A battery charger is a device that charges batteries from AC or DC power. They vary in price from 10 US dollars for a simple wall charger, to hundreds of US dollars for a top of the line microprocessor-controlled charger. Most models on the market are so-called Delta-Peak chargers. The simplest models are Timed chargers.
Constant- or non-pulsed ChargingEdit
Most of the devices on the market recharge batteries with either a constant current, or a current that varies according to the charging progress. This method is relatively simple to implement, but has been proven to be less healthy to a battery-pack's total expected lifetime.
Pulse chargers are usually a lot more expensive due to higher currents and increased software requirements. One of the most prominent pulsed charge methods is the so-called "Reflex"-charge. This patented method uses pulsed charge/discharge cycles to defeat the memory effect of NiCd/NiMH cells. The cycle consists of a 1s charge, followed by a 5ms discharge pulse with a current that equals at least three times the charging current, which is then followed by a short measuring period. Obviously, this charge method is not to be used for LiPo cells, but only for NiCd and NiMH accumulators. The big advantage of reflex chargers is that there is virtually no memory effect, allowing the immediate charging of, for example, a half-empty battery pack without the need to discharge it first. Reflex charging is much healthier for the accumulator cells and can extend their lifespan considerably. Despite the high burts of discharge-current, battery packs generally remain cooler than those charged with regular chargers. True Reflex chargers don't use Delta-Peak method to measure when the cell is fully charged, but rather the dU/dT-method, effectively switching off when the cell is at 98% of it's specified capacity. Delta-Peak is usually also implemented, to serve as a backup method in case the change in the voltage curve is not distinct enough for the charger to determine the switchoff point via dU/dT. Good models use a combination of regular charge, reflex charge, and careful final charge phases to bring the cells to 99% capacity without overcharging. "Cheap" Reflex chargers use only Delta-Peak, have a different charge cycle, or only support a relatively low-amp discharge pulse, resulting in suboptimal performance. Reflex chargers are usually more expensive than regular chargers, in part because of the patent fees attached to this charge method, and run for around US $200.- for basic, high-quality models. With the advent of LiPo cells, the Reflex charge method is loosing some of it's importance. In addition, few hobbyists even know about it's existence or advantages.
Actually, terms like "Delta-peak" do not say anything about the actual charge process - you could have a pulsed- or constant-current charger and both could be called a "Delta-peak charger". This term simply denotes the way the charger determines when to stop charging.
Delta-Peak (or just "Peak") chargers use digital circuitry that will detect when the battery has surpassed its maximum capacity, or its "peak". Depending on the charger and the selected charge parameters, the switchoff point may be more or less behind the peak, thus slightly overcharging the cells. Overcharging can result in damage to the cells, and undercharging results in shorter runtime. Most peak chargers are capable of using AC and DC for input power, but some, usually higher-end models, accept only DC power, necessitating the use of DC power. The voltage charge curve of NiMH and NiCd is different, thus requiring the charger to "know" what type of cell is to be charged. Using the wrong type of cell is likely to result in permanent damage to the cell. Delta-Peak chargers are the most common kind of charger on the market.
Another method of determining when a battery is full is the dU/dt-Method. By measuring the steepness of the voltage curve, it is possible to determine when the battery pack is at 98-99% of its capacity, thus avoiding overcharging entirely. The advantage is that while the pack is not 100% charged, the missing 1-2% don't make much of a difference while this method does extend the life expectancy of the cell considerably. Most dU/dt-chargers use trickle-charging to slowly bring the cell to 100% without overcharging and keeping it there.
Timed chargers use a simple mechanical or electronic timer to stop charging. They cannot detect when a battery is completely charged, so it is possible to inadvertently overcharge a battery if it is left unattended. This can result in permanent damage to the battery. In addition, fires can be caused by the battery overheating. Timed chargers are almost always capable of using either AC or DC power, and are generally cheaper than peak chargers. The low price of some of the Delta-Peak chargers has made most timed-chargers obsolete.