Skip to content
Sign inGet Started

IoT Hardware at 50 °C: Why the Temperature Rating on the Datasheet Is Not the One That Applies

By Saeed Baatiah · Published:

Short answer

The temperature range printed on a tracker datasheet usually excludes its own battery. Teltonika labels that row "Operating temperature (without battery)" and rates the FMC920 at -40 to +85 °C without the cell but only -20 to +40 °C with it fitted, so a device sold as +85 °C rated has an effective ceiling below Gulf summer ambient.

Key takeaways

  • Teltonika names the headline row "Operating temperature (without battery)", the +85 °C figure is battery-excluded by definition.
  • The FMC920 wiki prints both ratings in one table: -40...+85 °C without the battery, -20...+40 °C with it.
  • Charge and discharge are two separate limits on the same cell, charging stops at +45 °C while discharge continues to +60 °C.
  • An IP rating grades dust and liquids only; IEC does not cover temperature in the IP code at all.
  • Enclosure colour is the largest controllable factor in direct sun: black absorbs 0.97 of incident solar radiation, white 0.14.

01The rating that is printed, and the rating that applies

Almost every tracker datasheet and reseller listing quotes a single operating temperature range, and for the Teltonika FMC and FMB families that number is -40 to +85 °C. The manufacturer's own wiki, however, does not present it as a single figure. The row is literally named "Operating temperature (without battery)" in the source template for the FMC6xx family, which makes the headline number battery-excluded by definition rather than by interpretation. The FMC920 page is the clearest case because it prints both values side by side in the same table: -40 to +85 °C without the battery, and -20 to +40 °C with the battery installed. That is a 45 °C reduction in the upper limit, caused entirely by fitting a component the device ships with. For a buyer in Riyadh or Dammam this is not a footnote, because the effective ceiling as shipped sits below the ambient temperature the device will meet every summer afternoon.

02Charge and discharge are two different limits

The internal cell carries its own table, and it contains two distinct temperature limits rather than one. On the FMC920 and FMC003, the 170 mAh Li-ion battery is rated for charging between 0 and +45 °C and for discharging between -20 and +60 °C. The gap between them is 15 °C, and it has a consequence that is easy to miss when reading a spec sheet as a single pass/fail number: there is a temperature band in which the device continues to run and report on battery power while it is no longer permitted to replenish that battery. A unit in that band does not fail loudly. It keeps working, walks its stored charge down, and goes silent later, at which point the outage looks like a network or firmware problem rather than a thermal one. The FMC650 uses a different chemistry, a 550 mAh Ni-MH pack, and its wiki table lists only a charging window, again 0 to 45 °C.

03Why the cell limit is chemistry, not a vendor choice

It would be reasonable to suspect a conservative vendor margin, but the same ceiling appears in the general lithium-ion literature. Battery University states that Li-ion can be fast charged from 5 °C to 45 °C, with best results between 10 °C and 30 °C, and notes that many chargers prohibit charging above 50 °C altogether. The same source defines a battery dwelling above 30 °C as being at elevated temperature, a threshold Gulf ambient exceeds for most of the year, which means devices deployed there operate permanently inside the accelerated-degradation regime rather than visiting it occasionally. The consequence is measurable in retained capacity. A fully charged cell held at 25 °C retains about 80 percent of its capacity after a year; at 40 °C that falls to about 65 percent; and at 60 °C it reaches roughly 60 percent after only three months. Storage of spares in an un-airconditioned storeroom therefore consumes shelf life before the device is ever installed.

04The storage limit that shortens as time passes

The FMC920 battery table adds a detail that most spec comparisons drop entirely: the permitted storage temperature is not a single band but depends on how long the storage lasts. The wiki lists -20 to +45 °C for one month of storage and -20 to +35 °C for six months. The ceiling falls by 10 °C as the duration extends, which inverts the usual way a procurement team reads a temperature spec, as a fixed envelope a component either sits inside or does not. Here the envelope narrows the longer the part waits. For a fleet holding spare trackers against failures, that turns warehouse conditions into a real variable: a box of units kept through a Gulf summer in an uncooled storeroom is outside the six-month specification, and the capacity figures in the previous section describe what that costs. Seeding spares in smaller, more frequent orders is a cheaper answer than cooling a storeroom.

05Why two devices with the same rating fail differently

Because the binding limit lives on the cell rather than the enclosure, the battery chemistry decides the failure mode, and a fleet often owns more than one. The FMC920 and FMC003 both carry a 170 mAh lithium-ion cell with the charge and discharge limits described above. The FMC650, positioned as the professional tracker of the range, instead carries a 550 mAh nickel-metal-hydride pack at 8.4 to 10.0 volts, and its wiki table publishes only one thermal figure for it: a charging temperature of 0 to 45 °C. No discharge or storage row appears for that pack at all, which is a documentation difference rather than a physical guarantee, the absence of a published discharge limit is not evidence of a wider one. A procurement comparison that reads only the device row would conclude these three devices are thermally identical. Read the cell rows and they are three different components with three different published envelopes.

06Designing the cooling margin rather than the average case

When an enclosure does need active cooling, the question becomes what conditions to size it for, and the nVent white paper answers that explicitly rather than leaving it to judgement. Its stated worst-case assumptions are three illuminated sides, no wind, and a sky temperature equal to ambient, and on top of those assumptions it recommends selecting a system with 25 percent additional capacity. Each of those three assumptions describes a real Gulf installation rather than a pessimistic abstraction: a pole-mounted or rooftop enclosure genuinely does take sun on more than one face through the day, still desert air genuinely does remove the convective term the calculation would otherwise credit, and a clear sky at ambient removes the radiative sink. Sizing on an average day and hoping the margin absorbs the difference inverts the method, because the margin exists to cover component tolerance and ageing, not to cover a design case that was chosen too optimistically.

07An IP rating says nothing about heat

Gulf deployments need sealing against dust, and IP67 is the usual answer. It is worth being precise about what that rating certifies. IEC, the body that owns the IP code, states that the first numeral grades protection against solid objects on a scale from 0 to 6, and the second grades protection against liquids on a scale from 0 to 9. Temperature appears nowhere in either numeral. An IP67 device therefore carries no heat-tolerance guarantee from that rating at all, and the two properties can pull against each other: the sealing that keeps dust out also prevents convective cooling, so the enclosure that best satisfies the dust requirement is the one that best traps heat. Treating IP and temperature as one combined "ruggedness" score is the error, because they are independent specifications that happen to be printed next to each other.

08Solar loading and the colour of the box

For an enclosure in direct sun, incident radiation adds heat on top of air temperature. nVent's enclosure white paper gives the solar constant above the atmosphere as 1,366 W/m² and the typical extreme on a horizontal surface as 1,044 W/m², noting that higher extremes are possible in high-altitude, clear, dry climates, the same climate class as inland Saudi Arabia. How much of that becomes heat depends heavily on finish. The same paper tabulates solar absorption coefficients: black absorbs 0.97 of incident solar radiation, dark grey 0.95, medium grey 0.75, light grey 0.50, cream 0.25, and white 0.14. Black to white is nearly a sevenfold difference, which makes finish the largest single controllable factor for a sealed box in the sun. The paper's stated worst-case assumptions for evaluating an enclosure are three illuminated sides, no wind, and sky temperature equal to ambient, and it recommends selecting cooling with 25 percent additional capacity.

09Heat degrades the data, not only the device

A thermal specification is usually read as a survival question, but two documented behaviours show it is also a data-quality question. Teltonika states plainly in the FMC920 electrical characteristics that the analog input error margin can increase if temperature varies, so a heat-soaked installation degrades the accuracy of anything measured on an analog channel, an analog fuel-level sender included, before any device failure occurs. Separately, Teltonika documents an active thermal-protection behaviour on the RUT956 router: prolonged use in a poorly ventilated space raises ambient temperature and leads to automatic shutdown of the device or disconnection of the mobile network connection, with recovery requiring the unit be cooled in a well-ventilated place before being switched on again. That router is also rated to +75 °C rather than +85 °C, a lower ceiling than the trackers, so a site mixing both device classes does not have one thermal limit.

10What an externally powered sensor changes

The pattern in the previous sections points at one structural conclusion: the battery, not the electronics, is the thermally weakest part. A device without an internal rechargeable cell is not subject to the +40 °C effective ceiling or the +45 °C charge limit, because neither applies to a component it does not have. The Eurosens Dominator capacitive fuel-level sensor is a useful illustration: it is rated across the full -40 to +85 °C band with IP67 protection and is externally powered at 9 to 32 volts, so it has no charge window to violate. This is why the same nominal "-40 to +85 °C" means genuinely different things on two products, and why comparing two datasheets on that single line is not a comparison at all. For a fixed installation with reliable power, specifying the battery-less or externally powered variant removes the constraint rather than managing it.

11How to specify hardware for a Gulf site

The practical sequence follows from the sections above rather than from a vendor recommendation. Read the temperature row and check whether it names the battery; if it does not, assume the figure excludes it and look for the cell's own table. Treat the internal Li-ion as a 40 °C component regardless of the device rating. Where power is available, prefer an externally powered variant and remove the cell from the thermal calculation entirely. For anything mounted outdoors, specify a light-coloured enclosure and shade it, since finish is worth roughly a sevenfold difference in absorbed solar radiation and costs nothing at order time. Do not read an IP rating as thermal evidence. Order spares in smaller, more frequent batches rather than holding stock through a summer, because the six-month storage ceiling is 10 °C lower than the one-month ceiling.

12What ENTEK.AI does not claim here

One number that belongs in this discussion is missing on purpose. A figure for how much hotter a sealed enclosure in direct sun runs than the surrounding air is widely quoted as 20 to 30 °C above ambient, and it appears on many vendor pages. We could not read it in any primary source. The nVent white paper presents enclosure temperature rise only as unlabelled graph curves whose units are ambiguous in the extracted text, and the pages repeating the numeric figure are secondary and were not readable directly. The absorption coefficients and the irradiance values above are quoted primary-source text, so they are used and the derived temperature rise is not. We also do not publish a figure for device failure rates across our own Gulf deployments, because we have not run that as a dated, methodologically stated study, and an estimate presented as a measurement would be worth less than saying so.

The same nominal rating, three different effective ceilings
DeviceNominal device ratingBattery limit that actually bindsEffective ceiling as shipped
Teltonika FMC920 (battery fitted)-40...+85 °CLi-ion charge 0 to +45 °C+40 °C, stated in the same table
Teltonika FMC003 (battery fitted)-40...+85 °CLi-ion charge 0 to +45 °CCharge window closes at +45 °C
Teltonika FMC650 (Ni-MH back-up)-40...+85 °CNi-MH charging 0 – 45 °CCharge window closes at +45 °C
Teltonika RUT956 router-40...+75 °Cno internal cell+75 °C, plus documented thermal shutdown
Eurosens Dominator (external power)-40...+85 °Cnone, no internal cell+85 °C, the full rated band

Effective temperature ceiling as shipped

Teltonika FMC920 (battery fitted)40 °C
Teltonika FMC003 (battery fitted)45 °C
Teltonika FMC650 (Ni-MH back-up)45 °C
Teltonika RUT956 router75 °C
Eurosens Dominator (external power)85 °C

Comparison table in this article, from Teltonika and Mechatronics documentation

Solar absorption coefficient by enclosure finish

Black0.97
Dark grey0.95
Medium grey0.75
Light grey0.5
Cream0.25
White0.14

nVent HOFFMAN white paper WP-00005B, solar absorption table

Figures and sources

Questions

Does a +85 °C rating mean the device works at 85 °C in Saudi Arabia?

Not as shipped with a battery. Teltonika names that row "Operating temperature (without battery)", and the FMC920 page states -20 to +40 °C with the battery fitted in the same table. The battery, not the electronics, sets the effective ceiling.

Why does the device keep working but then die weeks later?

Because charge and discharge have separate limits. The cell may discharge up to +60 °C but only charge up to +45 °C, so in that 15 °C band the device runs on stored charge it cannot replenish. It reports normally until the reserve is gone, which makes the failure look like a network problem.

Does IP67 protect a device from heat?

No. IEC defines the first IP numeral as protection against solid objects and the second against liquids. Temperature is not part of the IP code at all. Sealing can make heat worse by preventing convective cooling, so IP and thermal ratings must be read as two independent specifications.

What is the cheapest change that reduces heat on an outdoor enclosure?

The finish. nVent tabulates a solar absorption coefficient of 0.97 for black against 0.14 for white, close to a sevenfold difference in absorbed radiation, and specifying a light colour costs nothing at order time. Shading it as well removes the direct-sun component entirely.

How should spare trackers be stored in a Gulf warehouse?

Note that the storage ceiling depends on duration: Teltonika lists -20 to +45 °C for one month but -20 to +35 °C for six months. Ordering smaller batches more often keeps stock inside the shorter-duration window and avoids the capacity loss that a summer in an uncooled storeroom causes.

Sources

Related insights