Pick the battery by its charge limit, not its discharge limit — that one rule settles most of a solar enclosure in a cold climate. An AGM battery charges down to about -20 C, so a -18 C morning is still inside its datasheet. A standard lithium pack must sit out any morning below 0 C. NiCd carries the garden-light record, but its printed charge floor is 0 C too. Put 25 mm of closed-cell foam around the battery, give the charge controller temperature compensation, and mount the enclosure in the shade. The shade does more for battery life than any heater you can bolt in.
The design case in five numbers
You are sizing for a solar camera that sleeps most of the day. The average load is about 10 mA at 12 V, or 120 mW, and the climate does the rest of the talking. Five numbers define the case, and each one rules something out.
| Number | What it does to the design |
|---|---|
| -18 C (0 F) charging morning | standard lithium stops below 0 C; AGM holds to about -20 C |
| -34 C (-30 F) discharge night | every printed discharge floor sits warmer, so deep cold means the pad or a sleep-through night |
| 38 C (100 F) summer afternoon | a box in full sun adds roughly 10-20 C at midday |
| 87-100% relative humidity | filter fans and vents turn into maintenance contracts |
| Dust at eave and farm height | every gasket and gland has to hold the full rating |
Two of the five do the real damage. The -18 C charge morning disqualifies standard lithium unless the controller blocks the charge, and the 38 C afternoon shortens battery life faster than any cold night does. The -34 C discharge looks worse than it is: a cold discharge borrows capacity for one night and hands it back when the pack warms, while the permanent damage all happens on the charge side.
Charge floors decide, discharge floors do not
Charging below a chemistry's floor does damage that never heals. Lithium plates metal onto the anode below 0 C, the lost capacity never comes back, and the plated metal raises the risk of an internal short. Lead-acid charged cold without voltage compensation chronically undercharges and sulfates; charged hot without it, it overcharges and gasses. That is why AGM datasheets require temperature compensation whenever the battery sits below 10 C or above 30 C for long periods, at the usual charger figure of about 3 mV per cell per C. Discharge is gentler: the pack gives up capacity for the night and takes it back at breakfast.
| Chemistry | Charge floor | Discharge | Heat behavior | Verdict here |
|---|---|---|---|---|
| Standard LiFePO4 / Li-ion | 0 C; colder charging plates lithium | to about -20 C on cell datasheets | ages quickly above 30 C | only with a 0 C charge lockout |
| AGM (VRLA) | about -20 C on small-sealed datasheets | about -20 C printed; capacity down when cold | float life halves every 8-10 C above 25 C | the default pick |
| NiCd | 0-45 C as printed by cell makers | -20 to 65 C printed | tolerates 65 C, the heat champion | garden-light proven; design to the printed floor |
Your -18 C morning clears the AGM floor with 2 C to spare, so AGM carries this case without a heater. Lithium wins on weight and cycle life, and a camera cycles shallowly, so if you want it, buy the 0 C lockout, not hope. NiCd has earned its garden-light reputation: at 10 mA into a 7 Ah brick the charge current is C/700, a trickle gentle enough that cells ride through cold snaps far outside their printed numbers. Design to the datasheet anyway and let the trickle be the margin.
Worked example: how long a 12 V 7 Ah AGM rides out a -34 C night
The inputs, with their sources:
- Battery: 12 V, 7 Ah AGM at 2.5 kg. Example assumption — datasheets for bricks this size list 2.4-2.8 kg, so use your pack's real mass.
- Compartment: 8 x 6 x 6 in (203 x 152 x 152 mm) around the brick, all six faces lined. Example geometry; measure yours.
- Insulation: 25 mm closed-cell polyethylene foam at about 0.035 W/m·K, the figure insulation datasheets print.
- Night: -34 C ambient, the design-case low.
- Start: +5 C inside at dusk. Assumption to replace — a shaded winter box holds a few degrees above the afternoon air.
- Floor to protect: -20 C, the AGM charge floor this article designs to.
The rule, in words: a warm body cooling to steady air falls on an exponential, so the time to slide from T0 to T1 is (C / UA) x ln((T0 - Ta) / (T1 - Ta)), where C is mass times effective specific heat, UA is k times area divided by thickness, and Ta is ambient.
The arithmetic:
- C = 2.5 kg × 1000 J/kg·K = 2500 J/K. An AGM's effective specific heat is about 1000 J/kg·K: acid and case dominate, and lead's 130 J/kg·K pulls the average down from water's 4180.
- U = 0.035 / 0.025 = 1.4 W/m²K. Area = 2 x (0.203 x 0.152) + 2 x (0.203 x 0.152) + 2 x (0.152 x 0.152) = 0.17 m². UA = 1.4 × 0.17 = 0.24 W/K.
- Headroom at dusk: 5 - (-34) = 39 K. Left at the floor: -20 - (-34) = 14 K.
- t = (2500 / 0.24) x ln(39 / 14) = 2.9 h × 1.02 ≈ 3 h.
Result: the battery sits above its charge floor for about 3 h after sunset and spends the rest of the night below it. Foam buys hours, not the night.
The decision, with its threshold: holding the whole night on insulation alone needs UA = 2500 × 1.02 / 86,400 ≈ 0.030 W/K, which at 0.035 W/m·K over 0.17 m² means about 200 mm of foam — a different product. Above 3 h of cold, add a 5 W silicone pad inside the foam: 5 / 0.24 = 21 K of lift settles the pack near -13 C, over the floor. Budget it honestly: 5 W for a 12 h night is 60 Wh, while a 10 W panel makes 20-40 Wh at 2-4 winter sun-hours at US latitudes, so run the pad on a thermostat that closes below -20 C. Your electronics cannot do this job: 0.12 W into this compartment lifts it 0.12 / 0.24 = 0.5 K. For the same construction sized around a lithium pack, sizing an insulated battery box for LiFePO4 walks the same numbers.
Heat is the shorter clock: shade the box
38 C of air is already 13 C above the 25 C reference every battery datasheet uses for calendar life. A sun-hit box adds roughly 10-20 C at midday, so the pack floats at 50-58 C through the afternoon. Battery datasheets print the halving rule: life halves for every 8-10 C above 25 C. A pack rated 5 years at 25 C lasts about 6 months at 55 C — (55 - 25) / 9 ≈ 3.3 halvings, and 2^-3.3 is about a tenth. Held at 30 C instead of 50 C, the same pack lives roughly four to five times as long.
Mount under the eaves or on the north face and a shaded stainless shell tracks air temperature within a few degrees. The solar heat load on an outdoor enclosure arithmetic shows what the sun adds when shade is out of reach, and the solar shield numbers for an outdoor enclosure show what a shield buys. NiCd heads the hot list with a printed 65 C discharge ceiling; AGM runs at 38 C but at about a third of its 25 C float life there.
Seal it: dust, 100% humidity, and no fan
Nobody climbs a ladder to wash a fan filter, and at 120 mW the electronics gain nothing from airflow anyway. Seal the box and drop every moving part: a sealed board drawing camera power stays cool, which whether a low-power board overheats in a sealed outdoor enclosure works through. Fit cable glands rated for the full IP66, because the weakest part sets the rating — one IP54 gland turns the whole assembly into an IP54 box.
Keep the enclosure closed through charging. A small VRLA recombines its own gas at these trickle currents, which is why sealed batteries go in sealed boxes; for the big-battery version of the argument, vented or sealed battery charging enclosures works the arithmetic. Swap in desiccant sachets at each site visit to dry the air the door seals in.
Our stainless wall-mount boxes come manufacturer-rated NEMA 4X / IP66; they are not UL listed. In US installations, NEC 110.28 says an IP code is not a substitute for the enclosure type number, so a US specification should carry both; the outdoor enclosure ratings guide maps the type numbers to conditions.
What to put on the order
Order the box once, with the thermal work already decided. This table is the whole list:
| Line | To specify | The number that matters |
|---|---|---|
| Enclosure | 304 stainless wall-mount inland, 316L within salt air, gasketed door | manufacturer-rated NEMA 4X / IP66, not UL listed |
| Size | brick plus controller plus 25 mm of foam on every face | a 12 V 7 Ah AGM measures about 6 x 2.6 x 3.7 in (151 x 65 x 94 mm) |
| Battery | AGM 12 V, 7 Ah class; NiCd for heat-soaked sites | charge floor about -20 C |
| Controller | temperature compensation; charge lockout for lithium | compensation below 10 C and above 30 C; lockout at 0 C |
| Insulation | 25 mm closed-cell foam, foil-faced | about 0.035 W/m·K, 0.24 W/K on the example compartment |
| Heater, cold sites | 5 W silicone pad on a thermostat | closes below -20 C, 60 Wh per 12 h night |
| Mount | under eaves or north face, always shaded | keeps the shell within a few degrees of air temperature |
| Consumables | desiccant sachets | swap at each site visit |
Then the decision by site, so you can close the tab:
- Most of the United States: mornings rarely below -10 C, summers to 35 C — AGM, 25 mm of foam, shaded mount, no pad.
- Northern winters: -18 C charge mornings — still AGM, clearing its -20 C floor with 2 C to spare, compensation on the controller; lithium only behind a 0 C lockout.
- Continental sites: -30 to -34 C nights — the 5 W pad on its thermostat, or a camera that sleeps until the sun warms the pack. NiCd is the alternate chemistry, and the cold temperature electronics enclosure plan lists the rest of the winter work.
Size the enclosure in the stainless enclosure size finder, which filters every wall-mount and HMI box by grade and outside size; give the battery its foam allowance before you fix the depth. If you would rather start from environment and listing, the enclosure selector sorts the same catalog the other way. The box is made to order and delivered in 10 business days, counted from your order or, for the custom layout, from your approval of the drawing.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 10 × 8 × 6 in | 304 stainless | 10 x 8 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 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 |
Questions
Can I charge a LiFePO4 battery below freezing?
Charge below 0 C plates metallic lithium on the anode. The capacity loss never returns, and the plated metal raises the risk of an internal short, so a standard pack has to sit the cold snap out. Discharge down to about -20 C stays inside most cell datasheets, which is why the fix blocks the charge, not the discharge: a controller that holds off below 0 C and releases the moment the cells warm.
Which battery chemistry charges at -18 C?
AGM. Datasheets for small sealed AGM bricks set the charge floor near -20 C, so a -18 C morning clears with 2 C to spare; the same datasheets require charge-voltage compensation whenever the battery sits below 10 C or above 30 C for long stretches. NiCd's printed charge floor is 0 C. Its cold reputation comes from trickle charging in solar garden lights, not from the datasheet.
How much foam does a battery need for a -34 C night?
Foam buys hours, not the night. Over the 0.17 m² compartment of the worked example, 25 mm of closed-cell foam at 0.035 W/m·K gives 0.24 W/K and holds a 2.5 kg AGM above its -20 C charge floor for about 3 h at -34 C ambient. Holding 24 h on insulation alone takes about 200 mm of foam, so for continental cold you add a 5 W thermostat pad or lean on the chemistry.
Will the camera electronics keep the battery warm?
No. The design case draws an average of 10 mA at 12 V, or 120 mW, and 0.12 W into a 0.24 W/K compartment lifts the inside about 0.5 K. At camera power the battery's temperature is set by the sun, the foam and any pad; electronics heat only matters from roughly 10 W up in a box this size.
Does heat or cold kill a solar enclosure battery faster?
Heat, over the years. AGM float life halves for every 8-10 C above 25 C, the figure battery datasheets print, so a pack held at 30 C lives roughly four to five times as long as one at 50 C. Cold borrows capacity for a night and threatens the charge floor; heat burns calendar life that never comes back. Shade is the cheapest life extender you can buy.
