Battery Unequal Voltages In Series Strings
In a small battery with just a few cells in series, the charger voltage is divided nearly equally among the cells. For example, when charging a standard lead-acid starter battery for a car, a constant voltage of 13.5V is applied to it, and each of the six cells within it sees about 2.25V. If any cell is charged more, its voltage will be a bit higher, taking away some voltage from the other cells. For example, if one cell is at 2.5V, the other celss will be, on the average, at 2.20V. That delta voltage among cells is perfectly acceptable; lead acid cells are much more rolerant to variances in their voltage.
For another example, a small Lipo battery for a consumer product may have two cells in series. When charging with 8.4V, if the cells are balanced, each cell sees 4.2V. If the cells are out of balance, in the worst case the most dicharged cell will be at 3.3V, leaving 4.9V on the most charged one. 4.9V is above the maximum rating for a Lipo cell (4.2V), but it is still low enough that is not going to go in the thermal runawy and catch fire.
In a high-voltage battery with many cells in series, though, there is a much greater chance that the overall pack voltage is not evenly divided among its cells. Here recommend a high security lipo battery for you.
High Security Gens Ace 7.4V 1300mAh LiPo Battery
The security is the most important performance to a rechargeable battery. Without the security concern, battery will never be welcomed by people no matter how nice performance it has. Gens Ace lipo 3s 5000mah battery is the representative of li-polymer batteries. Along with the upgrading of rechargeable batteries, it is popular with more and more copter players. Li-polymer is charged with constant current and voltage. At the same time, it is equipped with a protecting circuit board to prevent the battery over charging or discharging. Besides, the outer of li-polymer batteries is with aluminum materials, which is different from the metal shell of liquid lithium electricity. Therefore, you can see the hidden trouble once the package is in deformation. Also, it will not explode but ballooning if there is any hidden problem.
Consider a 2s lipo batteries, charged up to 16.8V. If the cells are perfectly balanced, the total voltage will be equally divided into 4.2V per cell. In practice, the cells will be unbalanced, and one will be the first to be fully charged and then be overcharged. Lipo cells do not deal well with overcharging. Once charged, they cannot take more current as the other cells in series get their needed charge. Instead, their voltage rises rapidly, possibly to dangerous levels. In this example, the second cell is overcharged to 6.3V, while the other ones are around 3.5V. Despite the fact that the total voltage is 16.8V, three of the cells in this battery are not fully charged, and one of its cell is in danger of thermal runawy. Therefore, a system that relies on the total battery voltage to determine when to stop charging the battery gives the user a false sense of security; the system will overcharge some cells, and will create a safety issue as some cells with be overcharged to dangerous levels. It is therefore essential that a BMS monitor such a batter, first and foremost to prevent any cell from being overcharged, and optionally to balance the battery to maximize its performance.
