LiPo cell structure and principles
A lithium polymer (LiPo) cell consists of an anode (graphite), a cathode (lithium oxide) and a polymer electrolyte. The nominal voltage of a single cell is 3.7 V, the fully charged voltage is 4.2 V, and the minimum safe discharge voltage is 3.0 V (although in practice you should not go below 3.3 to 3.5 V under load).
LiPo packs connect cells in series, denoted by the symbol S. A 3S pack is three cells in series giving 11.1 V nominal (12.6 V charged). A 4S pack is 14.8 V nominal (16.8 V charged). A parallel cell arrangement (denoted P) increases capacity. For example, 3S2P is six cells, three in series and two in parallel.
C-rate and discharge current
The C-rate specifies the maximum discharge current relative to the pack's capacity. If a pack has a capacity of 2200 mAh and a rated C-rate of 25C, the maximum continuous discharge current is 2.2 A x 25 = 55 A.
Manufacturers usually list two parameters: continuous C-rate and burst C-rate. Continuous is the current the pack can deliver for an extended period without overheating. Burst (for example 50C) is the current allowed for short pulses (a few seconds). Use the continuous value when sizing the power system.
Real C-rates are often overstated by manufacturers. A practical rule is to assume the actual continuous C-rate is 30% to 50% lower than advertised. A pack labeled 30C may in reality safely deliver current corresponding to 15C to 20C. Symptoms of overloading include a sharp voltage drop under load and heavy heating of the pack.
Balance charging
Balance charging is the only safe method for charging multi-cell packs. The charger monitors each cell's voltage independently and equalizes them so that all cells reach 4.2 V at the end of the charge.
Without balancing, cells can drift apart in voltage. One cell may reach 4.3 V (dangerous overcharge) while another is only at 4.1 V. Overcharging a LiPo cell above 4.25 V risks swelling, electrolyte leakage and fire.
The standard charge current is 1C (for a 2200 mAh pack that is 2.2 A). Fast charging at 2C is acceptable for packs rated for it by the manufacturer, but it shortens cell life. Never exceed the maximum charge current stated by the manufacturer.
Storage voltage
LiPo packs should not be stored either fully charged (4.2 V per cell) or fully discharged (3.0 V per cell). The optimal storage voltage is 3.80 to 3.85 V per cell, corresponding to roughly 40% to 50% capacity.
Most modern chargers have a storage mode that automatically charges or discharges the pack to the storage voltage. Storing a fully charged pack for an extended period (weeks) accelerates cell degradation and increases the risk of swelling.
Store packs in a dry, cool place (15 to 25 degrees Celsius). Avoid temperatures above 40 degrees (for example inside a car in summer). High temperatures speed up the chemical aging of the cells.
Fire safety
LiPo cells contain a flammable electrolyte. Mechanical damage (puncture, crush), overcharging, deep discharge or a short circuit can trigger a thermal runaway, an uncontrolled thermal reaction that ends in fire.
Always charge packs in a fireproof LiPo bag (LiPo Safe bag) or on a non-flammable surface (ceramic, metal). Never charge packs unattended. Never charge on a carpet, a wooden table or near flammable materials.
Keep a dry powder or sand extinguisher within reach. Water is ineffective on LiPo fires (and may make things worse). A burning LiPo pack releases toxic fumes, so in case of fire ventilate the room or carry the burning pack outside using metal tongs.
Signs of pack damage
A swollen (puffed) pack is a sign that gas is being generated inside the cell from electrolyte decomposition. A swollen pack must not be used or charged any further. Even slight puffing is a warning sign.
Other damage symptoms include: a sharp voltage drop under load (internal resistance increase), uneven cell voltages after charging (a difference above 0.05 V despite balancing), physical casing damage (dents, cuts) and heat generated during storage.
Parallel charging risks
Parallel charging means charging several packs at the same time through a parallel charging board connected to a single charger. It saves time but carries significant risks.
Before connecting packs for parallel charging, verify that all packs have the same cell count (for example all 3S), similar capacity and similar voltage (the voltage difference between packs should not exceed 0.1 V per cell). Connecting a pack at a significantly different voltage causes a large equalization current rush between packs, which risks overheating and fire.
Parallel charging does not replace balancing. The parallel board should have fuses on each port. If you are not experienced with LiPo charging, charge packs one at a time. It is slower but much safer.
Disposing of used packs
Used LiPo packs must not be thrown into regular waste. They contain heavy metals and flammable electrolyte. Before disposal, discharge the pack to a safe voltage (below 3.0 V per cell) using a charger's built-in discharge function or a 12 V car bulb.
Take discharged packs to a battery recycling collection point. In Poland these are available at electronics stores, hardware stores and municipal selective waste collection points (PSZOK). Never throw LiPo packs into fire, as this risks explosion.