LiPo & LiHV batteries

Lithium-polymer is the default combat chemistry: high power density, high discharge rate, cheap. Li-ion (18650/21700) appears where energy density matters more than peak current; LiFePO₄ is rare in insect classes.

LiPo vs LiHV

Most current beetleweight packs are LiHV (high-voltage lithium-polymer), not standard LiPo. LiHV cells charge to a higher voltage — 4.35 V/cell vs 4.20 V — and sit at 3.8 V nominal, so a LiHV pack delivers more energy and power for the same size and is generally preferred.¹

Set the correct chemistry on your charger. Charging a LiPo pack on the LiHV setting overcharges it and can start a fire; charging a LiHV pack on the LiPo setting just leaves it partly full.¹

Nomenclature

• S — cells in series. Sets voltage. LiPo: V_full = 4.2 × S. LiHV: V_full = 4.35 × S. A 4S LiHV pack is 15.2 V nominal, 17.4 V full.
• P — cells in parallel. Multiplies capacity and current. Rare in insect-size packs.
• Capacity (mAh / Ah) — charge stored. An 850 mAh pack delivers 0.85 A for one hour, or ~51 A for one minute.
• C-rating — claimed max discharge as a multiple of capacity. I_max = C × capacity. Manufacturers heavily overstate this; it is only useful for comparing packs from the same maker.¹
• Watt-hours (Wh) — total energy, V_nominal × Ah. The right number when comparing packs across voltages: swapping to a higher S at the same Wh (and a proportionally lower motor Kv) gives the same power at lower current.

Sizing

1. Voltage (S): beetleweights run 3S–6S. Higher S = lower current for the same power = thinner wire and less sag, at the cost of cell mass and higher-voltage electronics. 4S is the common baseline.¹
2. Peak current: I_peak ≈ weapon_spinup_current + drive_stall_current. Weapon spin-up dominates; drive draw is small by comparison.
3. C-rating: pick a pack whose real sustained capability (well below the label) exceeds I_peak. A pack run near its rating sags hard and heats up.
4. Capacity for endurance: size so you use ≤ 70–80% of the pack over a 3-minute match. Deep-cycling every match kills packs fast.²³
5. Mass: energy density is ~150–180 Wh/kg. Beetleweight packs are commonly 8–12% of robot mass — see Weight budgeting.

Typical picks (beetleweight)

Robot	Pack
Wedge / lifter / grabber (no spinner)	3S–4S, ≥450 mAh, C-rating not critical
Spinner — baseline	4S 850 mAh
Big horizontal, beater bar, or unusually fast weapon	4S 1100 mAh, or higher S at equal Wh

GNB / Gaoneng LiHV packs above ~60C are the common go-to for price and availability; the plain 60C (“purple”) packs can’t feed most spinners. Tattu, Palm Beach Bots, BetaFPV Lava, and Bonka are other known-good brands.¹

Sag

Under load, terminal voltage drops (internal resistance × current). Symptoms of too much sag: slow weapon spin-up, ESC low-voltage cutoff mid-match, brownout resets of the receiver. Fixes: higher C-rating, higher capacity, higher S (lower current), shorter/thicker wiring, better connectors.

Health and retirement

Retire a pack when it puffs (“spicy pillow”), a cell drifts >0.05 V from its siblings at rest, internal resistance climbs, or it can’t hold full charge. See Battery safety & charging.

References

Footnotes

1. Ping, L., with L. Skotiniotis, I. Talbert & D. Tran. Beetleweights for Dummies (Fall 2025 revision) — builder-maintained beginner guide distributed via the NHRL Discord. Sections: Electronics & Wiring → Batteries; Testing & Safety → LiPo Battery Safety. ↩ ↩² ↩³ ↩⁴ ↩⁵

2. Team Run Amok — the Tentacle Torque & Amp-Hour Calculator estimates match current draw and required pack capacity. ↩

3. Battery University (Cadex), BU-808: How to Prolong Lithium-Based Batteries — charge/discharge limits, depth-of-discharge, and cycle life. ↩