Lithium Polymer Batteries

Lithium Polymer (LiPo) Batteries: How to Choose the Right Pack

LiPo batteries offer high power-to-weight and flexible shapes for drones, RC models, robotics, wearables, and DIY electronics. This guide explains how to pick the correct voltage, capacity, and discharge rating, match connectors, and use safe charging and storage practices.

Quick Spec Decoder

• Voltage / Cells: 1S=3.7V, 2S=7.4V, 3S=11.1V, 4S=14.8V (nominal). Full charge is 4.2V per cell.
• Capacity: mAh (e.g., 2200 mAh = 2.2 Ah). Higher = longer runtime, more weight/size.
• C-Rating: Discharge capability. Max continuous current ≈ C × Ah. Example: 2.2 Ah × 35C ≈ 77 A.
• Connectors: Main power (e.g., JST-PH 2.0, JST-RCY, XT30, XT60, EC3, Deans/T-plug) + balance lead (usually JST-XH) for 2S+ packs.
• Form Factor: Soft pouch (lightweight), hardcase (more impact protection).
• Protection: Protected single-cell packs include a small BMS; RC/FPV packs are usually unprotected and rely on the device’s ESC/BMS.

How to Choose a LiPo Battery

1) Match Voltage (Cell Count)

• Use the voltage specified by your device or ESC/flight controller. Mismatched voltage can damage electronics.
• Common picks: 1S for wearables/IoT and micro drones, 2S–3S for small RC, 4S–6S for FPV quads and powerful robots.
• Set cutoffs: avoid dropping below ~3.3–3.5 V per cell under light load (3.0 V absolute minimum).

2) Size and Fit

• Check dimensions and weight against your battery bay. Leave 1–2 mm clearance for wires and heat expansion.
• Choose hardcase packs for vehicles with impacts; soft pouches for maximum energy density when protected.

3) Capacity and Expected Runtime

• More capacity = longer runtime but adds mass, which can reduce performance in drones/RC.
• Quick estimate (RC): runtime (hours) ≈ capacity (mAh)/1000 ÷ average current (A). Example: 2200 mAh at 22 A ≈ 0.1 h ≈ 6 minutes.
• Electronics (watts): runtime (hours) ≈ (Voltage × Ah × 0.9 efficiency) ÷ Load (W).

4) Discharge Rating (C)

• Compute needed current: peak A × safety headroom (20–50%).
• Choose a C-rating that supports your continuous draw. Example: Need 40 A continuous on a 2.0 Ah pack → 40/2.0 = 20C minimum; pick 30–45C for margin.
• “Burst C” is short-lived; prioritize continuous rating.

5) Connectors and Wiring

• Match the main connector to your device: micro builds often use JST-PH 2.0 or JST-RCY; mid-power uses XT30; higher power uses XT60/EC3/Deans.
• 2S+ packs include a balance lead (typically JST-XH). Ensure your charger supports the same balance connector.
• Avoid frequent adapter chains; they add resistance and heat.

6) Protected vs. Unprotected Packs

• Protected 1S packs: Best for IoT/portable electronics. They limit current and add over/under-voltage protection. Not suitable for high current RC motors.
• Unprotected RC packs: Deliver high current but require device-level cutoff and careful handling.

Charging, Storage, and Care

Charging

• Use a LiPo-specific CC/CV charger. Balance-charge all multi-cell (2S+) packs.
• Charge rate: typically 0.5C–1C unless the pack explicitly supports faster rates. Approx. time (hours) ≈ (Ah ÷ charge A) × 1.2.
• Temperature: charge 0–45°C (32–113°F); discharge −20–60°C (−4–140°F). Let packs cool before recharging.
• Charge in a LiPo-safe bag on a nonflammable surface. Never leave unattended.

Storage

• Store at 3.75–3.85 V per cell (≈40–60% state of charge) in a cool, dry place (10–25°C / 50–77°F).
• Check voltage every 1–3 months; top up to storage level if needed.
• Retire packs that are puffed, damaged, or show large cell imbalance (>0.05–0.10 V difference at rest).

Safety and Transport

• Do not puncture, crush, or short terminals. If a pack swells, disconnect, isolate, and dispose of properly.
• Air travel (typical rules, verify with your airline): spares in carry-on only; under 100 Wh generally allowed; 100–160 Wh may require airline approval; protect terminals and use individual bags/caps.
• Recycle at e-waste or battery collection points; do not throw in household trash.

When to Choose LiPo vs. Alternatives

• Choose LiPo for maximum power-to-weight and custom shapes: drones, FPV, RC cars/boats/planes, compact robots.
• Choose Li-ion (18650/21700) for higher cycle life and energy per volume in rigid packs: e-bikes, power banks, tools.
• Choose LiFePO4 for improved thermal stability and long cycle life where weight is less critical: UPS, solar storage, starter batteries.

Pros and Cons

• Pros: Excellent power-to-weight, flexible form factors, high current output, slim profiles.
• Cons: Sensitive to over/under-voltage, lower cycle life under high loads, puncture/swelling risk, stricter charging requirements.

Comparison at a Glance

Chemistry	Nominal V/cell	Typical cycle life	Discharge capability	Form factor
LiPo	3.7 V	200–500	High (25C–100C)	Pouch (soft/hardcase)
Li-ion (NMC/NCA)	3.6–3.7 V	300–800	Moderate (5C–15C)	Cylindrical/prismatic
LiFePO4	3.2–3.3 V	1000–3000	Moderate	Cylindrical/prismatic

Use-Case Quick Picks

• Micro drones / whoops: 1S, 300–650 mAh, high C, JST-PH 2.0.
• 5" FPV quads: 4S–6S, 850–1500 mAh, 75–100C, XT60.
• RC cars (1/10): 2S–3S, 4000–6000 mAh, 50–100C, XT60/Deans/EC3.
• DIY electronics / wearables: 1S, 500–4000 mAh, protected pack, JST-PH 2.0.

FAQ

How do I read “3S 2200 mAh 35C”?

3S = three cells in series (≈11.1 V nominal, 12.6 V full). 2200 mAh = 2.2 Ah capacity. 35C ≈ 2.2 Ah × 35 = 77 A max continuous (approx.).

How long will my pack run a 20 A load?

Runtime (hours) ≈ Ah ÷ A. A 2.2 Ah pack at 20 A ≈ 0.11 h ≈ 6–7 minutes. Real-world times vary with throttle, temperature, and efficiency.

Can I parallel or series-connect packs?

Only if they are the same cell count, similar age, capacity, and state of charge. Balance first. Incorrect pairing can cause dangerous currents.

What charger do I need?

A LiPo CC/CV balance charger. Balance-charge 2S+ packs via the balance lead; use the correct cell count and charge rate (0.5C–1C unless specified).

What if my LiPo is swollen?

Stop using it immediately. Isolate in a fire-safe location, discharge safely per local guidance, and recycle. Do not puncture or compress a puffed pack.