Most of us look at a battery’s capacity and current ratings when choosing a battery for a regulated device. Staying under a battery’s true current rating can help lower the risk of using these batteries, improve performance, and help to extend their life. But does the capacity rating of a battery help us pick the best one for the way we vape? The short answer is no. It only gives you a very, VERY rough idea of the amount of vaping time you might get. The capacity rating of a battery, in milliamperes-hours (mAh), tells us how long it will run only when discharged at a very low current level. Typically, this is around 0.5A for the batteries we use. When discharging a battery at the much higher current levels we use when vaping the capacity rating doesn’t apply anymore. Due to the voltage sag of the battery it ends up delivering much less capacity than if it was discharged very slowly. Making matters worse is that different batteries have different levels of voltage sag, even if their current and capacity ratings are the same. For example, with two 3000mAh 20A 18650 batteries one of them could deliver 1100mAh when discharged at 20A down to 3.2V (a typical regulated mod low voltage cutoff point) but the other might only deliver 400mAh! This means one will run almost three times longer than the other before needing recharging. These two batteries have the same current and capacity ratings but the difference in their efficiency and voltage sag (determined by their internal resistance) results in hugely different performance at higher current levels. So how can we choose a good battery for use in a regulated device? We can use their watt-hour (Wh) specifications. Capacity uses milliamperes-hours (mAh), a combination of current and time. But it doesn’t take into account how efficiently a battery runs or how badly its voltage sags. It also doesn’t take into account the differences in chemistry between different batteries. Some are much more efficient when warm and others aren’t. This greater efficiency helps the battery run longer before the voltage drops too far. The capacity rating doesn’t tell you anything about this. The watt-hour (Wh) spec gives you more information about how the battery actually behaves when discharged at different current levels. It combines the current and time numbers that capacity gives us along with voltage over the entire discharge to tell us the total amount of energy a battery can deliver. You can see this by remembering that Watts = Volts x Amps. That means watt-hours are volt-amp-hours or volt-milliampere-hours. That’s our milliamperes-hour capacity spec with the voltage also included. How does that help us choose a battery? Well, with the Wh specs for two batteries we can easily pick the better performing one for a regulated device, that is, the one that will deliver more energy for the way we vape. The higher the Wh spec, the better. For example, let’s assume you’re vaping at 60W per battery, about 20A***, down to the mod’s cutoff voltage of 3.2V and let’s compare the two batteries in the image below. The 20A-rated 3000mAh LG HG2 delivers 4.9Wh of energy at 20A (shown below, circled in red). But the 25A-rated 2400mAh Samsung 24S delivers 5.7Wh of energy at 20A (shown below, circled in red)! Even though the Samsung 24S is a lower capacity battery it will run for longer in a regulated device at 60W each (120W in a two battery device, etc). This is because the more efficient Samsung 24S can hold its voltage up for longer than the LG HG2. At high power levels it can even hold its voltage up long enough to compensate for the HG2’s much greater capacity. At lower power levels though the greater efficiency of the Samsung 24S doesn’t matter as much. At 10A, about 30W***, the LG HG2 delivers 7.5Wh of energy but Samsung 24S only delivers 6.5Wh. At lower power levels the LG HG2 is the better choice of these two batteries for a regulated mod. At around 15A/45W each they will both run for about the same amount of time. The Samsung 24S’ higher current rating of 25A means it will run a little cooler though and this can help extend overall battery life. So while the capacity rating of a battery might give you a very rough idea of how two batteries might compare the Wh spec can tell you which one will actually perform better at the power levels you vape at. I don’t have Wh specs for every battery I have tested, just the ones I have tested in the past 1-1/2 years or so. You can check the Wh spec in the ratings graphic for those batteries in the test reports I haves posted at ECF (List of Battery Tests | E-Cigarette Forum To summarize...If you want the best performance and longest running time for your batteries then don’t shop for batteries just by choosing the highest capacity (mAh). Choose the batteries with a continuous current rating high enough for the way you vape and then choose the one that has the highest Wh spec at, or near, the level of current you will be drawing from each battery. _________________________________________________________ ***To calculate APPROXIMATELY how much current you will draw from each battery in a regulated device just divide the wattage each battery handles by three. For example, if you have a two-battery device and you are vaping at 120W that is 120W / 2 batteries = 60W per battery. The current drawn from each battery is then 60W / 3 = 20A (approximately). It does not matter if the batteries are wired in series or parallel, each handles an equal share of the total wattage you are using.