# Ohm's law and rebuildables: vaping safely with cells > Ohm's law applied to rebuildables: the relationships that matter, the measurement that comes first, and how to read a cell's discharge rating. A rebuildable build is not judged on flavour alone. Behind every coil sits a question of electricity, namely how much current the cell has to deliver without complaint. Ignoring that question leaves your safety to chance, and on a mechanical mod there is no electronics standing between a bad decision and a lithium cell. **Ohm's law** is not abstract theory. It is the tool that ties the value of a resistance to the current being demanded, and therefore to the limit printed on the cell you are using. This piece turns it into concrete habits and checks rather than a set of numbers to copy. ## What Ohm's law says about a vaping build **Ohm's law** links three quantities in a single relationship: voltage equals resistance multiplied by current, where voltage is in volts, resistance in ohms and current in amps. For a rebuildable vaper the interesting rearrangement is current equals voltage divided by resistance. The lower the resistance, the higher the current climbs. Power follows from the same family of relationships, since power equals voltage squared divided by resistance, which is also voltage multiplied by current. Those relationships are enough to accept or refuse a build before the first draw. The decisive part is the comparison, not the arithmetic. The current your build will draw has to stay below the maximum continuous discharge current of the cell, a figure the manufacturer publishes for that exact model. A build that exceeds it pushes the cell beyond what it was designed for, with a real risk of thermal runaway. That is why the check happens before the build is fired, never after, and why it is always done with a comfortable margin rather than at the edge. A cell run close to its stated limit heats up, ages faster and loses the very current capability that made the sum work in the first place. ## The relationships to know by heart Two or three expressions cover almost everything a rebuildable vaper needs, and knowing them saves you depending on a calculator at the exact moment you should be paying attention to the deck. The first gives the current: current equals voltage divided by resistance. The second gives the power dissipated in the coil: power equals voltage squared divided by resistance. The third, derived from the others, gives the lowest resistance a given cell can support: minimum resistance equals voltage divided by the maximum current of that cell. These relationships are exact and universal. They depend on no brand and no model, which is precisely what makes them useful when a listing is vague or a forum post is confidently wrong. ## Reading the data sheet rather than the wrap This is where most accidents begin, and it deserves stating plainly. The number that goes into the calculation is the continuous discharge current of the exact cell in your hand, taken from the manufacturer's data sheet for that model. It is not the figure printed on the plastic wrap, which is frequently a rewrapped and generously rounded claim, and it is not a number remembered from a different cell. Rewrapped cells are common, and an inflated rating turns a careful calculation into a false sense of security. Look for the line that names continuous discharge, sometimes abbreviated as CDR, rather than a pulse rating. A pulse figure describes a brief burst under laboratory conditions and has no business in a safety calculation. Independent testers publish measured figures for the common cells, and where they disagree with the wrap, the wrap is the one to distrust. ## Regulated mod or mechanical mod, two different worlds **Ohm's law** applies everywhere, but what it protects you from changes completely with the type of device. Confusing the two produces false confidence, which is the most dangerous state to build in. On a regulated mod an electronic circuit manages the power, refuses abnormal values and cuts out on a short circuit. The vaper has a safety net, even though the wattage range of the coil still has to be respected. On a mechanical mod there is no electronics at all. The cell sees the resistance directly and delivers whatever raw current the relationship dictates. Nothing will intervene between a build that is too low and a cell pushed past its rating, which is why rebuildables on mechanical mods belong to vapers who can do this check without hesitating. ## Ohm's law assumes a cell in good condition The relationships assume a cell in perfect condition. A tired or damaged cell upsets every calculation, however rigorous the work on the deck has been. A torn wrap, a corroded terminal or a cell whose voltage sags quickly will not hold the discharge current it claims. A damaged cell is set aside, a split wrap is replaced before any use, and an old or swollen cell goes to a collection point rather than back into a device. The safety of a rebuildable build rests as much on the mechanical health of the cell as it does on the sums. The best calculation in the world does not rescue a failing cell, and no reading on the screen will warn you in time. ## Carrying and looking after your cells The electrical check counts for nothing if the cell degrades in a pocket. Everyday handling is part of the safety of a rebuildable build, not an afterthought bolted on to it. A bare cell never travels loose with keys or coins, because contact across both terminals creates a direct short with no resistance to limit the current. Spare cells go in a dedicated hard case, the insulating wrap is inspected regularly, and any split wrap is replaced before the cell is used again. In a device that takes two cells, they are used as a matched pair: same model, same age, same state of charge, always together. Charging on a decent external charger, rather than permanently through the device, also preserves the life and the current capability that the calculation assumes. Those habits keep the assumptions behind **Ohm's law** valid. A healthy cell holds its stated current, a mistreated one does not, and a device that flatly refuses to start is never forced. Electrical safety begins in the pocket as much as in the sum, which is why the same discipline runs through our guide to [building a coil](/uk/build-a-coil-step-by-step/) and our piece on the [Short Atomizer message](/uk/short-atomizer-vaporesso-fix/). ## Frequently asked questions ### Where do I find the maximum discharge current of my cell? On the manufacturer's data sheet for that precise model, expressed in amps and often labelled continuous discharge rating. Treat figures printed on generic wraps with suspicion, since an inflated claim invalidates the whole calculation. Independent test results are the useful cross check. ### Is Ohm's law useful on a regulated mod? Yes, even though the circuit protects you. Understanding how resistance, current and power relate helps you stay inside the wattage range of the coil and choose sensible values. On a mechanical mod it stops being merely useful and becomes essential. ### Why calculate using the voltage of a fully charged cell? Because that is the worst case. A cell at full charge draws more current through the same resistance than one part way through its discharge. Calculating at full charge keeps you under the limit throughout the cycle rather than only at the end of it. ### Does a dual coil build change the calculation? Yes. Two coils in parallel lower the total resistance of the build, which raises the current demanded from the cell. You have to work from the resulting resistance of the complete build, not from one coil, before applying the relationships at all. ### What happens if I go too low in resistance? The current demanded can cross the limit of the cell, which then heats dangerously. On a mechanical mod nothing intervenes. That is precisely the scenario the calculation exists to prevent, and a value too low for your cell is corrected on the deck rather than tried out. ## Making the calculation a reflex **Ohm's law** is not an obstacle reserved for electronics enthusiasts. It is a compass that ties the measured value of a coil to the current your cell has to absorb, and it takes minutes to apply once the habit is there. Measure on an ohm reader before anything is powered, read the continuous discharge line on the data sheet for the exact cell, keep a margin, and check the physical state of the wrap and terminals. That routine turns a risky rebuildable build into a controlled one, and it is the same discipline that shows up again when a build misbehaves, as our [diagnosis of a spitting coil](/uk/spitting-coil-burnt-taste-diagnosis/) sets out. Editorial content for adult readers only. Vaping products usually contain nicotine, a substance that causes addiction. Not for sale to under-18s in the United Kingdom. This article is not a buying recommendation.