4/5 Sub C Batteries

What is a 4/5 Sub‑C battery?

4/5 Sub‑C (often labeled 4/5SC) is a compact, high‑drain cylindrical cell that’s about four‑fifths the length of a standard Sub‑C. It’s commonly used to build rechargeable packs for RC, cordless tools, and repairs where a full Sub‑C won’t fit.

Form factor	Approx. 34 mm (L) × 22.5–23 mm (D); flat‑top common
Nominal voltage	1.2V per cell (NiMH or NiCd)
Typical capacity	NiMH: ~1200–2000 mAh; NiCd: ~1000–1500 mAh
Discharge	High‑drain capable; check datasheet C‑rating/maximum current
Common options	With/without solder tabs; loose cells or pre‑welded sub‑packs

When to choose 4/5 Sub‑C

• Use if your pack bay limits length but needs Sub‑C diameter and high current (e.g., compact RC chassis, stick packs, handheld vacuums).
• Good for rebuilds where the original design used 4/5SC cells or very short Sub‑C cells.
• Avoid if you need maximum runtime in the same diameter; full Sub‑C (≈43 mm length) generally offers higher capacity.

How to choose the right 4/5 Sub‑C

Chemistry

• NiMH: Higher capacity, lower environmental impact, minimal memory effect. Best general choice.
• NiCd: Robust and tolerant of high currents and abuse but lower capacity; contains cadmium—must be recycled properly.

Capacity vs. discharge

• For sustained high current (e.g., RC racing), choose cells with low internal resistance and higher continuous current rating, even if capacity is slightly lower.
• For longer runtime (lights, tools), prioritize higher mAh within your device’s current needs.

Tabs and pack building

• Select pre‑welded solder tabs (nickel) for safer assembly. Avoid direct soldering to bare cell ends if possible.
• Match the series count to your device voltage (e.g., 6 cells ≈ 7.2V, 8 cells ≈ 9.6V, 10 cells ≈ 12V).
• Use insulating rings/caps and heat‑shrink; maintain consistent cell orientation and polarity.

Charger compatibility

• Use a charger specific to NiMH or NiCd with delta‑V detection for fast charge.
• Standard charge: ~0.1C for 12–14 hours; trickle: ~0.03–0.05C. Fast charge only if the cell’s datasheet allows it.

Common uses

• RC packs (cars, boats) where short pack length improves fit and weight distribution.
• Cordless tool and handheld vacuum rebuilds that originally specified 4/5SC.
• Hobby robotics and compact lighting where high current in a short cell is required.

Compatibility notes

• Not a drop‑in for standard Sub‑C if the device relies on the full cell length for physical spacing; confirm fit.
• Do not mix chemistries (NiMH with NiCd) or old and new cells in the same pack.
• Li‑ion cells of similar size exist (≈3.6–3.7V), but they are not voltage‑compatible replacements for NiMH/NiCd packs.

Quick runtime estimate

Runtime (hours) ≈ Pack capacity (Ah) ÷ Average load (A). Example: a 7‑cell NiMH pack built from 1800 mAh (1.8 Ah) 4/5SC cells powering a 10 A RC load ≈ 1.8 ÷ 10 = 0.18 hours ≈ 11 minutes. High‑drain bursts and inefficiencies reduce actual runtime.

Care and safety

• Store around 50–70% charge in a cool, dry place; expect ~10–20% self‑discharge per month (NiMH).
• Avoid deep over‑discharge; recharge when performance drops. NiCd may benefit from occasional full cycles to reduce memory effect.
• Prevent short circuits; transport cells/packs with terminals covered.
• Recycle responsibly—especially NiCd—at an approved collection point.

Pros and cons

Pros

• High current capability in a shorter format.
• Rechargeable and cost‑effective over many cycles.
• Available with solder tabs for easy pack assembly.

Cons

• Lower capacity than full Sub‑C cells.
• Less common than AA/C sizes; may require specialty sources.
• NiCd variants require hazardous‑waste recycling.

FAQ

Can I replace a Sub‑C with a 4/5 Sub‑C?
Only if the shorter length still fits your holder and the pack wiring supports the change. Expect reduced runtime versus full Sub‑C.

Which should I choose: NiMH or NiCd?
NiMH for higher capacity and lower environmental impact; NiCd for harsh use cases that demand extreme robustness and where proper recycling is available.

What charge rate is safe?
Standard: ~0.1C for 12–14 hours. Fast: 0.5–1C with delta‑V and temperature monitoring, if the cell allows it. Always follow the datasheet.

Do I need solder tabs?
Yes for pack building. Pre‑welded tabs make assembly safer and reduce heat damage compared with soldering directly to cell ends.

Why does my pack lose charge in storage?
NiMH self‑discharges over time. If long shelf life is critical, look for low self‑discharge (LSD) NiMH variants or top off before use.