Increasingly developed portable electronic products require higher and higher capacity and cycle life of rechargeable batteries. At present, there are four kinds of commonly used batteries, nickel-cadmium batteries and lead-acid maintenance-free batteries (SLA) which have been widely promoted in the 1960s, and nickel-hydrogen batteries and lithium-ion batteries (lithium-containing polymer batteries). These four types of batteries have their own advantages. The table below gives a basic reference. The specific values of different battery manufacturers may be quite different. Consult manufacturers when using. The data in the table are for comparative reference only (the nominal capacity of batteries in table C, unit AH (ampere-hour), A (ampere-hour). Represents charging constant current.

project	Lead-acid maintenance-free battery	Nickel-cadmium battery (NICD)	Nickel metal hydride battery(NIMH)	Lithium ion battery(LI-ION)
energy density(Wh/kg)	30	40	60	100
Cyclic life (secondary)	300	800	500	800
Working Temperature (C)	0~35	0~45	0~40	0~50
Maximum charging current (A)	0.25C	2C	1C	1C
Charging mode	Constant voltage after constant current	constant current	constant current	Constant voltage after constant current
Charging time (hours)	C/A + 2 hours	C/A + 20%	C/A + 20%	C/A × 2

 

 Of course, people hope that the charger can be charged in a short time, but because of the high current and the difficulty of control, there is a certain cost in cost and technology. If the use conditions permit, the night slow charging charger with about 10-14 hours charging end will have the advantages of low price and compact size.

 For SLA batteries used in parallel on-line, it is recommended that 2.27V/cell (corresponding to 12V battery is 13.7V) be charged at constant voltage. This scheme will not overcharge batteries, but also help to stabilize equipment voltage. For cases where SLA batteries are often used offline, 0.2C constant current should be used first, then 2.45V/cell constant voltage should be used to charge the batteries properly. After that, a "three-stage" charging method with constant voltage of 2.27V/cell is instantly entered. The whole charging time is about 8 to 10 hours.

 NICD batteries allow a certain current to be overcharged, so it is economical to choose a simple constant current source charger with 0.1C current.

 NIMH batteries are sensitive to overcharge. Even if the current of 0.1C is used, if the battery is charged for more than 14 hours or if the battery is charged for several more hours, the life of the battery will be greatly damaged. It is very important to turn off the current or reduce the charging current to less than 0.02C in time when the battery is charged, even if it is slow at night for 0.1C. The same is true for NIMH chargers.

 There are many successful cases of intelligent fast charger with maximum charging current of about 2C. This kind of charger must monitor the battery voltage, temperature and other parameters at any time during the charging process. When the battery is about to be full of electricity, it can automatically reduce the rate of charging current, so that the overheating caused by battery overcharging can be achieved. The risk of overpressure is minimized.

 The charging characteristics of NICD and NIMH batteries are very similar, but the charging process of NIMH batteries is more heated and the peak voltage is less obvious. Both types of batteries use one of the following conditions to terminate fast charging: voltage rise slope (dV/dt), negative voltage growth (-dV) and temperature rise slope (dT/dt). There are also three conditions for operation under abnormal protection conditions: the maximum battery temperature, the maximum battery voltage and the built-in timer of the charger. In order to achieve these monitoring and intelligent switching actions, charger costs must rise, but without these measures, batteries will face potential safety hazards such as capacity decline, cycle life reduction and even 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 higher voltage accuracy (< 1%) in the charging constant voltage stage. Because the safety of lithium batteries is a fatal hidden danger, special care should be taken in the control and protection of charging end. For different applications, the lithium-ion battery pack will have a corresponding charge-discharge protection circuit built-in, so as to protect the battery more safely from happening outside.