Lithium Battery Pack Composition

- Jan 06, 2019-

The lithium battery pack is composed of several single lithium batteries in series. Due to the difference of the individual, the inconsistent voltage rise at the end of the series charging will lead to overcharging of some single batteries and undercharging of some single batteries. The ideal state is that the voltage of each battery rises synchronously during the charging process, which is exactly the same. When the charger is close to full, the charger turns the lamp and the charging stops. Regular balancing of lithium batteries can basically achieve this ideal state, which is the effect pursued by people who dislike lithium-ion protection panels. Lithium battery protection plate itself is not necessarily reliable, protection plate damage lithium batteries are rare. The parallel manual equalization method I experimented with has simple and reliable circuit, good effect and practical value. The basic principle is that all batteries are connected in parallel when charging equally, and in series when charging routinely and using electricity. The parallel voltage of all batteries is equal when charging equally, and the forced equalization of all batteries is realized.

1. Diode Isolated Parallel Charging Equalization Method

See Figure 1 of the circuit. Take six single batteries in series as an example, disconnect the switch S1-S5 and then connect the charging power supply. The diode is 1N5401-5408. The measured forward voltage drop of the diode is 0.8V at rated current of 3A. The current flowing through the diode is very small when the forward voltage drop is 0.7V. For lithium iron phosphate batteries, the maximum charging voltage is 3.65V. In practice, considering prolonging battery life, the maximum charging voltage is 3.5V, charging voltage is 3.5+0.7+0.7=4.9V and line voltage drop, it is suitable to choose 5V power supply. The maximum charging voltage of ternary and polymer lithium batteries is 4.25V, and the charging voltage is 4.1+0.7+0.7=5.5V. In both cases, the batteries can stop when they are close to full charge. Although the individual batteries are isolated by the diodes during charging, it does not affect the balance of the batteries, because the charging current with high voltage is small and the charging current with low voltage is large. Disconnect the equalized charging power supply and close the switch S1-S5 battery to discharge in series. When the load current of lithium battery pack is small, it is feasible to select switch for S1-S5. In the case of high current discharge, it takes less time to tighten and loosen the screw than to pull the switch to replace the switch with pressure connector because of its small size, small contact resistance, short wiring and low cost. This equalization is done once a month to three months depending on battery usage, which is generally not troublesome.

2. Direct parallel charging equalization method

As shown in Figure 2 of the circuit, the isolation diode is cancelled. The charging voltage of lithium iron phosphate batteries is 3.5-3.6V, and that of ternary and polymer batteries is 4.1-4.2V. The red alligator clamp leads are welded together to connect the positive pole of the charging power supply; the black alligator clamp leads are welded together to connect the negative pole of the charging power supply. When charging equally, the S1-S5 is disconnected, and the red crocodile clamp holds the positive poles of the individual batteries separately; the Black Crocodile clamp holds the negative poles of the individual batteries separately, and all batteries are connected in parallel. In circuit 2, plug-in diagram P1-P12 is used to represent the connection of crocodile clamp. The effect of replacing crocodile clamp with plug-in is the same. The advantage of this connection method is that without the heat loss of the diode, even if the battery without charging power supply can be balanced to a certain extent, the battery with high voltage will discharge to the battery with low voltage and force balancing.

Both balanced charging voltages are set in a way that is full of self-stop without supervision. If short time, large current, fast balancing and supervision are needed, the charging voltage can be increased. The second method has no concern about the heating of the diode. It can save the equalization time by using a large charge current. It can use a common low voltage and large current rectifier power supply without voltage stabilization. Battery packs with good balance are usually charged with ordinary chargers. It is advisable to reduce the output voltage of the charger appropriately so as to turn the lamp. There is no need for the protective board and there is no fear of overcharging. It is suggested that the charging voltage of lithium iron batteries should be 3.5V multiplied by series number and that of ternary batteries should be adjusted by 4.1V multiplied by series number. Although the two balancing methods are simple in principle, clumsy in operation, they are real, effective and reliable. At present, no balanced lithium battery protection board can achieve fast and efficient balancing, even if it is expensive, it can not afford to play. Interested friends may as well toss and turn.