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Battery Chargers
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Chargers for lithium-based batteries are more defined in terms of charge method and charge time. This is, in part, due to the tight charge regime and voltage requirements demanded by these batteries. There is only one way to charge Li-ion/Polymer batteries and the so-called 'miracle chargers', which claim to restore and prolong battery life, do not exist for these chemistries. Neither does a super-fast charging solution apply.
The pulse charge method for Li-ion has no major advantages and the voltage peaks wreak havoc with the voltage limiting circuits. While charge times can be reduced, some manufacturers suggest that pulse charging may shorten the cycle life of Li-ion batteries.
Fast charge methods do not significantly decrease the charge time. A charge rate over 1C should be avoided because such high current can induce lithium plating. With most packs, a charge above 1C is not possible. The protection circuit limits the amount of current the battery can accept. The lithium-based battery has a slow metabolism and must take its time to absorb the energy.
FEATURES OF BATTERY CHARGER :
Automatic battery conditioning
NiMH batteries perform best when they are conditioned through a discharge/charge cycle. Many chargers do this automatically. On some smart chargers you simply press a button for the conditioning to begin.
Rapid chargers
Charge batteries in a couple of hours. If the batteries and charger are specifically designed for quick charging, battery life should not be prematurely reduced.
Dual carging units
Charge two different sets of batteries simultaneously; eg: NiMH AA and NiHM AAA.
World voltage capable
Some chargers have a built-in 100~240V 50/60 Hz power supply. They can be used in other countries with a only an electric plug adapter.
BATTERY CHARGER PLUGS:
Transformer - Battery chargers with transformers can be less expensive than others, but are not convenient if you want to carry the charger in your camera bag.
Fold in - Plugs fold into the charger when not in use. They are compact and easy to pack in a camera bag. If you use this type of charger, check the instructions. Some must be removed from the outlet once batteries are fully charged.
Performance Measurement of NiMh Battery Chargers:
All the Battery Chargers can be measured against two parameters:
%age Charge Completion and Speed: %age Charge Completion depends on many factors like:
- Capacity
- Age
- State of Charge already in Cell when charging started
Typically all the rechargeable cells are rated in milliamp-hours(mAh). A fully charged 1000mAh cell can in principle deliver 1000mA for one hour. Manufacturers of rechargeable batteries usually measure their products' mAh ratings using a load that discharges the battery over 10 hours (this discharge rate is more technically called "0.1C", where C is the ostensible one hour current capacity). This slow discharge rate tends to result in fairly optimistic ratings; higher loads (such as those imposed by the medium- and high- drain devices that get the most benefit from NiMH batteries) can usually extract less total energy out of the cell than the rated capacity would suggest.
Another issue is that mAh does not measure energy capacity per se. While NiMH cells deliver almost constant voltage throughout their discharge cycle, there is still a small reduction in voltage over time, which in turn reduces the amount of energy delivered to a constant current load. The mAh rating does not take into account this voltage drop and therefore is not a precise indication of the total energy that the charged cell can deliver. To get total actual available energy, the relevant measure is the total capacity in Watt-seconds (Joules), which unfortunately is not generally specified by the cell manufacturers (but is easy enough to measure). The energy to current capacity ratio is probably more influenced by the construction and chemistry of the individual cell than by its most recent charge. However, since the charger does alter the cell chemistry over time, I report both measured current capacity and energy capacity here; compare whichever metric you prefer.
2. Damage done while charging
Each charge-discharge cycle damages the batteries somewhat; excessive or improper charging can result in a reduced number of charge cycles over the batteries' lifetime and in reduced energy capacity. Different chargers are said to vary widely in this regard. One of the most commonly cited factors in charger-related battery damage is excessive heat. Charging tends to heat the battery somewhat, due to internal resistance and cell chemistry. Excessive current as the cells become more fully charged, among other factors, tends to generate excessive heat, and a properly designed charger must be careful to back off the current as the cells become increasingly "full." Better chargers use a variety of heuristics to estimate cell state, including the rates of change in temperature and voltage, the aim being fill the cells as close to capacity as possible without overcharging.
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