C-rate refers to how much current it takes to discharge the pack in 1 hour. It’s essentially the inverse of the pack capacity. A 10,000 milli-amp-hour (mAh) battery can supply 10 Amps of current for 1 hour, theoretically. If that pack is rated for 1C discharge rate, then it can do that. If it’s rated for 2C, that means that it can actually source 20A for a half hour. At 4C, it can source 40A, at 100C it can source 1000A.

This is, of course, a simplification. There are different rates for very short duration pulses, somewhat short duration pulses (~10s), and continuous (>10s). The shorter duration ratings are often the ones marketed, but the continuous rating is what is usually important. Note that continuous is only defined as 10s duration. When you get into that time scale you have to deal with chemical diffusions, which are a slow process.

There are two ways to look at your C rating, and it’s useful to look at both. One is your peak current. If you need 100 Amps of current for more than 10s (continuous), then you need a pack that’s rated for 100 / capacity  C-rate. But it can be difficult to estimate the continuous current, so if you know your minimum flight time, you can use that to directly calculate the C rate. If you fly for 15 minutes minimum with your biggest payload and highest wind conditions, then you need a 4C pack. Realistically, you’ll want to add some margin since you might have some short bursts of higher current. It’s easy to get into a situation where your power system is theoretically capable of draining your pack in 2 minutes, but in reality you have 15 minutes of flight time so you do need to look at things both ways and use your judgment to select an appropriate pack if you don’t have the capability to simulate or measure actual requirements.

S is series and P is parallel. Series raises voltage and parallel raises Amp-hour capacity. Either configuration raises total energy capacity by the same amount. More parallel cells increases the Amp-hours and current rating of the pack, and more series cells increases the voltage of the pack.

There is a tradeoff between power and energy density that can be compensated for with added cost. High power cells and high energy cells can be made relatively cheaply, but high power cells that are also high energy require complex, expensive design. It’s important to look at the continuous C-rating of the cells and consider that higher voltages might make the whole system more efficient.

When looking at the C-rating, make sure to consider that there is a pulse rating and a continuous rating. Generally anything over 10 seconds is considered continuous so if your peak current happens at takeoff or landing, you need to spec your pack based on that.

A smart battery that can’t communicate with the vehicle has little value.

Cycle life of a pack is worse than that of the cells that it comprises. The weakest link determines the fate of the pack. So looking at average cycle life of cells isn’t directly useful in estimating your pack cycle life. Pack cycle life may be 50% or even worse due to variation in cells and thermal and electrical imbalance in the pack design. If you have a target pack cycle life in mind, make sure to investigate what that means when compared to the cell cycle life because they are not the same thing.

You can increase voltage by adding cells in series. The tradeoff is that for the same weight of battery, you’ll have lower amp-hour capacity, but higher voltage and the same amount of energy. The benefit is that at higher voltages you can draw less current for the same amount of power. This is good because efficiency is inversely proportional to current, so we increase system efficiency by raising voltage. You can use smaller, lighter wires and more compact motors with higher voltage. Higher voltage does come with costs, high voltage electronics tend to cost more, so ultimately voltage tends to scale with vehicle size. However, it’s usually useful to design toward higher voltages if your goals are higher efficiency and longer endurance.

Energy density is simply the energy capacity (voltage * amp-hour capacity) of the pack divided by the weight. The energy of the pack is proportional to your endurance and the weight of the pack is also, inversely, proportional. The battery is usually one of the largest weights and so is targeted for optimization. Energy density at the pack level includes the weight of the enclosure, wires, connectors, BMS, and other packaging materials, and results in a somewhat lower energy density than you would see at the cell level. Cost tends to increase exponentially with increased energy density so, while high energy density packs are extremely valuable for long endurance aircraft, it’s one part of an overall weight reduction strategy that should be discussed and considered carefully.

Generally speaking, yes. Unless you’re discharging so quickly that cooling won’t come into play, maybe 2-5 minutes flight time, cooling can make an important difference in system performance. High energy cells tend to produce more heat and efficiently removing that heat means that you can keep those cells cool and last for many flights. High power, high energy, and sealed systems with no cooling are the worst situation for batteries and typically result in the shortest cycle lives.

Smart batteries process data and communicate with the aircraft and therefore are or will be restricted by country of origin for government use.

• Water resistance
• Shock & vibe
• Temperature extremes, both for charging and discharging
• Required cooling