What this sheet works out
What the pack does under load
How these are worked out — the equations behind this sheet, their sources, and what each one assumes.
The pack at each condition
Where people go wrong
The errors this method actually invites, and what it does about each. Every one of them is already a warning somewhere — in a note that appears when your own figures trip it, in the scope statement, or in the equations behind the page. Collected here so you can read them before you need them.
Working the current out as power over voltage. The pack sags as it works, so the current is the smaller root of a quadratic rather than a division. On a big pack loafing the difference is a fraction of a per cent; on a small or tired one it is not, and this sheet shows both so you can see which case you are in.
Reading the power ceiling as available power. At the peak of that parabola half the energy is going into heat and the voltage has halved. A third of the theoretical maximum is a usable ceiling; the peak itself is a number to stay well away from.
Assuming the internal resistance holds all the way down. A fixed resistance describes a pack well at moderate load and badly near the end of a discharge or in the cold. Real cells sag more as they empty, more when cold, and roughly twice as much by the end of their life.
Sizing to the label. When the sheet says no solution exists it means the pack physically cannot deliver that power, whatever the C rating printed on it claims. That is an answer, not a failure to compute one.