#3 · How Many Hours Does a 100Ah Battery Last?
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- Ah (ampere-hours) and Wh (watt-hours) are both units of capacity. Ah is capacity in terms of current (A); Wh is capacity in terms of power (W) — energy (Wh) = nominal voltage (V) × capacity (Ah). So the same 100Ah is 1,280Wh in a 12.8V pack and 2,560Wh in a 25.6V pack.
- Runtime is "capacity ÷ rate of use". When you know the machine's current draw (A), it is Ah ÷ average current (A); when you know its power draw (W), it is Wh ÷ average power (W). The other way round, the capacity you need is current × hours (Ah) or power × hours (Wh).
- Real selection adds a margin to the calculated figure. Leaving 10–20% unused at each end is better for battery life and safety than running from 0% to 100%, so take 1.2–1.3× the required capacity as the minimum margin. Use that rule when choosing a standard pack; for a custom build, send the requirement and POWERCRAFT builds the margin into the quote.
1. Ah and Wh — two units of capacity
A spec sheet lists "nominal capacity 100Ah" and "energy 1,280Wh" side by side. Both describe how much the battery holds; the difference is whether the capacity is measured in terms of current (A) or power (W). Ah is capacity in terms of current — how many hours the pack can keep delivering current (A). Wh is the same capacity expressed in terms of energy — how many hours it can keep delivering power (W).
Power (W) = voltage (V) × current (A)
Energy (Wh) = nominal voltage (V) × nominal capacity (Ah)
2. Machines with a fixed voltage — calculate in Ah
Once you have confirmed the nominal voltage of the battery for your robot or AGV, the next question is "how many amps for how many hours".
Runtime (h) ≈ capacity (Ah) ÷ operating current (A)
Required capacity (Ah) ≈ operating current (A) × target runtime (h)
Worked example. A 24V-class machine drawing an average of 15A on a DM-PWR-25.6V-100Ah runs 100 ÷ 15 = about 6.7 hours. Conversely, if the same machine must run 8 hours per charge, it needs 15 × 8 = 120Ah (plus a margin — see section 4).
"Operating current" here means the average current drawn to run the connected machine. Refer to the "rated current" on the machine's datasheet, but for an accurate capacity estimate the machine's actual operating data, including standby periods, is the most reliable source. When you ask POWERCRAFT how long a standard pack will last, the same method applies: the first question is what current you draw, and the capacity is divided by it.
3. Machines running AC loads — calculate in Wh
Machines that run AC power such as 220V through an inverter usually state their load in W. Here Wh is the convenient unit of capacity.
Runtime (h) ≈ energy (Wh) ÷ operating power (W)
Required energy (Wh) ≈ operating power (W) × target runtime (h)
Worked example. "We run about 3,000W for 3 hours" means 3,000 × 3 = 9,000Wh, i.e. 9kWh is required. Conversely, a DM-PWR-51.2V-50Ah (2,560Wh) under an average 1,000W load runs 2,560 ÷ 1,000 = about 2.6 hours (going through an inverter adds conversion loss, so a further margin is needed — see section 4).
Whether you speak in Ah or Wh, the principle is the same. Multiply current or power by hours and you get the capacity you need; divide capacity by current or power and you get the runtime.
4. Don't pick the calculated figure as-is — a 1.2–1.3× margin
The calculations above assume the pack is charged to 100% and its full nominal capacity is drawn down to 0%. In real operation POWERCRAFT recommends using the battery with its life and safety in mind.
- Leaving 10–20% at each end is better for life and safety. Discharging to 10–20% remaining and charging to 80–90% is easier on the battery than cycling from 0% to 100%.
- Capacity falls with the operating environment and the number of charge cycles. The nominal capacity on the spec sheet can fall further with cold environments, high-discharge operation, and the SoH effect from article #2 (capacity falling as cycles accumulate) (current conditions are covered in article #4; temperature in a later article).
So take 1.2–1.3× the required capacity as the minimum margin and choose a model at or above it. If 15A for 8 hours is required, 120Ah × 1.2–1.3 = 144–156Ah, so select a battery pack of that capacity or more. Allow a further margin where low temperature, high discharge or long service life has to be considered. For a custom build, just put the current (or power) and the target hours in the specification request and POWERCRAFT will quote with a margin suited to your operating conditions.
Once voltage, current (or power) and runtime are settled, pick from the standard lineup by voltage, then capacity. → See all 49 standard battery packs — filter by voltage, capacity and battery type
- Units. Ah and Wh are both units of battery capacity. Ah is based on current (A), Wh on power (W), and voltage links the two (Wh = V × Ah).
- Runtime. "Capacity ÷ rate of use" — Ah ÷ A when you know the current draw, Wh ÷ W when you know the power draw — and required capacity is current or power multiplied by the target hours.
- Margin. Don't pick the calculated figure as-is: allowing for 10–20% unused at each end and for capacity loss from the operating environment and charge cycles, take 1.2–1.3× the required capacity as the minimum margin.
- Selection. Use that rule for a standard pack; for a custom build, just tell POWERCRAFT "how many A (or W) for how many hours" and POWERCRAFT will quote with the margin built in.
— all 49 standard packs can be filtered by voltage, capacity and battery type. If capacity is hard to decide, browse the POWERCRAFT custom build projects and, in the "Required capacity" field of the specification request, give your operating current (or power) and target runtime instead of an Ah figure. POWERCRAFT does the calculation, margin included.