Short answer: vent it. Build the outdoor charging locker as a Type 3R enclosure with louvered vents low and high, and it holds in the low 40s C on a 35 C afternoon; sealed, the same box climbs to about 54 C. The arithmetic is lopsided. A 100 W e-bike charger wastes 10 to 20 percent of its input power as heat, and the battery adds a few watts of its own, so the locker has to shed about 20 W during bulk charging. A sealed 20 x 6 x 6 in stainless box passes about 1 W to the air per degree C of rise, and that forces a rise of about 19 C. Two pairs of 3 in louvered vents carry that heat out by natural convection and hold the rise near 10 C. Order it in 304 stainless, or 316L where salt air reaches your wall.
The heat the locker has to shed
An e-bike charger is a small switching supply running 80 to 90 percent efficient at full load, so 10 to 20 percent of its input power leaves as heat in the case. The battery turns about 5 percent of the charge power into heat as well, from cell resistance and the protection board. Add both for the bulk phase, when current peaks; in the last hour of the charge the current tapers and the heat tapers with it.
| Charger input | Charger heat at 10-20 % loss | Battery heat at ~5 % of throughput | Total in bulk |
|---|---|---|---|
| 50 W | 5-10 W | 2-4 W | 8-14 W |
| 100 W | 10-20 W | 4-5 W | 15-25 W |
| 200 W fast charger | 20-40 W | 9-10 W | 30-50 W |
The 100 W row is the working case for this article, and the ceiling comes from the battery: most battery management systems charge only between 0 and 45 C, and most chargers carry a 40 C ambient nameplate. Hold the locker at or below 45 C and you stay inside the pack's window, which at 35 C ambient leaves you a 10 C rise budget.
Why the sealed version climbs past the charge window
A sealed box cools only through its skin, and this skin is small. Use 3.7 W per square metre per degree C for a bare stainless wall — the working coefficient for enclosure walls, set mostly by the air films on both sides of the metal. The wall behind the box blocks the back face, so count the five exposed faces of the 20 x 6 x 6 in body: 432 in², or 0.28 m².
The bulk heat through that skin needs a rise of about 19 C, and 35 C plus 19 C puts the locker at 54 C — nine degrees past the charge window on a day that is merely warm. Even at 30 C ambient you land near 49 C. Sun makes it worse rather than better: direct sun loads a small enclosure with more heat than the charger does, so keep the locker in the structure's shade. Solar heat load on an outdoor enclosure has those numbers, and a sun shield buys most of the gain back.
The wall material barely rescues the sealed design. A plastic wall works at 3.5 W/m²K and an aluminum wall behaves like stainless, because the air films dominate; the full radiation, convection and conduction heat-loss formulas show where the coefficient comes from. Sealed cooling is a surface game: a 24 x 12 x 8 in body exposes about 0.56 m², sheds about 21 W at the same 10 C rise, and covers the load. When the site forces a sealed box, that surface — or a charger mounted on a conduction path, as in using a sealed enclosure as a heat sink — is the way out.
Type 3R is the rating that fits
NEMA 250 defines Type 3R for outdoor use: it keeps out falling dirt, stands up to rain, sleet and snow, and the box stays sound with ice forming on it. That is the right fit for a locker under an open-sided roof — an outdoor rating that tolerates ventilation. Louver kits sold for NEMA 3R service are standard hardware: they pass air, equalize internal and external pressure and temperature, and shed falling water while the design keeps its rating.
One rule settles the paperwork: the weakest part sets the rating. Louvers in the wall make the whole locker a 3R design, so start there instead of converting a 4X box — what ventilation holes cost a sealed rating works through that trade. A membrane breather vent keeps more of the rating but passes vapor for pressure equalization, not airflow; whether breather vents keep a NEMA 4X rating covers them, and they will not move this much heat.
Every NEMA type and IP code in this article is a manufacturer rating of the design; our enclosures are rated by us and not UL listed. A UL listed enclosure has been evaluated by UL to UL 50E. If your site wants a listed box, order one from the start, because louvers cut into a listed enclosure afterwards leave you holding a manufacturer rating anyway.
Sizing the vents for a 20 x 6 x 6 in locker at 35 C ambient
Inputs, each with its source:
- Body: 20 x 6 x 6 in [508 x 152 x 152 mm], the 20 in dimension vertical, wall-mounted under an open-sided roof — the working size for a standing e-bike pack plus its charger.
- Heat: 100 W charger at 85 % efficiency gives 15 W; the pack's ~5 % of 85 W gives 4 W; from the table above, call it 20 W in bulk.
- Ambient 35 C, ceiling 45 C, so the rise budget is 10 C — the design day for a hot July afternoon.
- Air at 35 C: density 1.15 kg/m³, specific heat 1005 J/kg·K.
The rule in words: the airflow you need is the heat divided by (density × specific heat × the air temperature rise you allow on the way through). The flow a vent pair delivers comes from stack effect — air 15 C warmer inside weighs less, and the pressure difference pushes cool air in the bottom louver and warm air out the top.
- Airflow needed: 20 ÷ (1.15 × 1005 × 10) = 0.0017 m³/s, about 3.7 CFM [6.2 m³/h].
- Stack drive with louver centers 15 in [381 mm] apart: 1.15 × 9.81 × 0.38 × (15 ÷ 308) ≈ 0.2 Pa. That pushes air through two 3 in [76 mm] louvers — counting half the opening as free area — at roughly 2 CFM [3.4 m³/h] per pair.
- Carried heat per pair at a 10 C air rise: 0.00094 m³/s × 1.15 × 1005 × 10 ≈ 11 W.
Two pairs — one louver low on each side face, one high — give about 4 CFM and 22 W of carry, which covers the bulk load with 10 percent margin. The locker peaks near 45 C during bulk on the hottest afternoon and falls into the 30s as the charge tapers.
Take the rule with you: one 3 in louver pair per 10 W. Under 10 W, one pair; from 10 to 30 W, two pairs; above 30 W, fit 3 x 6 in [76 x 152 mm] louvers rather than more holes, or give the locker more surface. Two details buy real margin. Mount the 20 in dimension vertical — a sideways box has 4 in of stack and moves barely 1 CFM. And if you want the charger inside its 40 C nameplate on the design day, size for a 5 C rise, about 7 CFM here, which means roughly double the free area. Sizing inlet and outlet vents for a naturally ventilated enclosure carries the stack-effect math further for taller boxes.
Rain, cords and cold mornings
Falling rain is what 3R louvers are shaped against: the slats face down and shed it. Under an open-sided roof the locker sees even less, so put the louvers on the faces the weather reaches last. Wind-driven spray is past what 3R promises — no hose test stands behind it — and that is the one condition that pushes you to a sealed 4X build.
Cords are the other water path. Bring the charger cord through a 1 in [25 mm] hole in the bottom face with a grommet, and hang a 4 in [100 mm] drip loop below it so water tracking the cord drops off at the loop. The receptacle behind the locker wants a weatherproof cover that closes over the plug, plus the GFCI protection the NEC requires for outdoor receptacles. Kill the breaker and prove the receptacle dead before you touch the wiring.
Winter looks after itself. The battery management system refuses to charge below 0 C, so a frozen locker idles until the pack warms. A vented box also tracks outdoor air, so it traps less moisture than a sealed box breathing humid air through daily temperature cycles — enclosure vent or sealed box: which stays dry outdoors has the detail.
The decision by where the locker lives
The 3R versus 4 comparison covers the ratings in depth; the short version for this job:
| Where it lives | Rating and cooling | Material |
|---|---|---|
| Sheltered wall under an open-sided roof | Type 3R, two 3 in louver pairs, low and high | 304 stainless |
| Rain and snow beat directly on the wall | Same 3R build, louvers on the sheltered faces, gasketed door | 304 stainless |
| Coastal air or winter road salt | Same vented 3R build | 316L stainless |
| Hose-down or heavy blowing dust | Sealed 4X, no louvers; double the surface with a 24 x 12 x 8 in body or mount the charger on a conduction path | 304 or 316L |
The enclosure selector walks the same choice by environment and inside size when you want to compare boxes side by side.
What to put on the order
| Line | What to write |
|---|---|
| Material | 304 stainless, #4 brushed (316L where salt reaches it) |
| Outside size | 20 x 6 x 6 in [508 x 152 x 152 mm], 20 in vertical |
| Body | One-piece, hinged door, lockable stainless latch |
| Rating | Type 3R by the manufacturer, vented design, not UL listed |
| Cutouts | Four 3 in [76 mm] square louver openings, two low and two high on the side faces, centers about 15 in [381 mm] apart, 3R louver kits fitted |
| Cord entry | One 1 in [25 mm] hole in the bottom face, grommet fitted |
| Delivery | 10 business days anywhere in the world, counted from your approval of the drawing |
The catalog stainless line comes as a manufacturer-rated NEMA 4X-type / IP66 box, which is the right shell for the sealed rows above; the stainless size finder lists every size from 6 x 4 x 4 in up. The vented 3R locker is a custom enclosure: you set the material, the dimensions and every cutout, we send the drawing, and the 10 business days start when you approve it. Orders run through the Industrial Monitor Direct checkout, which builds, ships and supports every one.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 6 × 4 × 4 in | 304 stainless | 6 x 4 x 4 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 30 × 24 × 10 in | 304 stainless | 30 x 24 x 10 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Will rain get through the louvered vents?
Falling rain, no: 3R louver kits are built with downward slats that shed it, and under an open-sided roof they see little weather. Wind-driven spray is past what Type 3R covers, because there is no hose test behind the rating. If the wall takes spray or washdown, go sealed 4X and cool the charger by conduction instead.
How hot can a lithium-ion battery get while charging?
Most battery management systems charge only between 0 and 45 C and stop outside that band. That 45 C is the design limit used throughout: a 10 C rise at 35 C ambient lands right on it. Heat also ages the pack — as a rule of thumb, every 10 C above 25 C roughly doubles the aging rate — so a locker that runs cool holds its capacity longer.
How much heat can a sealed 20 x 6 x 6 in box shed?
About 1 W per degree C of rise over ambient, counting the five exposed faces at 0.28 m² and 3.7 W/m²K for stainless. At the 10 C rise budget that is only 10 W, so the bulk load forces a rise of about 19 C. Twice the surface sheds twice the watts.
Do the louvered vents lower the enclosure's rating?
They set it: an assembly carries the rating of its weakest part, so a louvered locker is a Type 3R design from day one. A membrane breather vent keeps a higher rating, but it passes vapor for pressure equalization, not airflow, and it moves essentially none of the charging heat. Louvers for cooling, breathers for pressure.
Would an aluminum or plastic locker run cooler than stainless?
Not meaningfully. The wall coefficient is 3.7 W/m²K for stainless or aluminum and 3.5 W/m²K for plastic, because the air films on each side dominate. That 5 percent difference moves a 19 C sealed rise by about 1 C. Pick the material for corrosion, strength and locking hardware, not for heat.
