Here is a strange fact: a battery does not contain a fixed amount of electricity. The same battery gives you MORE if you take it out slowly and LESS if you grab it fast. That sounds like cheating, but it is honest physics, and it was measured carefully back in 1897 by a German engineer named Wilhelm Peukert. His little law quietly runs the battery world — every forklift shift, every backup room, every sizing calculation we do — and it can be understood completely with one picture: a runner.
The runner in every battery
Ask a runner to walk, and she can go all day — thirty kilometres, no drama. Ask the same runner to SPRINT, and she is finished in a few hundred metres. Same runner, same legs, same breakfast — but the total distance she covers before exhaustion depends entirely on how fast you make her go. A battery is exactly this runner. Drain it gently and it delivers its full promise of ampere-hours. Drain it hard and it gives up early — not because the energy was never there, but because rushing wastes it.
Why rushing wastes it
Two things happen inside a hurried battery. First, the chemistry needs its ingredients DELIVERED: the acid must soak into the deep pores of the plates to react there. Take current slowly and the acid keeps up, feeding the whole thickness of the plate. Take current fast and only the surface layers get fed — the deep material sits unused while the voltage collapses, like a kitchen serving only the tables near the door. Second, every battery has a little internal resistance, and the heat it wastes grows sharply with current — sprint, and more of your energy leaves as warmth instead of work. Slow is thorough; fast is shallow and hot. That is the entire secret.
The law itself — one line, no fear
Peukert wrote the behaviour as a formula: Ik × t = a constant — current (raised to a power k), times time, always multiplies out the same. The only character you need to meet is k, which is the battery’s honesty number. If k were exactly 1, the battery would be perfect: double the current, exactly halve the time, no penalty. Real lead-acid batteries have a k typically quoted around 1.1 to 1.3 — a little more than 1 — and that small excess is the sprint tax. The higher the k, the more the battery punishes hurry. In practice the effect looks like this: a cell rated 100 ampere-hours at the gentle 20-hour rate might hand you only around 70 if you demand everything in a single hour. Nothing is broken; that is simply the law collecting its tax. (How makers turn this into the C20, C10 and C5 ratings on every datasheet is the story of the C-rate guide — this article’s practical twin.)
Out of breath, not empty
Here is the kindest part of the law, and the least known. The capacity a sprint “loses” is not destroyed. Stop the discharge, let the battery rest, and the acid quietly finishes its journey into the deep pores — and a good share of the missing capacity reappears, ready to be used at a gentler pace. The sprinting runner wasn’t empty; she was out of breath. This is why a forklift battery that sags at the end of a brutal shift can still light a lamp all evening, and why a rest pause is a legitimate tool in battery operations, not a superstition.
Where the law stops — and what to do instead
A storehouse article owes you the fine print. Peukert’s k is not a perfect constant: it drifts at temperature extremes and at very high or very low rates, so serious engineers never stretch the formula far beyond the rates it was measured at — they read the manufacturer’s rate table, which is Peukert’s law with the guesswork removed. And the law is chemistry-specific: lithium-ion cells hold their capacity much more stubbornly as rates climb, one of that chemistry’s real advantages. None of this weakens the lesson; it sharpens it. When you tell us your load and your hours, we look up YOUR rate in OUR tables — measured, not extrapolated — and size the bank so the sprint tax is already paid. That, in one sentence, is what professional sizing is. A battery, like a runner, will happily tell you what it can do — as long as you ask it at the pace you actually intend to run. Ask us with your real duty, and the answer comes back honest.