Cordless String Trimmer Battery Voltage, Ah and Wh: How Buyers Should Compare Platforms

A cordless string trimmer is not just a motor and a shaft. It is a battery system. The voltage, capacity, charger, and platform compatibility affect spare-battery planning, serviceability, and range compatibility — not just how the tool performs on day one.

Voltage, Ah and Wh — What Each Number Tells You (and What It Does Not)

Three numbers appear on every cordless tool battery: voltage (V), amp-hours (Ah), and sometimes watt-hours (Wh). Each measures something different, and none of them tells the full story on its own.

Voltage (V) is the electrical pressure the battery can deliver. In cordless garden tools, common platforms include 18V, 20V, 24V, 36V, 40V, 58V, and 80V. Higher voltage can support higher-power system designs, but actual power output depends on the motor, controller, current limits, battery capability, and load — not voltage alone.

Amp-hours (Ah) is the battery’s capacity — how much current it can deliver over time. A 2.0Ah battery can theoretically deliver 2 amps for one hour, or 4 amps for 30 minutes. But this is a capacity figure, not a runtime prediction.

Watt-hours (Wh) is the total energy stored in the battery. Wh is calculated as nominal voltage × Ah. Using nominal voltage gives a fair basis for comparison across platforms. However, not all suppliers label their batteries with nominal voltage — some use maximum voltage (the voltage at full charge), which is higher. This means a battery labeled “20V Max” may use the same cell configuration as one labeled “18V” — the difference is whether the supplier is quoting the nominal voltage (~18V for a 5-cell series pack) or the maximum charge voltage (~20V for the same pack).

For comparison purposes, buyers should ask the supplier to clarify whether the stated voltage is nominal or maximum, and calculate Wh using nominal voltage:

The first two rows may represent the same cell configuration — a 5-cell series pack with nominal voltage of approximately 18V and maximum charge voltage of approximately 20V. Buyers should not assume that “20V Max” means more energy than “18V” — the actual energy storage depends on the cell configuration and nominal voltage, not the label.

What none of these numbers tell you: runtime. A 40Wh battery does not mean 40 minutes of trimming. Actual runtime depends on the motor efficiency, the load (thick grass vs. light weeds), the operating mode (boost vs. economy), the cutting line diameter, and the tool’s power management system. Two trimmers with the same Wh battery can have very different runtimes.

Nominal vs Maximum Voltage — Why Labels May Not Be Directly Comparable

This is a common source of confusion in cordless tool specifications.

A lithium-ion cell has a nominal (average operating) voltage of approximately 3.6V–3.8V and a maximum charge voltage of approximately 4.2V per cell. When a manufacturer builds a battery pack, they may label it using the nominal voltage or the maximum voltage at full charge.

Different brands may use nominal voltage, maximum voltage, or marketing labels on their battery packs. A supplier listing “18V” and another listing “20V Max” may be describing packs with similar cell configurations, or they may not — buyers cannot assume the cell configuration from the voltage label alone.

What to ask:

• Is the voltage listed nominal or maximum?
• What is the nominal voltage per cell and the maximum charge voltage per cell?
• How many cells are in the pack, and what is the cell configuration (series/parallel)?
• What is the cell chemistry (e.g., Li-ion NMC, LiFePO4, or other)? This is an inquiry item — the buyer should confirm what chemistry the specific model uses rather than assuming a particular chemistry is standard for garden tools.
• What is the exact battery model number?

Battery Energy vs Runtime — Why Wh Is Useful but Not Predictive

Watt-hours give you a fair basis for comparing total energy storage across different voltage platforms. But predicting actual trimming runtime requires more information than any battery label provides.

Factors that affect runtime beyond battery capacity:

• Motor type and efficiency: Brushless motors may improve efficiency depending on the specific motor design, controller, battery, and operating point — the “brushless” label alone does not guarantee longer runtime or higher efficiency.
• Load variation: Trimming light grass along a driveway draws far less current than cutting thick weeds against a fence. A “30-minute runtime” claim is meaningless without specifying the load.
• Operating mode: Some cordless trimmers have a boost or turbo mode that increases power at the cost of significantly reduced runtime. A runtime claim measured in economy mode is not comparable to one measured in boost mode.
• Line diameter and cutting swath: Thicker line and wider cutting swath increase the load on the motor, reducing runtime.

For buyers, this means: do not accept a runtime claim without test conditions. If a supplier says “runs for 45 minutes,” ask: Under what load? What mode? What line diameter? What cutting swath? Was the battery fully charged at the start? If the supplier cannot answer these questions, the runtime number is marketing, not data.

When comparing samples, the most reliable approach is to run the same test on both tools: same battery charge level, same grass type and height, same mode, same line diameter, and same cutting duration. Record what happens.

Battery Platform and Cross-Tool Compatibility — A Sourcing and After-Sales Decision

The battery platform is one of the most commercially significant decisions in cordless tool sourcing — and it is often overlooked in favor of comparing the trimmer itself.

When a supplier’s battery is compatible across multiple tools (trimmer, blower, hedge trimmer, chainsaw), the buyer gains several advantages:

• Spare battery sales: Customers who own two tools from the same platform only need one spare battery, which simplifies inventory.
• Range planning: A single battery platform lets you build a product line where each tool reinforces the others. The customer who buys a trimmer is more likely to buy a blower if the battery fits both.
• After-sales simplicity: One battery SKU to stock for warranty replacement, instead of a different battery for every tool.

Modern battery packs are designed with cross-tool ecosystems in mind. A typical pack includes terminals that connect to the tool’s circuit, a latch mechanism for secure attachment, and in some cases a microprocessor that monitors charge state, temperature, and battery chemistry. Some packs can communicate nominal voltage and chemistry information to the tool, which adjusts performance accordingly.

What to verify:

• Does the supplier’s battery fit other tools in their range? Which specific models?
• Can the customer buy a spare battery separately? What is the MOQ for spare batteries?
• Is the charger compatible with all batteries in the platform, or are there different chargers for different capacities?
• If the supplier changes the battery design in the future, will older tools still be compatible?

Charger, Plug and Transport Documentation — What Buyers Must Confirm

The battery does not ship alone. Buyers need to verify the charger specifications and the documentation required to ship lithium batteries to their destination market.

Charger specifications to confirm:

Lithium battery transport — what buyers need to confirm:

Lithium batteries are regulated as dangerous goods for air, sea, and road transport. The specific requirements depend on how the battery is shipped (standalone, packed with equipment, or contained in equipment), the transport mode, the battery’s Watt-hour rating, and the applicable regulations in force at the time of shipment.

What to confirm with the supplier and forwarder:

• Does the battery type meet UN38.3 test requirements? UN38.3 (UN Manual of Tests and Criteria, Section 38.3) is the transport safety test series referenced in the UN Recommendations on the Transport of Dangerous Goods. Confirm that a test summary is available — this is the document that carriers and authorities may request. A full test report may or may not be provided by the supplier depending on commercial terms; the test summary is the key transport document.
• Is an SDS/MSDS (Safety Data Sheet) available? Some carriers and forwarders may request an SDS as part of their shipping documentation requirements. Whether it is mandatory depends on the transport mode, carrier, and destination — confirm with your forwarder.
• How will the battery be shipped — standalone, packed with the equipment, or contained in the equipment? Each configuration has different packaging, labeling, and UN number requirements.
• What UN number applies to the shipment configuration (e.g., UN3481 for batteries packed with or contained in equipment, UN3480 for batteries shipped separately)?
• What are the specific packaging, labeling, and State of Charge (SoC) requirements for the chosen transport mode and destination? For air freight, these should be confirmed against the current IATA Dangerous Goods Regulations (DGR) in effect at the time of shipment. For sea freight, confirm requirements against the current IMDG Code with your forwarder. Air and sea transport rules differ — do not assume that air freight requirements apply to sea freight.

A supplier who cannot provide the UN38.3 test summary creates a logistics risk — carriers may refuse the shipment. Confirm documentation availability before placing the order, and verify specific transport requirements with the forwarder for each shipment.

Buyer Verification Checklist: Cordless Trimmer Battery Systems

Before committing to a battery platform:

1. Request the full battery specification pack — nominal vs maximum voltage, cell configuration, chemistry, charger specs, and UN38.3 test summary.
2. Compare battery platforms side by side — physical samples let you test runtime, cross-tool compatibility, and charger behavior under the same conditions.
3. Confirm transport documentation — verify that UN38.3 test summary is available and that transport requirements are confirmed with your forwarder for your destination market.

Research Sources Used

• UN Manual of Tests and Criteria, Section 38.3 — Lithium cell/battery type testing and test summary requirements. UNECE. https://unece.org/transport/dangerous-goods/manual-tests-and-criteria
• IATA Dangerous Goods Regulations (DGR) 67th Edition (2026) — Packing Instructions 965–970, including PI966 State of Charge requirements for batteries packed with equipment. https://www.iata.org/en/programs/cargo/dgr/
• IMDG Code (International Maritime Dangerous Goods Code) — Sea transport requirements for lithium batteries. IMO.
• Note: Lithium battery transport regulations are updated periodically. Buyers should verify current requirements with their forwarder at the time of shipment. Wh calculations should use nominal voltage for fair comparison — buyers should confirm with the supplier whether the stated voltage is nominal or maximum.