UPS inside a PLC enclosure: heat and battery life

Short answer: hold the UPS battery at 30 °C (86 °F) or below, and in most plants that means the UPS lives outside the sealed PLC panel, not inside it. Sealed lead-acid (VRLA) manuals publish the number that decides this: for every 10 °C (18 °F) above the 25 °C (77 °F) reference, expected float life roughly halves. At a 40 °C (104 °F) panel interior a 5-year battery reaches about 21 months and a 3-year battery about 13 months, and the typical office-grade 600 VA unit is rated to operate at only 0 to 40 °C, so its electronics are at their limit as well. The worked example below puts real numbers on a 600 VA UPS in a 24 x 20 x 8 in [610 x 508 x 203 mm] panel, then settles the placement for washdown sites and for a 20-30 panel expansion.

Battery life halves every 10 °C above 25 °C

The rule comes from the battery makers' own VRLA manuals: from a 25 °C (77 °F) reference, each 10 °C (18 °F) rise cuts expected float life about in half, because the plate corrosion rate speeds up the same way any chemical reaction does. Field rules that quote 5 °C per halving are the pessimistic version; the published manuals say 10 °C. Heat is the biggest lever you hold over battery life, bigger than brand or float-voltage trim.

Runtime and life are different animals: a hot battery still delivers its minutes of hold-up, then dies young. What heat costs you is years of service, not seconds of backup. Float voltage moves the same curve, because overcharge cooks the plates the way heat does, so the charger belongs on its datasheet float, compensated for temperature — the numbers are in the mounting section below.

What the UPS adds to the panel heat load

Inside the box, a UPS is a small heater that never switches off. The electronics convert power continuously, and the battery sits on float around the clock. Assume the unit wastes about 40 W: a 600 VA office-grade unit at a typical 0.6 power factor pushes 360 W through electronics running roughly 90 % efficient, which leaves about 36 W of loss, plus a few watts of float charge. Replace that assumption with the efficiency and charger draw on your unit's datasheet before sizing anything.

The electronics carry their own limit: office-grade 600 VA units typically list a 0 to 40 °C (32 to 104 °F) operating rating on their datasheets, so a panel that cooks batteries also ages or trips the charger itself. The UPS is rarely alone in there either. The PLC, I/O and a network switch add their own watts and ride the same rise that makes a driver box run hot inside a sealed enclosure.

Worked example: 600 VA UPS in a 24 x 20 x 8 in panel

Take a 24 x 20 x 8 in [610 x 508 x 203 mm] stainless wall-mount panel, 30 °C (86 °F) room ambient, and 60 W of internal dissipation: 20 W of PLC, I/O and network gear, plus the assumed 40 W UPS. The rise follows ΔT = Q / (k × A):

1. Surface area: 2 x (0.61 x 0.51 + 0.61 x 0.20 + 0.51 x 0.20) = 1.07 m². Minus the 0.31 m² back face against the wall: 0.76 m² effective.
2. Wall transfer: k = 3.7 W/m²K for stainless, the value used across this site's heat guides.
3. Conductance: k x A = 3.7 x 0.76 = 2.81 W/K.
4. Rise: 60 W ÷ 2.81 W/K = 21 K.
5. Interior: 30 °C + 21 K = 51 °C.

That fails twice. The charger runs 11 K past its 0 to 40 °C rating, and the battery factor at 51 °C is 2^2.6, about 6.1, so the 5-year battery lasts roughly 10 months and the 3-year battery about 6.

Move the UPS to the wall beside the panel and the numbers come back. Dissipation drops to 20 W, the rise to 20 ÷ 2.81 = 7 K, and the interior to 37 °C — 3 K under the PLC's own 40 °C rating. The battery now sits in 30 °C room air: 42 months for the 5-year unit and 25 for the 3-year, close to four times the life it had inside the box.

Four ways to place the UPS, and where each wins

For a 20-30 panel expansion the central option wins on arithmetic alone. One or two large units in a conditioned room hold their batteries near full rated life and give you one replacement cycle, instead of two dozen small batteries expiring in different months across the plant. Scattered small UPS units are the ones nobody checks until a power dip finds them dead, so set the battery replacement date at commissioning either way. Centralizing also frees the enclosure spec: the panel stays a sealed box with no cooling hardware at all.

For a single washdown panel, wall-mount the UPS beside it in its own small enclosure and run its output into the panel through one sealed gland entry. Where the UPS must stay inside, cooling keeps the seal: see the cooling options for a sealed enclosure in a hot ambient, and for going below ambient, an air conditioner or vortex cooler on a sealed panel. Typical vortex catalog data at 90 psi (6 bar) shows a 15 scfm (7 L/s) model delivering about 1,500 Btu/h [440 W] of cooling, an order of magnitude more than a 40 W UPS needs. The trade is clean, dry air, because oil or water in the supply rides the cold stream into the panel.

Rating and material for the panel

With the UPS out of the box, the enclosure spec stays simple. Manufacturer-rated NEMA 4X / IP66 stainless for washdown and outdoor, a dust-tight indoor type for clean rooms; our wall-mount line is rated NEMA 4X / IP66 by the manufacturer and is not UL listed. The rating holds only as built, because the weakest part sets the assembly rating: a fan-filter louver, an open vent or an unrated gland drags a Type 4X panel down to that part's level. A panel you specified for hose-down cools itself through sealed hardware only.

Material follows the chemistry of the room. 304 stainless covers indoor washdown, food and beverage lines and most outdoor sites; pick 316L for salt air, brine and chemical splash. If a shortlist helps, the stainless enclosure size finder filters wall-mount and HMI lines by grade and size, and the enclosure selector narrows by environment first. For a panel with its own gland layout, custom stainless enclosures are built to your drawing and delivered in 10 business days from your approval of the drawing.

Mounting, glands and the battery rules

Mount the UPS upright on the removable panel, on its own feet or a tray, with at least 4 in [100 mm] of clear air above the battery valves and nothing hot mounted directly above it. Heat stratifies, and the top of the box holds the hottest air in it, so stacking order matters in a sealed panel — the guide on component layout inside a sealed enclosure covers it.

The battery rules come from the battery, not the box. Under normal float a small VRLA battery gasses barely at all; a failed or overcharging charger can open the relief valve and release hydrogen, and a sealed Type 4X enclosure gives that gas nowhere to go. Keep the battery upright in the compartment its instructions call for, and treat the charger's health as part of the battery design — charging a battery inside a sealed waterproof enclosure walks through that balance in detail. Float compensation buys life back in warm rooms: charger manuals specify about −3 to −4 mV per °C per 2 V cell, roughly −18 to −24 mV per °C for a 12 V battery, and the sealed lead-acid charging voltage, current and load page shows the arithmetic. If the UPS sits outside, bring its output back through the same sealed bottom gland plate as the panel feed: one entry, rated glands, drip loop on the outside face.

What to put on the order

Decide by environment, then order once. A conditioned electrical room available: central or panel-adjacent UPS, and the panel stays a plain sealed box. A washdown floor with the UPS forced inside: Type 4X panel plus a rated sealed cooler. A clean indoor room only: a fan-filter panel is acceptable, at the lower dust rating.

Rerun the size against your own wattage with the enclosure sizing guide before ordering. The example above used 60 W in a 24 x 20 x 8 in box, and every 20 W you add or remove moves that panel's interior about 7 K.

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.

Questions