Lithium Polymer (LiPo) batteries are popular in many applications due to their light weight, high energy density, and flexibility in form factors. A fundamental understanding of their electrochemistry and design is crucial to grasp the reason behind their voltage characteristics.
1. Chemistry and Voltage:Each cell in a LiPo battery has a specific chemistry that dictates its nominal voltage and charge voltage. The nominal voltage (3.7V for most LiPo cells) represents the average voltage the battery will have during its discharge cycle. This is the average operating voltage for most of the battery's life.
The charge voltage (typically 4.2V for most LiPo cells) is the maximum safe voltage to which a LiPo cell can be charged. It represents the potential at which the battery is considered "full."
2. Why a 7.4V LiPo Battery is Charged to 8.4V:A 7.4V LiPo battery consists of two individual cells connected in series (2S configuration). Each cell has a nominal voltage of 3.7V, and together they sum up to 7.4V. However, when charging, each cell is filled up to its full charge voltage of 4.2V. So, for two cells in series: 2 x 4.2V = 8.4V.
3. Operation From Full Charge to Nominal Voltage:When a LiPo battery is fully charged (to 8.4V for our 2S battery), it contains the maximum energy the battery chemistry allows. As you use the battery, you're essentially extracting this stored energy to power your device. The voltage gradually drops as the energy is consumed.
Mechanism Behind Voltage Drop During Discharge:LiPo batteries, like other lithium-ion chemistries, work on the principle of moving lithium ions between the anode and cathode. When the battery is charged, the lithium ions are primarily at the anode. As the battery discharges, these ions move to the cathode, releasing energy in the process. As more and more ions move to the cathode during discharge, the potential difference (voltage) between the anode and cathode decreases, leading to the observed voltage drop.
To understand how they discharge their energy and why their voltage can drop below the nominal value under specific loads, we must delve into the battery's electrochemistry and operational characteristics.
1. LiPo Discharge Mechanism:
LiPo batteries operate based on the movement of lithium ions between the battery's two electrodes: the anode and the cathode.
2. Why Battery Voltage Drops Below Nominal Under Certain Loads:
The observed voltage of a battery under load (when it's powering a device) versus its open-circuit voltage (no load) can vary, especially under high loads.
In essence, while the nominal voltage provides a generalized midpoint for the battery's operational life, the actual observed voltage can fluctuate based on internal battery characteristics and external demands. It's essential to ensure LiPo batteries aren't subjected to excessive loads that could cause significant voltage drops, as repeatedly doing so can reduce the battery's lifespan or even pose safety risks.
In conclusion, the charging and operational voltages of LiPo batteries are intrinsic to their chemistry and design. Proper understanding and management of these voltages are crucial for ensuring the longevity and safe operation of these batteries.