# Battery charger: measuring real current and balance > Battery charger: how to measure the real current per bay, the balance between bays and the cut-off voltage, with a USB tester and a multimeter. A charger is sold on three arguments: a number of bays, a headline current and a screen. Not one of the three tells you what the unit actually does once four cells are sitting in it at the same time. A **battery charger** is judged on quantities you can measure, and those quantities are almost never published. Here is how to take them, with a tester and a multimeter. ## The three readings that qualify a battery charger Everything else is convenience. These three readings are enough to place any model on a scale, and none of them needs a laboratory. The first is the current genuinely delivered per bay, taken bay by bay, then again with every bay occupied. The second is the spread in current and in end voltage between bays with identical cells fitted, which is the balance reading. The third is the voltage at which charging stops, measured on the multimeter with the cell out of the cradle. The first two need a tester in line or a clamp meter. The third needs nothing more than a multimeter and a little patience, and it is the one that actually separates a good unit from a poor one. ## Cell and charger are qualified as a pair You never test a charger in the abstract. You test a pairing. The cell sets what it will accept, the charger offers what it offers, and a reading only means anything when both are written down. The reference point sits on the cell, not on the box. Every reputable cell maker publishes a datasheet giving a nominal voltage, a maximum charge voltage, a standard charge current and a maximum charge current, and those four figures are the ones a protocol works from. Reading that sheet is a skill worth acquiring, because it is the only document that describes the cell in your hand rather than the family it belongs to. We deliberately publish no charging value here as suitable. A bench states the method and the source: find the datasheet for the exact cell reference, note the charge figures it gives, and set the session from those. A number lifted from a review is not a substitute for the sheet that came with the cell. ## Why a charger rarely holds its headline figure This is the most common gap in the category, and the easiest to demonstrate. The current advertised on the packaging is almost always a total, or a current available in one configuration only. The listings of the most widespread models say so plainly once you read them to the end. A Nitecore i2 is quoted as one bay at 1 A or two bays at 0.5 A. A Nitecore i4 is quoted as one bay at 1.5 A, or two at 0.75 A, or two at 0.5 A, or four at 0.375 A. The same unit therefore delivers four times less per bay when it is full than when it holds a single cell. The protocol draws a rule from that. Every current reading is taken twice, at one bay occupied and then at all bays occupied, and both values are published. Quoting the better of the two, which is what the market does, produces a figure that is accurate and misleading at once. The same mechanism applies to a mod that charges its cells internally. On a dual cell chassis with a stated port current, that current is shared between the two cells, so the time taken lengthens accordingly. Comparing an internal charge against an external one therefore means measuring both, on the same cells, from the same starting state, which is the only way the comparison stays honest. ## Measuring the balance between bays Balance here means something precise: that each bay handles its cell independently of the others. A good unit has an autonomous charging circuit per position, and the retailer very rarely mentions it. The reading is taken like this. Two cells of the same model, brought to the same state of charge, are placed in two bays. You record the current in each and the voltage at the end of charge. Then you repeat with the positions swapped, which is what separates a faulty bay from a tired cell. A charger with independent circuits finishes its two bays at similar voltages but at different moments when the cells differ. A charger that cuts both bays at the same instant is handling its positions in series or in parallel, and that shows up immediately in the numbers. ## Checking the end of charge cut-off on a battery charger This is the least spectacular function and the most important one. Detecting and managing the end of charge is what distinguishes a good unit from one that should be set aside. The check has two stages. Let the charge run to its end, take the cell out as soon as the indicator changes, and measure its voltage on the multimeter within the minute. Repeat the measurement an hour later, once the voltage has settled to its resting level. The first value tells you where the charger stopped. The second tells you what the cell actually took. A reading that sits meaningfully above the maximum charge voltage printed on the datasheet is grounds for retiring the unit, not a nuance to be argued over. ## What the test and revive functions are worth Screen equipped models advertise extra functions, labelled test, revive, battery refresh or charge control depending on the brand. They are verified like everything else. A revive function aims to restart a cell that has fallen too low to be detected. A capacity test announces a figure in milliampere hours obtained through a charge and discharge cycle, and that figure depends entirely on the current used for the discharge: a test run gently flatters the cell. The protocol therefore never compares two capacities measured by two different units, and notes the discharge current every time it is shown. Without that parameter, the capacity on screen is a number with no frame of reference. ## What the British market puts on the shelf British retailers stock the same families as everywhere else, with Xtar, Efest, Nitecore and Gyrfalcon appearing side by side on the same listing pages, alongside cell formats running from 18650 through 20700 to 21700. Regulation in the United Kingdom lands on the liquid and the tank rather than on the charging accessory: refillable tanks are capped at 2 ml, nicotine containing e-liquid at 20 mg/ml, and those bottles at 10 ml. None of that touches a **battery charger**, which is why the charging aisle here looks so much like the one in a torch shop. What does apply is the age rule. Vaping products are not for sale to under-18s in the United Kingdom, and that covers the cells and chargers sold alongside them. ## Frequently asked questions ### Is a single bay charger more reliable? It mechanically removes the question of shared current, which simplifies the reading. It says nothing at all about the quality of its end of charge cut-off, which is verified in exactly the same way as on a multi bay model. ### Can a vaping cell be charged on a general purpose charger? Only where the chemistry and the format are explicitly listed by the unit. A charger built for nickel metal hydride cells does not follow the same curve as a lithium ion charger, and the stopping point is not the same. ### Which power source should be used during the readings? One only, stated, and kept from one session to the next. A charger fed from a computer port, a mains adapter or a power bank does not deliver the same current, which makes two readings incomparable. ### Should temperature be recorded during charging? It is a useful reading, taken without contact and always at the same point on the body of the cell, with the ambient temperature noted alongside. A temperature with no ambient figure next to it compares to nothing. ### How many cycles are needed to judge a charger? One cycle is enough to catch a failing cut-off, which is an immediate defect. Judging consistency takes a repeated series on the same cells, with the same current and the same source. ## Related reading - [Vaporesso Gen Max: the test grid for a 220 W box](/uk/vaporesso-gen-max-review/) - [Vaptio Cosmo 2: what we measure on a small pod](/uk/vaptio-cosmo-2-review/) ## What the bench keeps on choosing a battery charger A **battery charger** is qualified in three readings, the real current per bay, the balance between bays and the cut-off voltage, and all three can be repeated at home with a tester, a multimeter and two identical cells. The number of bays and the screen are conveniences of use; they do not enter the scoring. The same demand applies to devices that carry their own battery, such as the [tube pods that charge through a port](/uk/doric-60-pro-review/), where the curve is measured at the socket rather than at the bay. Cells travel in a dedicated case, wraps stay intact, and the datasheet remains the only document that says what a given cell will take. 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.