charging method is one of the most important aspects that influence the rechargeables. The charger must provide a 100% charging of the cells, irrespective of the cell's initial level of discharge. Full charge must be detected and signaled. Overcharge shortens the rechargeable's lifetime because of the raising of the inner pressure. This is a result of gas release in overcharging and overheating conditions. Extreme overcharge or wrong polarity can result in the explosion of the cells.
2.2. Methods of charging end detection (Ni-Cd; Ni-Mh):
Detection of voltage decrease for each element over a set value
During charge the voltage increases, reaches a maximum than tends to decrease; compared to the maximum value, when the decrease reaches a certain value/cell it indicates the rechargeable battery is completely charged.
Detection of temperature increase in time over a set value
During charge the temperature of the cells raises. At the end of the charging process the rechargeable has a tendency to increase the temperature/minute - at this point the charge must be stopped.
End of charging after a period of time is used mostly with slow charging processes (the slow charging - with currents of 0.1 from C5 - ends after 14-16 hours)
End of charging at a maximum voltage value correlated with the room temperature.
End of charging at a set cell temperature value (eg. Ni-MH at 60C).
3. Outage threshold - the minimum voltage down to which the cells can discharge.
Usually this final voltage is set at 1V. For rechargeable batteries it can also be set at 0,9V When the application requires high current discharge this voltage can be set at 0,65V/cell. This voltage value depends on the number of cells that compose the source and on their protection levels. These types of discharges must be avoided as they shorten the life of the rechargeable battery. Eg. A NiCad rechargeable 100% discharged has approx 800 cycles while one that is only 50% discharged has approx 2500 cycles.
4. Lifetime for a rechargeable battery represents the number of complete cycles until its capacity decreases under a certain level, set in accordance with the nominal capacity (NC):
• 80% NC for NiCad, Li-Ion cells
• 60% NC for Ni-MH cells
Lifetime varies with:
• Chemical structure: NiCad, Ni-MH, aso
• Technological performances
• Usage conditions
From the usage conditions point of view, there are many factors that determine the lifetime of the rechargeables. Some of them are:
• Range of deposit, charging and discharging temperatures
• Charging method
• Depth of discharge
• Value of discharge currents and the discharge method,
• Charging-discharging frequency as well as the time between them
The first three are the most important elements that influence the lifetime of a rechargeable battery.
Influence of temperature:
Temperature of usage:
• 20C - 100% lifetime
• 0C - 80% lifetime
• 60C - 10% lifetime
5. Primary/rechargeable galvanic elements. Typology:
The galvanic elements can generate electrical energy due to the electrochemical potential of the constitutive elements.
5.1. Primary galvanic elements - known as batteries - generate electrical power until they reach a minimal outage threshold from where cannot be recharged anymore.
5.2. The rechargeable galvanic elements - known as rechargeables - can be used until the voltage will lower under the recommended value. After this they can be recharged with devices that will help renew the rechargeable's capacity under the conditions imposed by the manufacturer.
5.3. Types of rechargeables, by chemical structure:
5.3.1. NiCad rechargeables
Advantages: Resistant; Longer lifetime; Wide operating teperature range; Resistant at medium intensity overcharging; Can be easily recharged;
Limitations: Contain heavy metals: Cad; under special working conditions can have "memory effect".
5.3.2. Ni-MH rechargeables
Advantages: Higher capacity for the same volume as NiCad rechargeables; Do not have memory effect; do not contain toxic metals.
Limitations: Shorter lifetime compared to NiCad rechargeables; narrower operating temperature range and have more restrictions regarding the way they can be used; Higher self-discharge rate.
5.3.3. Li-Ion rechargeables
Advantages: Highest energy density; Long lifetime; Can be used in a wide range of temperatures without significant changes in their performances; Low self-discharge; Do not have memory effect; Do not contain toxic metals.
Limitations: They can be used only with a protection circuit that ensures the protection of both the elements and the user; Requires a complex charger: does not allow overcharge; Higher price.
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