The increasingly developed portable electronic products have higher and higher requirements for the capacity and cycle life of rechargeable batteries At present, there are four commonly used categories, nickel cadmium batteries and lead-acid maintenance free batteries (SLA) have been widely promoted in the 1960s. Nickel hydrogen batteries and lithium-ion batteries (lithium polymer batteries) have only been commercialized in recent ten years Each of these four types of batteries has its own advantages. The following table gives a basic reference. The specific values of different battery manufacturers may vary greatly. Consult the manufacturer when using. The data in the table are only for comparison and reference (C in the table is the nominal capacity of the battery, and the unit is ah (ampere hour), and a represents the charging constant current.)
Comparison item lead acid maintenance free battery nickel cadmium battery (NiCd), nickel hydrogen battery (NIMH) lithium ion battery (Li-Ion)
Energy density (wh/kg) 304060100
Cycle life (Times) 300800500800
Operating temperature (° C) 0~350~450~400~50
Maximum charging current (a) 0.25c2c1c1c
Charging mode constant voltage after constant current constant current constant voltage after constant current
Charging time (hours) c/a + 2 hoursc/a + 20%c/a + 20%c/a × two
Of course, people hope that the charger can complete charging in a short time, but due to the large current and difficult control, it has a certain cost in terms of cost and technology If the service conditions permit, the slow charging charger at night after 10~14 hours of charging will have the advantages of low price and small size
For SLA batteries used online in parallel, it is recommended to use 2.27v/ unit (corresponding to 13.7v for 12V batteries) for constant voltage charging. This scheme will not overcharge the battery, but also help to stabilize the equipment voltage For the occasions where SLA batteries are frequently used for off-line charging and discharging, the "Three-stage" charging method of 0.2C constant current, then 2.45v/ unit constant voltage should be used. After the battery is charged, it immediately enters 2.27v/ unit constant voltage The whole charging time is about 8 to 10 hours
NiCd battery allows certain current overcharge, so it is economical to choose a simple constant current source charger with 0.1C current
NiMH battery is sensitive to overcharge, that is, it uses a current of 0.1C. If it is charged for more than 14 hours or the charged battery continues to be charged for several hours, the service life of the battery will be greatly damaged It is very important to turn off the current or reduce the charging current below 0.02c when the battery is fully charged, even for the 0.1C night slow charging NIMH charger
There have been many successful cases of intelligent fast charger with maximum charging current of about 2C Generally, this kind of charger must monitor the battery voltage, temperature and other parameters at all stages of the charging process. When the battery is about to be fully charged, it will automatically reduce the charging current multiplier, so as to minimize the risk of over temperature and over voltage caused by battery overcharge.
The charging characteristics of NiCd and NiMH batteries are very similar, except that NiMH batteries generate more heat and the peak voltage is less obvious during charging. Both of these two types of batteries adopt one of the following conditions as the conditions for fast charging termination: voltage rise slope (dv/dt), negative voltage rise (-dv) and battery temperature rise slope (dt/dt). Three other conditions are set to operate under abnormal protection conditions: maximum battery temperature, maximum battery voltage and charger built-in timer. To realize these monitoring and intelligent switching actions, the cost of the charger must rise. However, without these measures, the battery will face potential safety hazards such as reduced capacity, reduced cycle life and even liquid leakage explosion
The charging methods of Li-ion and SLA batteries are similar, requiring constant current before constant voltage. The difference is that lithium-ion batteries require high voltage accuracy (<1%) in the charging constant voltage stage. Since the safety of lithium battery is a fatal hidden danger, special care should be taken in the control and protection of the end of charging. For different applications, the lithium-ion battery pack will be equipped with corresponding charge and discharge protection circuit, so as to protect the battery more safely.
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