How to spot a fake capacity power tool battery
A 6.0Ah pack that weighs less than your old 4.0Ah, a 9900mAh 18650 cell for four dollars, an 18V pack the size of a soda can claiming 9.0Ah. All three are the same trick: a capacity number larger than the physics of the shell allows.
Capacity is the easiest spec to fake because almost nobody measures it. Voltage is easy to check with a five dollar meter. Milliamp hours need a controlled discharge to verify, which is why inflated numbers survive for years on marketplace listings.
The good news is that a fake mAh battery claim usually fails an arithmetic test before you need equipment. Stored energy per gram has a hard ceiling.
The energy density ceiling that ends most claims
Lithium-ion cells store a bounded amount of energy per kilogram. Battery University's chemistry reference lists 150 to 220Wh/kg for NMC cells, 150 to 200Wh/kg for lithium cobalt oxide with specialty cells reaching 240Wh/kg, and 200 to 260Wh/kg for NCA.
Take the top of that range as a generous ceiling: 260Wh/kg. Any commercial cylindrical cell claiming more energy per gram is claiming a chemistry that does not exist in production.
Now apply it to a cell claim. A single 18650 weighs about 45 to 49 grams depending on model, and reference cells cluster between 2.6Ah and 3.5Ah. For a 9900mAh 18650:
- Energy equals nominal voltage times capacity: 3.6V times 9.9Ah is 35.6Wh.
- Required mass at 260Wh/kg equals 35.6 divided by 260, which is 0.137kg, or 137 grams.
- An 18650 cell weighs about 47 grams.
The claim needs three times the mass the format allows. It is not a premium cell. It is a mislabeled lower capacity unit, usually a 2Ah cell in a printed wrapper.
The same arithmetic scales to packs. DeWalt's product page for the DCB204 4.0Ah 20V pack lists an assembled weight of 1.4 pounds, about 635 grams. At a nominal 18V and 4.0Ah it holds 72Wh, so pack-level energy density works out to roughly 113Wh/kg including the shell, latch and circuit board. That is a useful benchmark: a real tool pack lands well under 150Wh/kg once you count everything that is not a cell.
Apply 150Wh/kg to a claimed 18V 9.0Ah pack. Energy is 162Wh, so the pack should weigh about 1.08kg, or 2.4 pounds. A shell the size of a 4.0Ah pack cannot hold that.
The cell count test
Energy density is not the only constraint. Cell count shows in the pack's proportions.
An 18V pack is five cells in series. Capacity comes from parallel strings:
| Claimed capacity | Cells at 3.0Ah | Cells at 2.5Ah | Minimum mass at 47g per cell |
|---|---|---|---|
| 2.0Ah | 5 (5s1p) | 5 | 235g plus shell |
| 4.0Ah | 10 (5s2p) | 10 | 470g plus shell |
| 6.0Ah | 10 | 15 (5s3p) | 470 to 705g |
| 9.0Ah | 15 (5s3p) | 20 (5s4p) | 705 to 940g |
Five-cell and ten-cell packs have different shells, and the difference shows in the hand: a 2.0Ah compact is a single row, a 5.0Ah standard is two. A seller claiming 9.0Ah in a ten-cell shell is either using cells above 3.0Ah, which exists, or overstating. The way to tell is mass.
This is also why third-party packs that genuinely hold 6.0Ah are noticeably bigger and heavier than the OEM 4.0Ah they replace. If a replacement claims double the capacity at the same size and weight, one of those numbers is wrong.
The weight test in practice
The battery weight test resolves most of these questions in under a minute. Weigh a known-good pack of the same platform, then weigh the suspect.
A genuine pack's mass scales roughly with claimed capacity. If a 6.0Ah pack weighs 8 percent more than a 4.0Ah of the same series, the claim is suspect, because 50 percent more cells cannot fit into 8 percent more mass.
Cell-level checks are more precise, and legitimate sellers quote the per-model anchors. Samsung INR18650-30Q tops out at 48.0g, Sony Murata VTC6 is about 46.6g typical, Molicel P28A about 46g, LG HG2 47.0g max, LG MJ1 49.0g max. A "30Q" that weighs 38g is not a 30Q.
The discharge curve test
Weight and arithmetic catch most fakes. A discharge test catches the rest, including a genuine-looking pack with lower grade cells inside.
The method needs a load and a way to log voltage. A constant-current electronic load is ideal; a resistive load with a multimeter and a timer shows the shape. Discharge at the rated rate, log voltage every minute, plot it.
A healthy pack holds a plateau through most of its discharge, then falls off a cliff near the end. A pack with inflated claims usually does one of three things:
- It falls off the cliff early, delivering fewer amp hours than the label, because the cells are a lower capacity model.
- It sags hard in the first minute, because the cells have high internal resistance and are not rated for the tool's current. The link between discharge rate and sag is covered in the guide to C-rate in power tool batteries.
- It gets hot early, which is the same resistance problem appearing as heat instead of voltage drop.
For 18650 cells, a proper test also reads internal resistance, because a rewrap of an old high-resistance cell can pass a low-current capacity test and still be useless in a drill. Published anchors exist per model: 30Q at 26 milliohms AC maximum, P28A at 20, HG2 at 20, MJ1 at 40. A cell far above its model's anchor is either worn or not that model.
What counterfeit DeWalt and Milwaukee packs look like
Brand-name fakes are a separate category from generic overstatement: the seller imitates a specific product rather than exaggerating a house brand.
The physical tells repeat, and a counterfeit DeWalt battery shows them all: soft or misaligned wrapper printing, a date code in a font that does not match the OEM, a latch that binds, molding flash on the seam, and a label whose watt hour figure does not match volts times amp hours.
The electrical tells are more reliable. A counterfeit pack often lacks the thermistor the charger expects, so the charger refuses it or charges slowly in a default mode. Some chargers flash an error code instead of charging an unrecognized pack, which is the pack telling you something.
DeWalt publishes support guidance for identifying counterfeit batteries and directs owners to contact support with photos when a pack is suspect. For a pack you already own, that is the right escalation: stop running it in high-drain tools while you investigate.
The used market adds a twist, since a worn genuine pack and a fake look similar from outside. The checklist for buying used or refurbished packs covers condition checks, and the date code is the fastest way to separate old from fake.
Seller language that predicts a fake
The listing copy is often enough on its own. Certain phrases correlate with inflated claims and are worth screening before you look at the photos.
Capacity numbers with no voltage. "9900mAh" alone means nothing. Energy needs both numbers, and sellers who omit voltage usually do not want you doing the multiplication.
Capacity above the format's ceiling. Branded 18650 cells cluster between 2.6Ah and 3.5Ah. False capacity claims of 5Ah, 6.8Ah or 9.9Ah on a Samsung or LG cell mean a different cell inside or a misprint.
Voltage played as a marketing figure. A pack sold as 20V that computes watt hours at 20V is being generous by about 11 percent, a sign the spec sheet is being worked rather than reported.
No model number for the cells inside. Reputable rebuilders name the cell. Fakes never do.
Photographs that never show the label. Six angles of the box and none of the rating plate means the one surface with verifiable data is hidden.
When the pack is a compact one, size is the giveaway. A 6.0Ah pack in a shell the size of a Milwaukee M12 2.0Ah is not a capacity upgrade; the M12 platform guide walks through how capacity steps track physical size in that series.
Price is the signal most buyers use, and cheap versus expensive packs breaks down where the money goes. The brand question sits next to it, in aftermarket versus OEM.
FAQ
Are aftermarket batteries counterfeit? No. An aftermarket pack is a legal third-party product that fits a brand's tools, and many are honest about their cells. Counterfeit means imitating a brand's product or its claimed specs.
Can a 9900mAh 18650 be real? Not in the 18650 format at any chemistry in current production. The energy would need about 137 grams of cell mass against a 47 gram format limit.
Does a heavier pack always mean a real one? No. Counterfeiters add weights to beat the scale. Weight is a fast screen that catches careless fakes, not a certification.
Is a cheaper pack with a lower mAh rating a fake? Not necessarily. An honest aftermarket pack stating 3.0Ah where the OEM offered 5.0Ah is a downgrade, not a counterfeit. The problem starts when the label claims more than the shell can hold.
How do I check capacity without a load tester? Time the tool. Bore a set number of holes in the same material with a known-good pack and the suspect. The ratio of the times approximates the ratio of usable capacity, enough to catch a 30 percent overstatement.
Sources
- Battery University, "BU-205: Types of Lithium-ion" (specific energy ranges by chemistry): batteryuniversity.com/article/bu-205-types-of-lithium-ion
- 18650 Battery Store, "How to Spot Fake 18650 Batteries" (per-model weight, capacity and internal resistance anchors): 18650batterystore.com/pages/how-to-spot-fake-18650-batteries
- DeWalt, DCB204 20V MAX 4Ah battery product page (assembled weight 1.4 lb): dewalt.com/product/dcb204
- DeWalt support, counterfeit battery identification (contact support with photos of a suspect pack): dewalt.com/support
For a baseline to weigh against, the 18650 cell listing states its capacity and cell format, and the DeWalt-compatible collection lists packs with their rated amp hours.