The Pack as a Part: Contacts, Keying and the Third Wire

Take the pack off and look straight into its terminal block, as in the photograph on this page. Most modern packs mark the pins. Write down what yours has. - **B+ / B−** — the two outermost, heaviest blades. Everything the motor draws goes through these. They are wider and thicker than the rest for exactly that reason. - **TH** — the thermistor tap. Step 4 measures it. - **ID** — a resistor to B−, of a value that tells the charger which chemistry and capacity it is holding. This is how one charger safely serves several packs. - **C1, C2, C3, C4** — taps between the series cells, so a smart charger or balancer can read each cell separately rather than only the total. Rung 7 is entirely about why that matters. Measure the geometry with the calipers while it is in your hand: blade width, blade thickness, the depth of engagement, and the rib spacing. Then look at the **keying** — the asymmetric ribs and the latch. The pack physically cannot enter the wrong tool, cannot enter backwards and cannot fall out under vibration, and none of that depends on the user reading anything. Measure the voltage on each pin against B− with the meter, and note which read a steady voltage, which read nothing and which read something odd. Do not short anything: an 18 V pack will deliver over a hundred amps into a screwdriver blade and the blade will weld.

You cannot read a 2 mΩ contact with an ordinary meter on its resistance range, because the meter's own leads are worth more than that. Measure it the way it is actually measured: **push a known current through and read the voltage across**. Set the bench supply to constant current — 2 A is plenty and is safe — and pass it through the pack-to-tool contact pair with the pack fitted and the tool's own switch off. Put the meter directly across the two points either side of the joint, as close to the contact as your probes will reach. Resistance is the voltage you read divided by the current you set. This is a four-wire measurement: the current leads and the voltage leads are separate, so the lead resistance drops out. Do it on a clean pack and again on a neglected one. Then clean the neglected one's blades with isopropyl alcohol on a lint-free swab, let it dry, and repeat. The improvement is usually large and it is the single most effective repair anyone can do to a tired tool. Take the number into step 3 and see what it costs at 40 A. A joint that measures 25 mΩ is not a marginal joint; it is a joint that is on its way to a melted terminal block. **Do not** attempt this with the tool running, and do not bridge B+ to B− at any point.

Put the meter on resistance between **TH** and **B−** on a pack that has been sitting at room temperature. You should read something in the region of 10 kΩ. That is the pack's thermistor, and it is pressed against a cell inside. Now prove it is a thermistor and not a plain resistor. Warm the pack gently — a few minutes of ordinary work, or a heat gun on its lowest setting held well back and waved, never parked — and watch the resistance **fall**. Cool it and watch it rise. Write down three pairs of temperature and resistance, taking the temperature with the infrared thermometer on the case beside the terminal block. Then do the same with a loose 10 kΩ NTC from the thermistor kit, which you can put in iced water and in warm water and measure properly. Two fixed points and the beta equation from step 3 give you a calibration good enough to predict the third. Why this pin exists: charging a cold lithium cell plates metallic lithium on the anode instead of intercalating it, which is permanent damage and, eventually, the failure of rung 9. Charging a hot cell accelerates every side reaction it has. The charger needs to know, it cannot guess from voltage, and one cheap part in the pack answers it. **Never** put a heat gun on a lithium pack on anything but its lowest setting, and stop at 50 °C. You are demonstrating a slope, not testing a limit.

You are not going to open a pack — a slipped screwdriver across a cell tab is a short of a hundred amps or more, and in a lithium pack that is a fire. But you can build the same thing from loose cells and see exactly how it goes together. Put four 18650 cells in a four-cell holder. That is a *4S* string: four in series, 14.4 V nominal. Wire it as a pack would be wired: 1. **B+** from the top cell's positive, **B−** from the bottom cell's negative, both in heavy wire — at least 14 AWG for a tool-sized current. 2. A **tap from every cell junction**, in thin wire, brought out to a connector. These are the C1–C4 pins from step 1. 3. A **10 kΩ NTC** taped against the middle of the middle cell, its other leg on B−. That is TH. Measure B+ to B−, then each tap against B−, and check the arithmetic: the taps should step up by roughly one cell voltage each. Any tap that is out of step has told you something real, and rung 7 is about what. In a production pack the series links are not wire at all: they are **nickel strip spot-welded** to the cell ends. A weld is used rather than solder because the heat of soldering a cell tab damages the cell right under the joint, and because a welded strip has a far lower and far more repeatable resistance than a solder blob. If you have the spot welder, practise on strip and scrap ends only; cell-to-cell welding is a separate skill and a separate risk, and step 2's resistance measurement is the way to tell a good weld from a pretty one.

**Attribution, stated honestly.** The removable slide-on pack appears in the professional tool market around **1978**, with Makita's 3.6 V cordless driver usually cited as the first widely sold example and Black & Decker working the same ground in the same period. No single patent number is asserted here: the sources disagree, and the feature is a convergence rather than one filing. What is documented, and what matters, is the arrangement — a keyed, latched, multi-pin connector that separates the energy store from the tool. **Why it changed the industry and not just the tool.** Once the pack is a part, the *platform* becomes the product. A maker buys into a voltage and a connector, and every later tool in that family costs the price of a bare tool. That is why the pins multiplied: ID so the charger knows what it holds, TH so it knows whether it may hurry, and the cell taps so the pack can be looked after rather than merely filled. **Honest limits.** The connector is the weakest part of a cordless tool and always has been. It wears, it corrodes, it loosens, and it is the usual reason a pack 'goes bad' when the cells are fine. It is also proprietary by design — the same keying that stops a wrong pack entering also stops a compatible one entering, which is a commercial decision wearing safety clothing. And the pins that make a pack smart also make it repairable only with the manufacturer's charger, which is the part of this rung worth arguing about rather than admiring.