How does a Lithium Polymer (LiPo) battery work

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.

  • Initial Stage (8.4V to ~3.85V per cell): This phase experiences a relatively quick voltage drop. The battery leaves its peak charged state and starts providing power.
  • Middle Stage (~3.85V to ~3.6V per cell): Here, the voltage decrease slows down. This is the bulk of your battery's operational life, and it hovers around the nominal voltage. Most of the battery's capacity is used in this range.
  • Final Stage (<3.6V per cell): The voltage drop accelerates again as the battery approaches its lower limit. It's essential not to discharge a LiPo battery too much, as going below a specific voltage (often around 3V per cell) can damage the battery or reduce its lifespan. Special circuits, called Battery Management Systems (BMS), are often used to prevent over-discharge.

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.

  • Full Charge: At full charge, the anode holds a concentration of lithium ions. This accumulation results in the battery's voltage peaking (typically 4.2V per cell).
  • Discharge: When connected to a device or load, the battery completes an electrical circuit. Lithium ions migrate from the anode, travel through the external circuit (powering the device), and embed themselves in the cathode. This movement of ions from the anode to the cathode releases energy, which manifests as the electrical power supplied to the device.
  • Nominal Voltage: As the battery discharges and more ions move to the cathode, the voltage starts to drop. The point where the battery operates most of its usable life is termed its "nominal voltage" (commonly 3.7V per cell for LiPo batteries). It's an average or midpoint in the battery's discharge curve.

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.

  • Internal Resistance: Every battery has an intrinsic property known as 'internal resistance,' a result of the battery's internal components and chemistry. This resistance opposes the flow of current. When a load is applied, a portion of the battery's voltage is dropped across this internal resistance, leading to a phenomenon known as "voltage sag."Using Ohm's Law (V=I×R), the voltage drop (ΔV) across the internal resistance (Rinternal​) can be calculated as: ΔV=Iload​×Rinternal​ Where Iload​ is the current drawn by the external load. For high current draws (large Iload​), this voltage drop can be substantial, causing the observed battery voltage to momentarily dip below its nominal value.
  • Chemical Limitations: The rate at which ions can move between the anode and cathode is limited. Under heavy loads, the demand for ions can momentarily outpace the battery's ability to supply them, causing a temporary drop in voltage.
  • Battery State of Charge: As the battery discharges and gets closer to its depleted state, it has fewer ions to supply under heavy loads. This state can exacerbate voltage sag.

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.

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