A battery's nameplate tells you how much energy it holds when it is new, warm, and fully charged. That is almost never the number you should design around. The capacity you can plan to use each cycle — safely, repeatedly, and years from now — is smaller, and how much smaller is a set of decisions you make on purpose, not a single figure printed on a label.

This guide separates the different meanings of "capacity", explains what shrinks the usable figure, and walks through a short worksheet that turns a nameplate into a realistic planning number with every assumption on show. It assumes you can already convert between watt-hours and amp-hours.

Five words that all get called "capacity"

Confusion starts because one word is doing five jobs. Keep them apart:

  • Nominal capacity — the nameplate figure (for example 100 Ah, or 1 280 Wh at a nominal voltage). A reference value for a new battery.
  • Rated capacity — the capacity measured under a stated test: a particular discharge current, temperature, and cut-off voltage. Change the test and the number changes.
  • Usable capacity — the portion you deliberately plan to draw each cycle, after holding back a reserve. This is a design choice.
  • Available capacity — what the battery can actually give right now, given today's temperature, discharge rate, and age. It can be below your usable figure on a cold day or under heavy load.
  • Retained capacity (state of health) — the fraction of original capacity an aged battery still holds. It falls over the battery's life.

The gap between the first word and the rest is the whole subject of this article.

Usable capacity is a design choice

The single biggest reason usable is less than nominal is that most batteries should not be fully emptied every cycle. How deeply you discharge — the depth of discharge (DoD) — trades runtime against lifespan, and the trade differs sharply by chemistry.

For flooded lead-acid, the reserve is large and non-negotiable. Trojan, a long-standing manufacturer, states plainly that "50% (or less) discharges are recommended," that "80% discharge is the maximum safe discharge," and that you should "not fully discharge flooded batteries (80% or more) — this will damage (or kill) the battery." In practice that means a 100 Ah lead-acid battery offers roughly 50 Ah of comfortably usable capacity. Half the nameplate is reserve by design.

For lithium iron phosphate (LiFePO₄), you can use much more of the pack, but depth still trades against cycle life. A representative LiFePO₄ battery is rated for up to 3 000 cycles at 100% depth of discharge but up to 5 500 cycles at 60%. Nothing forbids the deep cycles; you simply get more of them if you leave some reserve. So "usable" for lithium is less about a hard safety limit and more about how long you want the battery to last.

The takeaway is the same for both: pick a depth of discharge deliberately, and treat the rest as reserve. Deeper means more energy now and fewer cycles later; shallower means the opposite.

The BMS sets the outer boundary

Most lithium batteries include a battery management system (BMS) that enforces hard limits and disconnects outside them. On the example battery, the BMS "monitors the voltage and temperature of the cells during both charging and discharging, and if dangerous values are exceeded it cuts off the power supply."

Two consequences matter for capacity planning:

  • The practically usable window ends where the BMS acts, not at a theoretical 0% or 100%. Even "100% DoD" leaves a protective margin the BMS holds back.
  • The BMS is a safety limit, not a suggestion. Reaching for reserve capacity by bypassing or defeating it removes the protection that makes deep cycling safe.

Available capacity: rate, temperature, and a hard charging limit

Even after you have chosen a usable window, the energy actually available depends on conditions on the day.

Discharge rate. Capacity is not a fixed number. As Trojan puts it, "the capacity of a battery, in Ah, is a dynamic number that is dependent on the discharge current" — a slower discharge yields more amp-hours than a fast one. Lead-acid is strongly affected; lithium much less so, but not immune. If your rating was measured at a gentle discharge and you pull hard, expect less.

Temperature. Cold slows the chemistry and lowers the capacity you can draw, so a battery gives less on a winter morning than on a mild afternoon. Use the manufacturer's temperature-derating figures rather than a guess.

A hard charging limit — this one is a safety point, not just a performance one. Charging most lithium-ion and LiFePO₄ batteries below about 0 °C (32 °F) drives metallic lithium plating on the anode. Peer-reviewed work describes this as causing "a drastic capacity loss, in addition to safety hazards" — and the damage is permanent, not a temporary cold-weather dip. This is why data sheets set a minimum charge temperature (0 °C for the example battery) that is stricter than the discharge range (which may extend to −10 °C / 14 °F or lower). Respect the manufacturer's charge temperature limit and rely on the BMS's low-temperature cutoff; never force charging into a cold battery.

Retained capacity: plan for an aged battery

A battery does not keep its nameplate. Capacity fades with every cycle and with calendar time, which is exactly why cycle life is quoted as a number of cycles — each rating implicitly runs until the capacity has fallen to an end-of-life threshold the manufacturer defines. Design around the capacity you will have late in the battery's life, not the day-one figure, or the system that was comfortable when new will fall short years later.

A usable-capacity worksheet

Put the pieces together on a real number. Start from a 1 280 Wh nominal LiFePO₄ battery and make each assumption explicit.

Step 1 — apply your design depth of discharge. Choose to use 80% and hold 20% in reserve for cycle life and margin:

Usable (new) = 0.80 × 1 280 Wh = 1 024 Wh

Check it the other way: reserve = 0.20 × 1 280 = 256 Wh, and 1 280 − 256 = 1 024 Wh. Consistent.

Step 2 — allow for ageing. Plan for a battery that has faded to about 80% of its original capacity near end of life (an assumption you should replace with your manufacturer's end-of-life figure):

Usable (planning) = 0.80 × 1 024 Wh = 819 Wh ≈ 820 Wh

So a 1 280 Wh nameplate becomes roughly 820 Wh of dependable planning energy — about 64% of the label — before you have even accounted for a cold day or a hard discharge, which would lower the available figure further still. Size your system around the ~820 Wh, not the 1 280 Wh.

Sensitivity. The assumptions drive the answer, so keep them visible. At a gentler 60% depth of discharge, Step 1 gives 0.60 × 1 280 = 768 Wh of usable energy when new — less per cycle, but many more cycles over the battery's life. There is no single "correct" number, only a documented choice.

Do not double-count your margins

A common planning error is to stack reserves that mean the same thing. If you have already set an 80% depth of discharge to protect cycle life, do not also subtract a separate unexplained "safety margin" for the same purpose — you would be reserving the reserve. Decide what each margin is for:

  • Depth-of-discharge reserve → cycle life and avoiding deep over-discharge.
  • Ageing allowance → capacity you will lose over the years.
  • Temperature derating → capacity unavailable on a cold day.

Apply each once, for its own reason, and note which is which. Hiding a recommendation inside an unexplained margin makes the design impossible to audit later.

Before you rely on a number

  • Do not infer a battery's health from voltage or a monitor's percentage alone; a state-of-charge estimate is not a direct measurement of retained capacity.
  • Never charge a lithium battery below its stated minimum charge temperature, and do not bypass the BMS to reach reserve capacity.
  • Stop and seek qualified help if a battery swells, gets hot, gives off odour, or if the BMS repeatedly disconnects for a reason you cannot explain.
  • The manufacturer's data sheet governs the real usable window, temperature limits, and derating for your specific battery.

Key takeaways

  • Nominal is the nameplate; usable, available, and retained capacity are all smaller and more conditional.
  • Usable capacity is a deliberate depth-of-discharge choice — about 50% for flooded lead-acid, deeper for LiFePO₄ but still a cycle-life trade-off.
  • Rate and temperature lower the capacity available on the day; charging below ~0 °C permanently damages most lithium batteries.
  • Plan around aged, not new, capacity, and apply each margin once for a stated reason. A 1 280 Wh nameplate can be closer to ~820 Wh of dependable energy.

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