½ D (Half D) Batteries: Choose the Right Fit, Chemistry, and Runtime

½ D batteries are short, D-diameter cells designed for devices that need D-level volume and contact spacing, but with a shorter length. This category guide explains how to select the best option by chemistry, voltage, pulse capability, temperature, and expected runtime.

Size and Fit

Typical dimensions: about 33 mm (1.3 in) diameter and 34–37 mm (1.34–1.46 in) length. Always confirm the holder style and terminal type (button top, flat, or tabs). ½ D is not the same as Sub-D or C size—do not substitute unless the device explicitly supports it.

Quick Comparison by Chemistry

Chemistry	Nominal Voltage	Typical Capacity (½ D)	Rechargeable	Shelf Life (stored, room temp)	Best For	Notes
Lithium Thionyl Chloride (Li‑SOCl2)	3.6 V	~5,000–9,000 mAh	No	Up to ~10–15 years	Long-life, low-average-current sensors, AMR/AMI, memory backup	Excellent cold/heat tolerance (often as wide as −55 to +85 °C). For pulsed loads, choose a "high-pulse" model or add a capacitor.
Lithium Manganese Dioxide (Li‑MnO2)	3.0 V	~2,000–4,500 mAh	No	~5–10 years	Moderate-drain medical and safety devices, alarms	Better pulse handling than some Li‑SOCl2 cells; moderate temperature range (typ. −20 to +60 °C).
Nickel-Metal Hydride (NiMH)	1.2 V	~2,000–5,000 mAh	Yes	~1–3 years (higher self-discharge)	Frequent-use gear, prototyping, robotics (with proper packs)	Rechargeable; prefers 0–45 °C charging. Lower voltage may affect devices expecting primaries.
Alkaline (rare in ½ D)	1.5 V	~3,000–6,000 mAh	No	~3–5 years	Budget, non-critical, low-drain applications	Limited availability; performance drops in cold and under high loads.

When to Choose Each Type

• Choose Li‑SOCl2 (3.6 V) if your device is a low-average-current sensor or meter that must run for many years without service, or if it faces extreme temperatures. Avoid if the device demands frequent high-current pulses without a buffer capacitor.
• Choose Li‑MnO2 (3.0 V) for moderate-drain devices that need good pulse performance and long shelf life, such as medical and safety equipment. Avoid for ultra-long (>10 years) maintenance-free deployments in very cold or hot environments.
• Choose NiMH (1.2 V, rechargeable) when you can recharge regularly and need higher current delivery or rapid prototyping. Avoid if the device expects a higher primary voltage or must sit unused for long periods.
• Consider Alkaline (1.5 V) only for low-stakes, low-drain uses where cost matters more than lifespan or temperature performance.

Runtime Planning (Simple Estimates)

Estimate runtime by dividing capacity (Ah) by average load (A), then derate for pulse peaks, temperature, and aging.

• Low-power sensor: 60 μA average on a 7 Ah Li‑SOCl2 cell ≈ 7 Ah / 0.00006 A ≈ 116,000 h (~13 years). Plan for 8–10 years after derating.
• Moderate drain device: 150 mA average on a 3.5 Ah Li‑MnO2 ≈ 3.5 Ah / 0.15 A ≈ 23 h total active time.
• Rechargeable use: 500 mA draw on a 4 Ah NiMH ≈ 8 h per charge (real-world 5–7 h after efficiency and voltage sag).

Always check the device’s minimum voltage and peak current needs; primary lithium cells may require a pulse-support capacitor for radios, alarms, or telemetry bursts.

Buying Checklist

1. Voltage match: 3.6 V (Li‑SOCl2), 3.0 V (Li‑MnO2), 1.5 V (alkaline), or 1.2 V (NiMH). Never substitute a different voltage unless the device manual allows it.
2. Load profile: Continuous current and pulse peaks (mA/A). Choose high-pulse variants or add a capacitor if needed.
3. Environment: Minimum/maximum operating temperature and storage temperature.
4. Terminals and fit: Button vs. flat top, spot-welded tabs, and holder style. Verify ½ D, not D or Sub‑D.
5. Compliance and brand: Look for datasheets with safety and compliance marks suitable for your application.

Storage, Handling, and Safety

• Store cool and dry (around 10–25 °C / 50–77 °F). Avoid heat and direct sun.
• Do not charge primary lithium or alkaline batteries. Use only appropriate chargers for NiMH.
• Cover terminals or keep cells in a case to prevent short circuits.
• For air travel, most spare lithium metal batteries must be in carry-on baggage only; check airline and TSA rules. Tape terminals or use protective cases.
• Recycle responsibly at battery collection points; follow local regulations.

FAQs

Can I replace a 3.6 V ½ D with a 3.0 V or 1.2 V cell?

Generally no. Many devices use the higher voltage for correct operation or accurate low-battery detection. Match the specified chemistry and voltage in the manual or datasheet.

Is ½ D the same as Sub‑D?

No. Both share a similar diameter, but Sub‑D is typically longer. Dimensions and terminals differ—use only the size your holder supports.

Why won’t my radio or alarm work well with a Li‑SOCl2 cell?

Some Li‑SOCl2 cells have higher internal resistance and struggle with large pulse currents. Use a pulse-rated version or add a parallel capacitor (per device guidance) to handle transmission bursts.

How long can a ½ D battery sit on a shelf?

Li‑SOCl2 can retain usable capacity for up to ~10–15 years; Li‑MnO2 typically ~5–10 years; NiMH self-discharges faster (often months to a couple of years depending on cell design). Actual life depends on storage temperature and brand.

Can I use a spacer or adapter to fit a different size?

Only if the device manufacturer approves it. Adapters may correct length but not voltage or current capability, and poor contact can cause heat or resets.