How to choose cordless tool batteries — voltage versus amp-hours, why platform choice matters more than any single tool, and how to make batteries last.
Buying your first cordless tool feels like buying a tool. It is not. You are choosing a battery platform, and you will probably live with that decision for the next ten years.
Every subsequent drill, saw, sander and blower you buy will be steered by which batteries are already on your shelf. Switching later means replacing not just tools but the entire battery and charger collection, which is why so many people quietly own three incompatible systems and a drawer of dead chargers.
The specifications do not help. Voltage is quoted two different ways for the same battery. Amp-hours get described as “power” when they are nothing of the sort. And “brushless” appears on everything without much explanation of why it matters.
This guide untangles all of it. 💡
Alt text: “Row of cordless power tool batteries in different sizes next to a charger on a workshop shelf”
- Platform is the real decision
- 18V vs 20V MAX — the same battery
- Amp-hours: runtime, not power
- Which size battery for which tool
- Brushless motors and why they matter
- Bare tools, kits and what to actually buy
- Third-party batteries
- Making batteries last
- Charging and storage safety
- Common mistakes (and how to fix them)
- Pro tips worth knowing
- What I recommend
- Frequently asked questions
- Your final checklist
Platform Is the Real Decision
Each manufacturer’s batteries fit only that manufacturer’s tools. There is no universal standard, and the physical connection differs deliberately between brands.
What that means in practice: the first cordless tool you buy sets the platform, and every tool after it is cheaper if you stay put. Once you own three or four batteries, you can buy “bare tools” — the tool alone, no battery, no charger — for substantially less than the kit price.
| Question to ask | Why it matters |
|---|---|
| How wide is the tool range? | ✅ A broad range means you never have to switch |
| Are bare tools widely available? | ✅ This is where the savings come from |
| Is the platform still current? | ⚠️ Discontinued platforms leave you stranded |
| Are replacement batteries easy to buy? | Batteries wear out before tools do |
| Do garden tools share it? | One system for drill, blower and trimmer |
| What do the batteries cost? | ⚠️ Often the biggest long-term expense |
The tool you are holding in the shop is a ten-year commitment to a battery shape. Choose the platform first and the tool second — it is the opposite of how almost everyone shops, and it is the reason so many garages contain three incompatible chargers.
Practical advice: pick a mainstream platform with a wide tool range and readily available bare tools, and stay on it. The differences between the major brands at the DIY level are far smaller than the cost of owning two systems.
18V vs 20V MAX — The Same Battery
This is the single most confusing thing in cordless tools, and it is entirely a marketing artefact.
A lithium-ion cell has a nominal voltage of about 3.6V and a peak, no-load voltage of about 4.0V when freshly charged. A pack with five cells in series can therefore be described two ways:
- 5 × 3.6V = 18V nominal — the working voltage under load
- 5 × 4.0V = 20V maximum — the peak voltage of a fresh pack at rest
These are the same battery. A “20V MAX” pack and an “18V” pack of the same cell count are electrically equivalent. Some brands quote nominal, some quote maximum, and it makes their marketing numbers look different when the hardware is not.
Where voltage genuinely does differ:
| Class | Also sold as | Typical use |
|---|---|---|
| 12V | 12V MAX | ✅ Compact drills, drivers, tight spaces |
| 18V | 20V MAX | ✅ The mainstream — most tools |
| 36V | 40V MAX | Garden tools, larger saws |
| 54V / 60V | Various | Heavy-duty and pro applications |
12V systems are genuinely underrated for home use. Much lighter, far less bulky, and completely adequate for assembling furniture, fitting shelves and general household jobs. If you are not drilling masonry or cutting timber all day, a 12V drill is more pleasant to use than an 18V one. Our guide to using a cordless drill covers the practical differences.
Amp-Hours: Runtime, Not Power
The second number on the label — 2.0Ah, 4.0Ah, 5.0Ah — is amp-hours, and it is widely misunderstood.
Amp-hours measure capacity, which translates to runtime. A 4.0Ah battery holds roughly twice the energy of a 2.0Ah battery of the same voltage, so it runs roughly twice as long. It does not make the tool twice as strong.
The nuance: larger packs often contain more cells, and more cells can deliver current more comfortably. On very demanding tools — a circular saw ripping thick timber, a large angle grinder — a bigger pack may sustain performance better under heavy load because it is less stressed. But this is a secondary effect. Amp-hours is fundamentally about how long, not how hard.
| Capacity | Weight | Runtime | Best for |
|---|---|---|---|
| 1.5–2.0Ah | ✅ Lightest | Short | Drills, drivers, overhead work |
| 3.0–4.0Ah | ✅ Balanced | Good | ✅ The all-rounder |
| 5.0–6.0Ah | Heavy | Long | Saws, grinders, sustained work |
| 8.0Ah+ | ⚠️ Very heavy | ✅ Longest | Garden tools, all-day use |
The weight point is real and underappreciated. A large-capacity pack adds noticeable weight to the bottom of a drill. For overhead work — fixing something to a ceiling, working up a ladder — a compact battery is genuinely more comfortable, and the shorter runtime rarely matters because you cannot hold your arms up that long anyway.
Which Size Battery for Which Tool
Different tools draw current very differently, and matching the pack to the tool makes a noticeable difference to both comfort and runtime.
Low draw — compact battery is fine
Drills, impact drivers, screwdrivers, work lights, small detail sanders. These sip current. A 2.0Ah pack will drive a great many screws. Light weight is worth more than runtime here.
Moderate draw — mid-size battery
Jigsaws, oscillating tools, random orbital sanders, reciprocating saws on light work. A 4.0Ah pack is the sweet spot — reasonable weight, decent runtime.
High draw — large battery
Circular saws, angle grinders, hammer drills in masonry, planers. These pull hard and drain a small pack quickly. A 5.0Ah or larger pack runs longer and is less stressed doing it.
Very high draw and long sessions — largest available
Leaf blowers, hedge trimmers, mowers, chainsaws. Garden tools run continuously rather than in bursts, and they empty batteries fast. This is where 8.0Ah and above earn their weight — and where a spare pack matters more than a bigger one.
Brushless Motors and Why They Matter
“Brushless” appears on almost every mid-range tool now, usually with no explanation.
A brushed motor uses carbon brushes pressing against a rotating commutator to switch the current. It works, it is cheap, and the brushes wear out with friction and heat.
A brushless motor uses electronic switching instead. No brushes touching, so less friction, less heat and less wasted energy.
| Brushed | Brushless | |
|---|---|---|
| Efficiency | Lower | ✅ Higher — more runtime per charge |
| Heat | More | ✅ Less |
| Lifespan | ⚠️ Brushes wear | ✅ Longer |
| Size and weight | Larger for same output | ✅ More compact |
| Price | ✅ Cheaper | More expensive |
| Worth it for | Occasional light use | ✅ Regular use, high-draw tools |
The honest assessment: brushless is genuinely better, and the runtime gain per charge is real. But for someone who uses a drill a few times a year to hang pictures, a brushed tool at a lower price is a perfectly rational choice. Brushless matters most on high-draw tools and on anything you use regularly — that is where the efficiency and durability actually pay back.
Bare Tools, Kits and What to Actually Buy
Cordless tools sell in three forms, and knowing which to buy saves a substantial amount of money.
| Form | Includes | Buy when |
|---|---|---|
| Bare tool | Tool only | ✅ You already own batteries |
| Kit | Tool + battery + charger | ✅ Your first tool on a platform |
| Combo kit | 2+ tools, batteries, charger, bag | ✅ Starting from nothing |
The sequence that costs least:
- Start with a combo kit containing the two tools you will use most — usually a drill and an impact driver — plus two batteries and a charger. Combo kits are almost always better value than the same items bought separately.
- Add bare tools from then on. Every subsequent tool costs meaningfully less because you are not re-buying a battery and charger you already have.
- Buy extra batteries only when runtime actually limits you, not pre-emptively. Batteries age whether you use them or not.
Third-Party Batteries
Non-branded replacement batteries sell for a fraction of the original manufacturer’s price, and the temptation is obvious. The honest position is that this is a real trade-off, not a simple “never.”
The genuine risks:
- Cell quality varies enormously. The cells inside are where the cost is saved, and capacity claims are frequently optimistic.
- Protection circuitry may be inferior. This is the part that prevents over-charge, over-discharge and thermal runaway. It is not a good place to economise.
- Warranty. Using a third-party battery may affect the tool’s warranty.
- Fit and communication. Some tools and chargers communicate with the pack; a generic pack may not work correctly or may charge poorly.
The reasonable middle ground: for an out-of-warranty tool you use occasionally, a well-reviewed third-party pack from a seller with a real returns policy is a defensible choice. For a tool under warranty, a high-draw tool, or anything you rely on, buy the manufacturer’s battery. And whatever you buy, charge it where a fault would not be catastrophic — not overnight in a hallway, not next to anything flammable.
Making Batteries Last
Lithium-ion packs degrade over time and with use. You cannot stop it, but the difference between careful and careless treatment is substantial.
Avoid extreme heat
Heat is the main enemy. A battery left on a dashboard in summer, or in a hot loft, ages far faster than one kept at room temperature. Never charge a pack that is hot from heavy use — let it cool first.
Avoid deep discharge
Running a pack completely flat repeatedly is harder on it than topping up. Modern tools cut out before real damage, but the habit of running to zero every time is not doing you favours.
Store partly charged for long periods
For extended storage — over winter, say — a partial charge is generally kinder than either full or empty. Around half to two-thirds is a common recommendation. Check every few months and top up if it has drifted low.
Do not store on the charger
Modern chargers stop when full, but leaving packs permanently docked in a warm garage is not the best long-term storage.
Keep the contacts clean
Dust and sawdust on the terminals cause poor connections, which cause heat. A wipe occasionally is enough.
Bring them indoors in winter
Cold batteries deliver less power and charge poorly. Most chargers refuse to charge a very cold pack at all, which is the correct behaviour — charging a frozen lithium cell damages it. Let a cold pack warm to room temperature first.
Heat kills batteries faster than use does. A pack that lives in a hot van or a sunlit shed will lose capacity in a couple of seasons; the same pack kept indoors at room temperature can still be working well many years later.
Charging and Storage Safety
Lithium-ion batteries store a lot of energy in a small space. Failures are uncommon, but when they happen they are serious, and a few simple habits remove most of the risk.
Warning signs to take seriously:
- Swelling or a bulging case — stop using it immediately
- Unusual heat during charging or use
- A smell, particularly sweet or chemical
- Physical damage after a hard drop
- Sudden capacity collapse — runtime that falls off a cliff
Disposal: never put lithium batteries in household waste or general recycling. They cause fires in bin lorries and at sorting facilities, and this happens regularly. Take them to a proper battery recycling point — most tool retailers and household waste sites accept them free of charge.
Common Mistakes (And How to Fix Them)
Mistake 1 — Buying tools before choosing a platform
The fix: decide on the battery system first, then buy tools within it. This one decision saves more money than any other.
Mistake 2 — Paying extra for “20V MAX” over “18V”
The fix: understand they are the same pack described differently. Compare amp-hours, motor type and tool specifications instead.
Mistake 3 — Assuming more amp-hours means more power
The fix: amp-hours is runtime. For more power, look at the tool’s motor and torque figures.
Mistake 4 — Buying two identical mid-size batteries
The fix: one compact and one large covers far more situations than two of the same.
Mistake 5 — Storing batteries in a hot loft or van
The fix: room temperature indoors. Heat is what actually kills them.
Mistake 6 — Charging a pack straight after heavy use
The fix: let it cool to room temperature first. Charging hot cells accelerates ageing.
Mistake 7 — Storing fully charged all winter
The fix: around half to two-thirds charge for long storage, checked occasionally.
Mistake 8 — Charging on a sawdust-covered bench
The fix: hard non-combustible surface, clear space, not unattended overnight.
Mistake 9 — Still using a swollen battery
The fix: stop immediately and recycle it properly. A bulging pack is a genuine hazard.
Mistake 10 — Binning old batteries
The fix: take them to a battery recycling point. They start fires in waste facilities.
Pro Tips Worth Knowing
- Label your batteries with the purchase date. When runtime drops, you will want to know whether a pack is two years old or seven.
- A second battery beats a bigger battery for garden tools — one charges while the other works.
- Bare tools go on offer more often than kits. Once you have batteries, watch for them.
- A fast charger transforms a two-battery setup. Often a better upgrade than a third pack.
- Check whether garden tools share your platform before buying a blower or trimmer on a new system.
- Keep one battery permanently in the drill. The tool you can grab and use is the tool you actually use.
- Store packs off the metal bench. Loose metal across terminals is a short circuit.
- Wall-mounted storage keeps packs visible and off the floor — dropped batteries are damaged batteries.
- Rotate your batteries. Using the same pack constantly while another sits idle wears one out and lets the other age unused.
- Do not buy a fourth battery to fix a runtime problem that is really a “the tool is wrong for the job” problem.
What I Recommend
Once you own several batteries, tools and chargers, the practical problem changes. It stops being “which battery” and becomes “where is the charged one” — and packs left loose on a bench get dropped, buried and shorted against metal.
YYR Heavy Duty Power Tool Organizer, Wall Mount
🏆 Best For Battery Storage★★★★☆ 4.7 out of 5 on Amazon
A wall-mounted rack that gets tools and batteries off the bench and into view. Once you are running a platform with several packs, keeping them mounted, visible and off the floor solves the two real problems — knowing which pack is charged, and stopping them being dropped or shorted.
- ✅ Everything visible at a glance — no digging through a drawer for a charged pack
- ✅ Batteries off the floor, which is where drops and damage happen
- ✅ Terminals kept clear of loose metal on the bench
- ✅ Frees up bench space in a small workshop where space is the constraint
- ✅ Heavy duty build that carries real tool weight, not just light items
Price and availability change often — the button shows the live Amazon price.
Frequently Asked Questions
Is a 20V MAX battery more powerful than an 18V one?
No — they are the same battery described two different ways. A lithium cell has a nominal voltage of about 3.6V and a peak of about 4.0V, so a five-cell pack is either “18V nominal” or “20V MAX” depending on which figure the brand chooses to print. The electrical output is identical. Compare amp-hours and the tool’s own specifications instead.
Do more amp-hours make a tool more powerful?
Mostly no — amp-hours measure capacity, so a 4.0Ah pack runs roughly twice as long as a 2.0Ah one at the same voltage. There is a secondary effect on very high-draw tools, where a larger pack with more cells can sustain heavy current more comfortably. But the main thing you are buying with amp-hours is runtime, not strength.
Can I use another brand’s battery in my tool?
No. Each manufacturer uses its own physical connection and electronics, deliberately. Adapters exist but are best avoided — they bypass communication between pack and tool and can cause charging and protection problems. This is exactly why choosing a platform matters so much: you are committing to one battery shape for the long term.
How should I store batteries over winter?
At around half to two-thirds charge, at room temperature, indoors. Avoid both fully charged and completely flat for long periods, and avoid heat above all — a hot loft or a van ages packs quickly. Check them every couple of months and top up if they have drifted low, and let a cold pack warm up before charging it.
Are cheap third-party batteries safe?
Quality varies a great deal. The savings usually come from cheaper cells and simpler protection circuitry, and the protection circuit is the part preventing serious failures. For an out-of-warranty tool used occasionally, a well-reviewed pack from a seller with a real returns policy is defensible. For anything under warranty or heavily used, buy the manufacturer’s. Either way, charge on a non-combustible surface.
Is brushless worth paying extra for?
For regular use and high-draw tools like saws and grinders, yes — brushless motors are more efficient, run cooler, last longer and give more runtime per charge. For a drill used a few times a year to hang pictures, a brushed tool at a lower price is a perfectly sensible choice. Spend the difference where the tool works hardest.
Your Final Checklist
- Choose the platform before the tool — it is a ten-year decision
- Check the tool range and bare-tool availability on that platform
- Ignore 18V vs 20V MAX — same battery, different label
- Consider 12V if most of your work is light and indoors
- Read amp-hours as runtime, not power
- Own one compact and one large pack, not two identical ones
- Start with a combo kit, then buy bare tools
- Choose brushless for high-draw and frequently used tools
- Never charge a hot pack — let it cool first
- Charge on a non-combustible surface, not unattended overnight
- Store at part charge, at room temperature, indoors
- Warm cold batteries before charging
- Stop using any swollen or damaged pack immediately
- Recycle old batteries properly — never in household waste
Next in this series: how to use a cordless drill and setting up a small workshop.