In short
Battery cabinets are rated in watt-hours because energy content, not physical size, determines how much a fire has to be contained. Read Wh from the pack label, or multiply voltage by amp-hours. Add up everything that will be inside at once, then leave headroom for growth.
This is the step most often skipped, and skipping it is how organisations end up with a cabinet that physically fits their packs but is rated for a fraction of their energy content.
What a watt-hour is
A watt-hour (Wh) measures energy: how much work the battery can do, and therefore how much energy is available to be released if it fails. It is calculated as voltage multiplied by amp-hours:
Wh = V × Ah
So a 36 V pack rated at 14 Ah holds 36 × 14 = 504 Wh. A 48 V pack at 14 Ah holds 672 Wh — same physical size class, a third more energy. That is why counting packs tells you very little.
Finding the figure
- Check the label first. Most e-bike and power tool packs state Wh directly, because transport rules require it. If it is there, use it.
- Otherwise multiply. Take the nominal voltage and the amp-hour rating from the label and multiply them. Use nominal voltage, not maximum charge voltage.
- Watch for mAh. Small packs are often rated in milliamp-hours. Divide by 1,000 to get Ah before multiplying — 2,500 mAh is 2.5 Ah.
- If the label is unreadable or absent, that is itself a finding. An unlabelled pack of unknown provenance is a procurement problem before it is a storage problem.
Worked example
| Item | Quantity | Per pack | Subtotal |
|---|---|---|---|
| E-bike packs, 36 V 14 Ah | 6 | 504 Wh | 3,024 Wh |
| E-bike packs, 48 V 14 Ah | 4 | 672 Wh | 2,688 Wh |
| Power tool packs, 18 V 5 Ah | 10 | 90 Wh | 900 Wh |
| Total | 20 | — | 6,612 Wh |
Note what the arithmetic shows: the ten power tool batteries are half the item count and a small fraction of the risk. If you are prioritising, the e-bike packs are the reason you are buying a cabinet.
Headroom
Size for what you will hold, not what you hold today. Three things reliably push the total up:
- Stock growth. Fleets and workshops accumulate packs faster than they retire them.
- Pack sizes rising. Successive generations of e-bike batteries have trended larger. A replacement pack is rarely smaller than the one it replaces.
- Peak versus average. The busy week, not the quiet one, sets the requirement.
A cabinet loaded to its rated limit has no margin at all, and the temptation when it is full is to leave the overflow outside it — which defeats the purpose. Buying a size up is usually cheaper than buying a second cabinet later.
Physical fit still matters
Energy content sets the rating you need; dimensions decide whether it works. Measure the tallest and widest pack you hold, check shelf spacing and whether shelves are adjustable, and confirm packs can be placed with terminals clear of each other. An e-bike pack is usually the awkward one — measure it rather than assuming.
Frequently asked questions
How do I calculate watt-hours from mAh?
Divide the mAh figure by 1,000 to get amp-hours, then multiply by the nominal voltage. A 5,000 mAh pack at 18 V is 5 Ah × 18 V = 90 Wh. Use nominal voltage rather than the maximum charging voltage, which would overstate the figure.
Should I size on total capacity or on the largest single pack?
Total, because propagation between packs is the scenario that matters — one cell failing can take its neighbours with it, so the cabinet may have to contain the lot. The largest single pack is useful for a different purpose: checking physical fit and shelf spacing.
Does state of charge affect the figure I should use?
Size on the rated capacity rather than on the charge level you intend to keep packs at. Storage discipline can slip, packs arrive fully charged, and the rating should cover the worst realistic case rather than the intended one. Partial-charge storage remains worth doing — it just is not something to take credit for when sizing.
What if my total exceeds the largest cabinet available?
Then the answer is not a bigger cabinet. Options at that point are multiple separated cabinets, a dedicated store or container away from the occupied building, or reducing on-site stock. This is the point at which the decision stops being a purchase and becomes a site design question — worth getting surveyed rather than guessing.