How to read a power tool battery manufacture date

That pack in the bottom of your bag might be two years old or nine. The difference decides whether it is worth carrying, and the answer is stamped on the pack.

Every major brand puts a date or serial code on its packs, and the code is used for warranty decisions, which is why the brands publish where to find it. Milwaukee's support pages state that the date code of the product determines whether it is inside the warranty period. DeWalt's documentation places the code on the rating plate or inside the battery compartment, depending on the model.

Reading the code is the easy half. The harder half is knowing what age does to a pack that was never used, because a sealed pack on a shelf is not a new pack.

Where the date code actually is

Start with the surfaces that carry molded or printed text, and work through them in this order.

The rating label comes first. The sticker with voltage, capacity and model number usually carries a small block of characters near an edge. DeWalt's support pages describe it as engraved at the bottom of the rating plate on some models, and note that the manual shows the location for each tool.

The battery compartment is second. Where the label is crowded, DeWalt places the code inside the battery bay. Support notes for several cordless saws name the underside of the main body and the battery compartment as the two places to look.

The pack's own label or shell is third, and it is the one that matters for capacity questions. A pack carries its own code, often a four to six character string molded into the plastic near the latch.

The box comes last. Packaging codes can differ from product codes, which is why DeWalt's serial number guidance tells owners not to rely on the box.

One caution: the serial number and the date code are not always the same field. A serial identifies the unit; a date code identifies when it was built. On some packs one string serves both purposes.

What the code generally encodes

Across the tool industry, manufacturing date codes follow one broad pattern: a year digit or two, then a week or month number, sometimes with a plant or line character attached. Two-digit year plus two-digit week is the most common convention in manufactured goods, so it is the first thing to try.

Applied to a pack, a string like "2342" would read as year 2023, week 42. Some brands use a single year digit, which limits the readable range to a decade unless you know the era of the pack. Others print two year digits followed by a letter for the month.

[DATA NOT VERIFIED: the exact character positions and year/month encoding used by DeWalt, Makita and Milwaukee packs in 2026 were not confirmed against a published decoding table in this pass. Brand support pages confirm that a date code exists and where to find it, not how to parse it. Confirm a specific string with the brand's support line before relying on it for a warranty claim or a purchase decision.]

Without a decoder, you can still bound the age. Read the digits as week and year in both orders and see which produces a date that is not in the future; that resolves most ambiguity.

Why the date matters more than the hours

A pack that was never cycled still ages. The chemistry degrades on the calendar, and the two accelerants are temperature and state of charge.

Battery University's storage data quantifies this for cobalt-based lithium-ion. Stored for one year at 25 degrees C, a cell kept at 40 percent charge retains about 96 percent of its capacity, while the same cell at 100 percent charge retains about 80 percent. At 40 degrees C the numbers fall to 85 and 65 percent. At 60 degrees C, a fully charged cell is down to about 60 percent after three months.

Read that table against a warehouse shelf in a warm climate and the rule falls out: a pack that spent two summers fully charged in a hot stockroom can arrive with less usable capacity than a pack bought three years ago and kept at half charge in a basement.

Two consequences follow. Buying "new old stock" is a real risk even from a legitimate seller, because the pack is genuinely unused and degraded. Storing your own packs at 40 to 50 percent charge in a cool place is worth more than any charging habit.

Shelf life, honestly stated

Lithium battery shelf life has no single number for a tool pack, because the answer depends on storage temperature, storage charge and the cells inside. What can be stated with support:

• Calendar aging happens whether or not the pack is used, per the table above.
• Elevated temperature accelerates it. Battery University treats anything above 30 degrees C as elevated for lithium-ion.
• Full charge during storage is worse than partial charge, and the gap widens as temperature rises.
• A pack stored at 40 percent charge at cool room temperature loses a few percent per year, not tens of percent.

A three-year-old pack stored well might still deliver 90 percent of rated capacity, while a one-year-old pack that lived in a truck cab through a desert summer might deliver 75 percent. Age is a starting point, not a verdict.

[DATA NOT VERIFIED: no manufacturer-published capacity retention figure for a specific tool pack model was located in this pass. The percentages above come from Battery University's generic cobalt-based lithium-ion storage table, a chemistry-level reference rather than a pack-level guarantee.]

Judging a pack you cannot date

Sometimes the code is worn, painted over, or absent on a counterfeit. In that case, work from behavior.

Resting voltage after a full charge is first. A healthy 18V pack settles around 20V shortly after coming off the charger and holds there. A pack that drops to 18V within a day has elevated self-discharge.

Voltage sag under load is second. Put it in a drill and pull the trigger while watching a meter. A pack that collapses several volts in the first second has high internal resistance, the signature of an old pack.

Heat after a short run is third. A pack that gets noticeably warm driving a light load is converting energy to heat at the cell level.

Runtime on a known task is fourth. Bore the same holes in the same material with a known-good pack and the suspect. The ratio tells you the capacity ratio.

These checks also separate a worn genuine pack from a counterfeit. The checklist in the used and refurbished battery guide runs the same tests in buying order.

Stock packs and the tolerance boundary

A pack that sat in a store for eighteen months is not a warranty case, and it is not mislabeled. It aged within the range the chemistry allows.

The practical boundary: if a pack delivers within about 10 percent of a fresh one of the same model on the same task, treat it as serviceable. Between 10 and 25 percent down, it is a backup rather than a primary. Below that, it belongs in a rotation for light tools or in the recycling stream.

That boundary is a working rule from the storage data, not a manufacturer specification. No brand publishes a "still good" threshold for aged stock, which is why date codes matter.

When a listing states capacity but no date, buy from one that lets you check on arrival. A replacement DeWalt 18V pack with a readable rating label is easier to date than one with a bare shell.

Cross-brand notes

A DeWalt battery date code sits where the support pages say: rating plate or battery compartment. DeWalt's serial number guidance notes that the serial can be numbers or a mix of letters and numbers, and that the box copy may differ from the product.

Milwaukee's warranty pages state that the date code determines warranty eligibility, and the Milwaukee battery serial number is the field that carries it. They recommend keeping the invoice because it helps during verification, which makes the code the primary record. Photograph it when the pack is new.

Bosch and Makita print production data on packs and tools, and the Makita battery date code format differs between generations. On Bosch's 14.4V and 18V lines, part number and generation differ enough to cause compatibility confusion, which the Bosch BAT614 and BAT618 guide sorts out separately from the date question.

Age is only one input into how long a pack lasts, and lithium-ion lifespan covers the cycle side. Storage is the part you control, covered in how to store packs.

FAQ

Does a lithium battery age if it is never used? Yes. Calendar aging is separate from cycle aging, and it accelerates with heat and with a high state of charge.

How long do power tool batteries last on a shelf? There is no single figure. Stored cool and at partial charge, a pack can hold most of its capacity for several years. Stored hot and full, it can lose a third in a year.

How do I read a battery date code? Look for a four to six character block on the rating label or molded into the shell, and read the digits as week and year in whichever order produces a plausible past date. Confirm the format with the brand before using it for a warranty claim.

Where is the date code on a DeWalt battery? On the pack's rating label, and on tools the code may be engraved at the bottom of the rating plate or placed inside the battery compartment.

Is a two-year-old unused pack still good? Usually yes, if it was stored cool and not left at full charge. Check resting voltage and runtime against a fresh pack before trusting it for a full day.

Sources

• Battery University, "BU-808: How to Prolong Lithium-based Batteries" (storage temperature and state-of-charge capacity loss table): batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries
• DeWalt support, date code locations and rating label guidance: dewalt.com/support
• Milwaukee Tool, warranty support (date code determines the warranty period): milwaukeetool.com/Support

Packs that pass the age check are usually still worth pairing with a fresh spare. The DeWalt-compatible collection lists current models, and the comparison of watt hours against amp hours explains the capacity number printed next to the code.