Cordless tool batteries look like simple black boxes, but the way lithium-ion cells are arranged inside determines runtime, weight, cost, and how hard a pack can work before it overheats. Most packs are built from cylindrical cells wired together in series and parallel configurations, and those two words explain why a 4Ah battery weighs more than a 2Ah pack, why some batteries fit the same tool but feel different in hand, and why spec sheets list codes like 5S2P. Learning to read that code makes comparing packs across brands straightforward. The same knowledge helps when you compare the value of a cordless tool combo kit, because the battery platform, not the tools alone, is often what locks you into a brand.
Series and Parallel Wiring: The Two Rules of Battery Packs
Every lithium-ion cell used in a cordless tool produces a nominal voltage of about 3.6 volts. A single cell cannot drive a motor, so manufacturers wire cells together in two ways. Cells connected in series, written with an S, link head to tail and combine their voltages. Cells connected in parallel, written with a P, combine their charge capacity instead. A pack that carries a 5S2P label contains ten cells arranged as five in series, with two of those series strings wired side by side.
Reading a Cell Configuration Code
The code is a three-part math problem.
- Find the number before the S and multiply it by 3.6V to get nominal voltage. A 5S pack delivers 5 x 3.6V, which is 18V.
- Find the number before the P and multiply per-cell capacity by it to get pack capacity. Two strings of 2Ah cells in parallel give 2Ah x 2, which is 4Ah.
- Multiply the two numbers to count total cells. A 5S2P pack holds 5 x 2, or 10 cells.
Why 18 Volts Is the Magic Number
The 18V figure appears because 5S is the most common series count in compact cordless tools. Five cells at 3.6V each produce 18V nominal, enough to run the motors in drills, drivers, and saws while keeping the pack small enough to hold in one hand. The same 5S math applies to the 18V tools sold outside North America, which use identical packs under a different label.
Wiring explains performance, but day-to-day habits decide how long a pack stays useful. How you store, charge, and discharge a battery shapes its lifespan, and common beliefs about conditioning and draining packs often do harm. The truth about cordless power tool battery care is simpler than the myths: lithium-ion cells prefer partial discharges and cool storage, and a pack charged when it reaches about 20 to 30 percent remaining capacity outlasts one that is run flat before every charge.
Read together, the three numbers give a quick profile of any pack:
- Voltage, the S number, sets tool compatibility and peak power output.
- Capacity, the P number combined with the cell rating, sets runtime.
- Total cell count sets weight, charge time, and price.
Why 20V Max Is Really 18 Volts
Battery labels do not always match the math. Many North American tools carry a 20V Max badge while the identical tools sold in Europe, Asia, and Australia are labeled 18V, and the packs are the same: five cells in series at 3.6V each. The 20V figure comes from the maximum voltage of a freshly charged cell, which reaches about 4.2V per cell before settling, so 5 x 4.2V equals about 21V at the absolute peak. Marketing rounds that number down to 20V Max, while the nominal 18V rating describes real working voltage.
Nominal Voltage vs. Maximum Charge
Nominal voltage is the average a pack holds during discharge, and it is the number that matters for compatibility. Maximum charge voltage is a brief peak that appears only right after charging finishes. If a tool is rated for an 18V nominal pack, the 20V Max label on the same chemistry does not change what fits in the battery port. Voltage ratings still matter for performance: the higher the pack voltage, the more power a motor can draw, which is why 20V and 60V platforms behave very differently on the same class of tool.
Batteries also power more than tools. A portable power station can convert corded tools to battery power by feeding a 120V outlet from a pack, which extends the platform to lights, radios, and tools that have no cordless version yet. The same cells that spin a drill motor can run a jobsite table saw, a reminder that the battery is a small power plant.
Cell Generations: 18650, 21700, Tabless, and Pouch
Inside the plastic shell, packs from different eras use different cells. First-generation packs use 18650 cells, named for their dimensions: 18mm in diameter and 65mm long. Second-generation packs use 21700 cells, 21mm by 70mm, which hold more energy and deliver higher current in the same voltage configuration. Third-generation cells come in two forms: 21700 cells with tabless electrodes, which shorten the current path inside the cell, and pouch cells, which trade the steel cylinder for a flat, flexible package with more capacity in a slim profile.
What the Numbers Mean
Cell names are metric dimensions. An 18650 cell is 18mm across and 65mm tall, and a 21700 cell is 21mm across and 70mm tall. The larger diameter of the 21700 increases both energy capacity and discharge capability, which is why a second-generation 3Ah pack can replace a first-generation 3Ah pack with fewer cells, or deliver more current from the same cell count.
Cell changes are one engine behind how cordless power tool platforms evolve. When a manufacturer moves a platform to a new cell format, voltage ratings stay the same but capacity, runtime, and weight all shift, and older tools often gain runtime when fitted with newer packs. That backward compatibility is why cell upgrades reach every tool you already own.
Tabless and Pouch Trade-offs
Tabless cells reduce internal resistance, so they run cooler under load and tolerate more charge cycles. Pouch cells allow flat pack designs that fit under a tool grip or in slim slide packs, but they need more protection in the housing because the soft wrapper is easier to damage than a steel can. Neither type changes the 5S wiring math; both just change how much energy fits in a given space.
Capacity, Runtime, and Watt-Hours
Capacity is rated in amp-hours, or Ah, which describes how much current a pack can supply for one hour. A 5Ah pack can deliver 5 amps for one hour, or 10 amps for 30 minutes. On the same platform, runtime scales simply: double the amp-hours and you roughly double runtime. Comparisons across platforms need a second number, because energy is voltage times capacity, which is why watt-hours, or Wh, is the honest cross-platform unit. A 20V pack at 5Ah holds 100Wh of energy, while a 60V pack at 6Ah holds 360Wh, and that difference explains much of the price gap.
Watt-hours also keeps comparisons straight when battery systems evolve across voltage transitions. A 100Wh pack is a 100Wh pack whether it is labeled 18V, 20V Max, or 60V Max, so the energy unit survives every marketing change.
Comparing Common Pack Sizes
| Pack label | Cell type | Configuration | Total cells | Capacity in 20V mode |
|---|---|---|---|---|
| 1.5Ah entry-level | 18650 | 5S1P | 5 | 1.5Ah |
| 2Ah | 18650 | 5S1P | 5 | 2Ah |
| 3Ah first generation | 18650 | 5S2P | 10 | 3Ah |
| 3Ah second generation | 21700 | 5S1P | 5 | 3Ah |
| 4Ah | 18650 | 5S2P | 10 | 4Ah |
| 5Ah | 18650 | 5S2P | 10 | 5Ah |
| 6Ah FlexVolt | 18650 | 5S3P | 15 | 6Ah |
| 15Ah FlexVolt | 18650 | 5S6P | 30 | 15Ah |
The table shows two patterns. First, a second-generation 3Ah pack reaches the same capacity with half the cells of the first-generation version. Second, FlexVolt packs rearrange the same 15 cells between modes: in 20V mode the 6Ah pack runs as 5S3P, and in 60V mode it switches to 15S1P, so one battery feeds tools at two voltages.
Matching Battery Size to the Job
Pack choice is a trade between runtime and weight. A 5Ah pack on a drill adds noticeable weight to every hole you drive, while a 1.5Ah pack keeps the tool light but dies halfway through a task. Contractors run mid-size packs on daily drivers and save big packs for high-draw tools, and a homeowner can use the same logic.
Battery systems that power modern construction work are sized by task, and crews carry two or three sizes instead of one. The pattern holds everywhere: small packs for drills and drivers, medium packs for saws and grinders, large packs for anything that runs continuously.
A practical sizing rule of thumb:
- 1.5 to 2Ah: light drilling, trimming, quick screw driving.
- 3 to 4Ah: all-day drilling and driving on a balanced tool.
- 5 to 6Ah: circular saws, grinders, and other high-draw tools.
- 8Ah and up: stationary use, demolition tools, and extended run jobs.
Entry-Level vs. High-Capacity Packs
Entry-level packs keep kit prices down and suit occasional users, but they strain under continuous load and charge more often. High-capacity packs cost more up front and deliver lower cost per watt-hour over their life because bigger cells run at lower stress. A heavy user who buys two 5Ah packs instead of four 2Ah packs usually spends less over two years.
Buying, Storing, and Getting the Most From Packs
Battery evolution has made packs cheaper per watt-hour with every cell generation, but the buying rules stay the same. Buy the smallest pack that finishes the job without frequent swaps, keep a spare charged for high-draw tools, and store packs at partial charge in a cool place. Heat is the main enemy: leaving a pack in a hot truck bed or on a sunny windowsill accelerates capacity loss faster than any charging habit.
Cordless power tool battery evolution keeps pushing capacity up and weight down, and the practical payoff is that a single platform can now handle everything from a trim screwdriver to a demolition hammer. When you standardize on one platform, every pack you own works in every tool, so the next battery you buy should be the size that fills the gap in your rotation, not the one with the biggest number on the label.
Storage and Rotation Checklist
- Store packs between 30 and 80 percent charge for long idle periods.
- Keep packs out of direct sun and away from heaters.
- Rotate packs so no single unit takes every job.
- Charge on the charger sold for the platform; generic chargers may skip cell balancing.
With the wiring rules, voltage story, and cell generations in mind, reading a battery spec sheet takes seconds: check the S count for compatibility, the Ah for runtime, and the cell type for generation. The pack that fits your workflow is the one that balances those three numbers.