Last week a customer phoned us while we were carrying out a shelf project; his drill had stopped working and the only thing he could say about the battery was that it was "the black one". This sort of thing occurs frequently and isn't his fault. The labels on the batteries appear to be a bunch of numbers until someone explains what those numbers actually mean.
Almost all aspects of electric tool batteries can be covered by three rules. You must match the voltage of the battery exactly to that of your tool. When looking at the amp-hours, think of it as corresponding to the capacity of the fuel tank rather than the size of the engine. Also, accept that a battery will only fit the brand for which it was made. If you observe all three of these points, then the rest is just a matter of shopping.
The guide explains the meaning of those numbers, why batteries from different brands cannot be used interchangeably, how long they usually last, and the four points you should check before purchasing an electric tool battery. Once you've finished reading it, you'll be able to examine any used battery, read the label on it, and order the correct replacement the first time.
The three numbers on your battery, decoded
If you flip the pack over you will see three figures worth knowing, typically in this order.
Voltage, marked with a V. With modern tools this usually refers to 18V, although you may also come across 12V, 20V MAX, 40V and a few other voltages. That is the figure you need to match since the tool has been designed around it.
Amp-hours, shown as Ah. The capacity ranges from 1.5Ah to 12Ah; this figure indicates how long the pack will last, not the force with which it strikes.
Watt-hours, marked Wh on some labels. The figure is simply voltage multiplied by amp-hours, and it's included merely so that you can compare energy values at different voltages. You usually don't come across it when you're shopping.
This is what you need to know about batteries: usually you don't need the manual since the label takes care of that for you. Just write down the voltage, note the Ah rating, take a picture of the connector, and that's all a seller needs in order to recommend the correct battery. If you'd like the full step-by-step explanation of that process, we have a separate guide which shows how to choose a replacement power tool battery.
Voltage: match it exactly
The voltage determines the amount of power the tool has and each tool is designed around a particular voltage. Therefore the principle isn't that more is better; it's that you should match them.
A quick way to think about the classes:
| Voltage class | What it's for |
|---|---|
| 12V | Light work: hanging shelves, small repairs, tight spaces. The tools stay light. |
| 18V (sold as 20V MAX in North America) | The mainstream. Drills, drivers, saws, grinders, most of the cordless world. |
| 36V to 40V | Yard tools and heavy outdoor work: mowers, blowers, chainsaws. |
That way of putting it is only a rough approximation, not a detailed specification. Some manufacturers also fit two 18V batteries side by side in order to power a 36V tool, which is the reason why a number of '36V' tools use the same batteries as those already in your drill.
Regarding the confusion about the labels, since it causes more trouble for buyers than anything else, an 18V pack and a 20V MAX pack are in fact the same. A footnote on DeWalt's official product pages states that the maximum initial voltage, measured with no load, is 20 volts, while the nominal voltage is 18 volts. It's the same battery but with two labels—one for use in North America and the other for use elsewhere. When you're shopping, you should regard 18V and 20V MAX as belonging to the same category.
What do you get if you make a mistake? A 12V battery will fail to fit into an 18V tool, and a 40V battery for sure won't fit into an 18V drill. Individuals contact us to ask whether a 40V battery will enable an old 18V drill to work, and the answer is no. Having more volts doesn't count as an upgrade; it just means you have a different machine.
Amp-hours: runtime, not power
Comparing the voltage to an engine and the amp-hours to a fuel tank, a battery with a capacity of 5.0Ah doesn't cause your drill to spin any faster than one that has a capacity of 2.0Ah; it only means that the drill will continue to run for a longer time.
With the same voltage, a higher Ah rating gives you a longer run time, and there are two real drawbacks associated with it. The pack becomes heavier and it is more expensive. The largest packs are actually quite heavy, a fact you'll notice the first time you hold one up overhead.
Quick sizing bands:
- 1.5 to 2.5Ah: light jobs and occasional use.
- 3.0 to 4.0Ah: the middle ground most people should buy.
- 5.0Ah and up: all-day work, saws, and yard tools.
After having done a bit of overhead work, I'd prefer to carry a 2.0Ah pack rather than a 6.0Ah one since it's the weight of this thing that no one talks about until your arm experiences it.
Here's another item to include in the toolkit: watt-hours. Since an 18V 2.0Ah battery has a capacity of 36Wh and a 40V 2.0Ah battery has 80Wh, watt-hours are important when comparing a 40V garden battery with an 18V one. As for the rest, the label can continue to do so. If you'd like to know more about what amp hours mean and how large a battery pack you really need, we have a separate article explaining it.
Lithium-ion vs the older chemistries
All products made in the past fifteen years use lithium-ion batteries, and the entire industry made the switch for good reasons. As the Power Tool Institute puts it, lithium-ion battery packs contain more energy in a smaller space, are lighter in weight, do not suffer from the memory effect which affected earlier chemistries, and retain their charge when they are not in use.
The older types of chemistry are still in use. In the 1990s, nickel-cadmium batteries were used to power drills, and they lost their capacity if you recharged them before they had been completely discharged. Nickel-metal hydride was the more user-friendly option in between. Both of these types are heavier and have a shorter lifespan than those which have taken their place.
And while we're at it, the previous recommendation of draining a battery completely before charging it has vanished with the obsolescence of nickel-cadmium batteries. Lithium batteries prefer the opposite approach: whenever it's convenient top them up and save the deep discharges for the trash can of history.
If the drill you currently have still works but its battery is now spent, then you'll have to make a choice between getting a replacement for the old chemistry or upgrading the tool to use lithium. We look at that option in a separate guide which deals with upgrading NiCad tools to lithium.
Why your old battery fits one tool and nothing else
Batteries appear to be universal but they aren't and two things are responsible for the locking.
As for the physical connector, most modern packs slide onto rails and click into position, whereas older ones used a stem or a post which plugged in. Packs that slide on will not fit into post tools and the same rule applies in the other direction. Indeed, this has changed within the history of a single brand, so the slide pack currently produced by that brand will not fit its own 1990s post tool.
Second—and more difficult to detect—each brand incorporates its own control circuitry into the pack, with the tool and the charger continuously communicating with that circuitry. The Power Tool Institute makes it clear that this circuitry is proprietary and that batteries will not be used across brands unless the manufacturer allows it. The same rule holds true within a brand as well; Ryobi's 18V ONE+ packs and its 40V packs constitute two separate systems that aren't interchangeable.
Which is why third-party battery adapters—the ones that allow you to use a battery from one brand in a tool made by another brand—always include asterisks. Many of these adapters omit the communication and safety features that the original system relies upon. If you want to know more about connectors and what actually fits what, see our power tool battery compatibility guide.
How long a pack lasts, in plain numbers
The typical lifetime of a lithium-ion battery pack on Black & Decker's support site is given as about two to three years, or 300 to 500 full charge cycles, whichever occurs first. A full cycle is defined as charging the battery from full to empty and then charging it back up again.
Consider them to be the manufacturer's ratings rather than genuine promises; heavy daily use, hot storage conditions, and repeatedly running the packs down to zero will all reduce the actual capacity, and the ageing process cannot be reversed. Once the capacity has decreased it will always remain lowered and you'll just get less running time per charge until it bothers you sufficiently to replace the pack.
Two points that most guides omit. The first is that the "whichever comes first" rule is more important than the number of cycles: a pack that has been sitting in a drawer for two years is more nearly finished than one that has been lightly used over the course of one year; what counts is the calendar age. The second is heat: if there's a single habit worth giving up, it's leaving a pack in a hot garage, since heat is the silent killer of lithium packs.
If you want the more detailed information about how long power tool batteries last or the symptom checklist that shows when a battery has failed, both guides can be accessed by simply clicking once.
The 5-minute battery audit
Before buying anything, take five minutes to decide if your battery is really dead; a surprising number of batteries believed to be dead are merely dirty or wrongly assessed, and a battery which is dying should still be given a decision before you spend any money.
First, examine the pack: if it's swollen, cracked, leaking or has a sweet smell, then stop right away since the pack is a safety concern and not something you should buy. Stick the terminals together and recycle it, using the steps that come later.
In step 2 you should clean the contacts. Dirt and sawdust on the terminals of the pack or charger are responsible for more 'won't charge' complaints than anything else, so first unplug the charger, wipe both sets of contacts with a dry cloth and then try again.
In step 3, carry out a cross-test by charging the faulty pack using a different charger and then placing a known good pack on the suspect charger. Just one swap will show you which of the two is at fault. If the good pack charges properly, then the charger in question is not guilty.
In step 4 you should check the voltage, provided that you have a multimeter; an 18-volt lithium battery that has just been charged shows a voltage of about 20 to 21 volts when at rest. If the voltage is below roughly 17.5 volts then the battery is very old. Treat those figures as a general rule for use in the workshop, not as specifications given by the manufacturer.
Step 5: Carry out a test. Fully charge the pack and carry out a job that you know well. If it fails after half the usual time then the capacity has been lost. If it shows that it is full but then cuts out when under load then one of the cells is weak and the pack is no longer usable.
Now make the call:
| Symptom | Verdict | Next step |
|---|---|---|
| Won't charge on any charger, even after cleaning | Dead pack or tripped protection | Replace it. Don't open it. |
| Charges, then dies in half the usual time | Capacity is gone | Replace, or live with the short runtime |
| Reads full, cuts out under load | Weak cell | Replace |
| Swollen or leaking | Safety hazard | Stop using it and recycle now |
| Charges and runs like before | Not the battery | Look at the tool or charger |
If your verdict is "replace," the 4-point check below is your shopping list.
How to pick a replacement: the 4-point check
Now the part that saves you money. Before you order, run these four checks against the pack sitting on your bench.
- Voltage matches. The V number on the new pack must be the same class as the old one. 18V for 18V. This is the one you can't fudge.
- The connector matches. Slide for slide, post for post. Some brands run more than one platform, so check the physical shape, not just the brand name.
- The capacity fits the work. Buy the Ah band that matches how you use the tool, not the biggest number on the page. A 2.0Ah pack for occasional use keeps the tool light and the price down.
- The seller passes the smell test. The Power Tool Institute's buying advice comes down to two lines: buy from authorized or reputable dealers, and if the price sounds too good to be true, it probably is. A real seller lists the models a pack fits, states the warranty, and answers questions. A blurry photo at half of everyone else's price is your cue to walk away.
As for factory versus compatible products, when it comes to a tool that you rely on every day the assurance provided by the factory pack——that is, it has been tested as a complete system——is worth the extra cost. But in the case of a drill that you only use on weekends, a compatible pack that passes this four-point test will do the same job at a great deal lower price. Since both options are valid, that is the reason for the check.
If you want the full numbers behind that trade-off, it's covered in our OEM vs aftermarket power tool batteries comparison. When you're ready to look at options, you can browse replacement batteries by brand across the twelve platforms we stock.
Safety and what to do with the old pack
Battery safety may not be complicated, but it's not something that can be left out either.
Do not use any battery that is swollen, distorted, leaking, or has a sweet smell. According to the Power Tool Institute's battery safety guidelines, swelling and leakage of liquid are cited as indications that a battery pack has failed, and it cautions that a damaged pack may result in burns or in fire, even sometimes weeks after the initial damage occurred inside. Do not charge it, do not open it, and do not throw it away.
That brings us to two further rules: you should never open a pack to repair it and you should never attempt to jump-start a dead one. Once a pack is no longer in use, cover the terminals with electrical tape to prevent them from short-circuiting and then take it to a battery recycling point. The Battery Network, the nonprofit organisation responsible for the Call2Recycle programme, has more than 20,000 recycling locations in North America, including large hardware stores.
When it comes to ordinary maintenance, there are two habits that are most important: store the packs in a cool, dry location, away from any flammable materials, and disconnect the charger once you've finished using it. The Power Tool Institute also cautions against keeping the packs in an enclosed vehicle near a window, as the sunlight can quickly raise the temperature.If your pack has stopped taking a charge entirely, start with our power tool battery not charging guide before you give up on it.
Brands at a glance
Replacement packs exist for every major platform, and here's how the twelve brands we carry line up:
| Brand | Main platform |
|---|---|
| Milwaukee | M18 (18V) |
| DeWalt | 20V MAX (18V nominal) |
| Makita | 18V LXT |
| Ryobi | ONE+ 18V and 40V |
| Bosch | 18V |
| Craftsman | V20 |
| Kobalt | 24V MAX |
| Ridgid | 18V |
| Metabo | 18V |
| Echo, Greenworks, Snow Joe | Yard platforms from 24V to 80V |
| Worx | 20V Power Share |
Two platforms deserve their own deep dives, because they have rules people get wrong. The Milwaukee M18 battery guide covers tiers and sizes, and the Ryobi battery guide explains why ONE+ 18V and 40V don't mix. You can also head straight to our Milwaukee compatible batteries and Ryobi compatible batteries collections.
Common questions about electric tool batteries
Does higher voltage mean more power?
Voltage is only one part of what makes a tool powerful, and the number that actually matters to you is simpler: the new battery has to match the voltage stamped on the old one. A higher voltage pack won't fit a lower voltage tool in the first place, so jumping voltage classes as an "upgrade" isn't a thing.
What does Ah mean on a battery?
Amp-hours measure capacity, the way gallons measure a fuel tank. A 5.0Ah pack runs your drill longer than a 2.0Ah pack at the same speed, and it weighs more and costs more to get there.
Can I use batteries from one brand on another brand's tools?
Almost never, and that's on purpose. Each brand builds its own connector and its own control circuitry, so one brand's pack won't fit or talk to another brand's tool. Third-party adapters exist, but most of them drop the safety features the original system relies on.
Should I buy OEM or aftermarket batteries?
Factory packs are the safest default, because the tool, battery, and charger were tested as one system. A good compatible replacement can work well too, as long as the seller lists the model numbers a pack fits, offers a real warranty, and isn't priced suspiciously low.
How long do power tool batteries last?
Most lithium-ion packs last about two to three years, or 300 to 500 full charge cycles, whichever comes first. Those are manufacturer ratings. Heavy use, hot storage, and running packs to zero shorten the real number, and the aging is irreversible.
How can I extend battery life?
Keep packs in a cool, dry place, off the charger, and out of parked cars and direct sun. Avoid running them all the way to empty, and protect the terminals so they can't short against keys or screws.
The entire subject actually can be summed up on a sticker, with one exception: the sticker doesn't indicate whether the pack is dead, dying, or merely dirty. That's why you have the 5-minute audit. After you've reached a conclusion, check the voltage, check the connector, select the Ah rating that suits the job, and carry out the 4-point check before making the order. If you do all that, the worst outcome is having to pay a bit more for capacity than you actually needed.
References & Sources
- Power Tool Institute battery safety and counterfeit battery guides (Power Tool Institute, the power tool industry's safety association)
- Battery Network (the nonprofit that operates the Call2Recycle battery recycling program)
- Black & Decker batteries lifespan support article (Black & Decker support)
- DeWalt official site (the 20V MAX nominal voltage footnote on DeWalt product pages)
Need a replacement?
If the old pack is done, browse replacement batteries by brand. We stock compatible packs for all twelve platforms above, so the matching part is already handled for you, and any question goes to a real person through our contact page.