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Why a Device Battery Outlives Its Charge Window, Two Limits, Not One

By Saeed Baatiah · Published:

Short answer

A lithium-ion cell carries two separate temperature limits. The Teltonika FMC920 battery table gives discharge from -20 to +60 °C but charge only from 0 to +45 °C. So a device keeps running in heat that already forbids recharging, and the narrower charge window is what actually bounds the deployment.

Key takeaways

  • Discharge and charge are two different limits on one cell, the FMC920 table gives -20 to +60 °C and 0 to +45 °C.
  • A device rated -40 to +85 °C often carries that rating only without its battery; with the battery the FMC920 is -20 to +40 °C.
  • Battery University defines elevated temperature as above 30 °C, which is ordinary shade in the Gulf.
  • Storage limits tighten with duration: -20 to +45 °C for one month becomes -20 to +35 °C for six.
  • The real fix for a hot site is a device with no internal cell at all, powered externally.

01One cell, two numbers, and they are not close together

The single most useful thing to know about a lithium-ion cell is that it has two temperature specifications rather than one, and they differ by a wide margin. The Teltonika FMC920 battery table prints both: discharge is permitted from -20 to +60 °C, while charge is permitted only from 0 to +45 °C. That is a fifteen-degree gap at the top, and it describes a real physical asymmetry rather than a conservative margin. Pushing current into a cell forces lithium ions to intercalate into the anode, and above the charge ceiling that process degrades into side reactions that plate metallic lithium and consume electrolyte. Taking current out is a gentler operation. The consequence for anyone specifying hardware is direct: a device can be entirely functional at a temperature where it has already lost the ability to replenish itself, and nothing on the display will say so.

02The datasheet headline usually excludes the battery

There is a second trap in the same table, and it catches buyers who read only the top line. The FMC920 wiki gives the device operating temperature as -40 to +85 °C without battery, and -20 to +40 °C with battery. Both figures are printed side by side, and the wiki row label for the headline number names the exclusion explicitly as "Operating temperature (without battery)". The FMC003 shows the same pattern: the device is rated -40 to +85 °C while its cell keeps the identical 0 to +45 °C charge and -20 to +60 °C discharge limits. So the impressive number belongs to the electronics, and the modest one to the chemistry sitting inside the same enclosure. If a procurement sheet quotes only the wide range for a device that ships with a battery, the range being quoted is not the range being deployed.

03Why 45 °C is a Gulf number, not a laboratory one

A charge ceiling of 45 °C reads as generous until you consider where the cell actually sits. The relevant temperature is not the weather but the internal temperature of the enclosure holding the cell, which is warmer than the air by whatever the solar load and the device's own dissipation contribute. Battery University's guidance sharpens this further: it defines a cell as being at elevated temperature above 30 °C, and its charging article gives the permitted fast-charge band as 5 to 45 °C with the best range 10 to 30 °C, noting that many chargers prohibit charging above 50 °C. Read together, those figures say that a Gulf installation spends much of the year above the ideal band and part of it above the permitted one, so the charge window is not an edge case here, it is the normal operating condition.

04The limit that tightens the longer you wait

A third limit is easy to miss because it applies before the device is even installed. The FMC920 battery table gives storage as -20 to +45 °C for one month but -20 to +35 °C for six months, which is a limit that gets stricter as time passes rather than staying fixed. Battery University supplies the reason in measured form: capacity recoverable after a year at full charge is about 80% at 25 °C, 65% at 40 °C, and 60% at 60 °C after only three months. That last figure is the one to sit with, three months in an uncooled store loses more than a year in a cool one. A batch of trackers waiting in a warehouse over a Gulf summer is therefore ageing on a schedule nobody is recording, and the loss is already in the cells on the day of installation.

05What the enclosure rating does not tell you

A recurring assumption in specification documents is that a sealed device is a protected device, and thermally the opposite can be true. The IEC ingress protection scheme grades solids with its first numeral from 0 to 6 and liquids with its second from 0 to 9 under IEC 60529, and temperature appears in neither. A well-sealed enclosure keeps dust and water out and also keeps heat in, because sealing removes convection as an escape route. So an IP-rated box in direct sun can hold a cell above its charge ceiling while a vented one at the same site does not. The rating is answering a different question than the one a thermal problem asks, and treating it as reassurance about heat is one of the more common ways a correct component ends up in an incorrect place.

06The honest answer for a hot site

If the charge window is the binding constraint, the cleanest engineering response is to remove the cell from the design rather than to nurse it. A device with no internal battery, drawing external power, has no charge ceiling to violate, no storage curve ageing in a warehouse, and no cell whose ratings shrink its host's published range. The Eurosens Dominator is a plain illustration of the shape: it is specified from -40 to +85 °C with IP67 and a 9 to 32 V external supply, and it holds that full range precisely because there is no cell inside to hold it back. Where a cell truly cannot be avoided, an asset with no host power, the design question shifts from which battery to how it is shaded, ventilated and sited, because those are the only variables left.

07What to ask before the order is placed

Four questions extract everything that matters, and all four have printed answers. First: what is the operating range with the battery fitted, not without it, and is the difference stated on the same page. Second: what are the separate charge and discharge windows for the cell, since only the charge one is likely to bind. Third: what is the storage limit at the duration your supply chain actually holds stock for, given that the FMC920 figure drops from 45 to 35 °C between one month and six. Fourth: does the device need a cell at all for its role, or is the cell there only to survive brief supply interruptions, a distinction the FMC650 makes visible by describing its Ni-MH pack as a back-up battery with a 0 to 45 °C charging temperature.

The three temperature limits on one battery-carrying device
LimitFMC920 figureWhen it binds
Device, battery not fitted-40 to +85 °COnly for a battery-less variant
Device, battery fitted-20 to +40 °CEvery deployment that ships with the cell
Cell discharge-20 to +60 °CRarely, it is the widest of the cell limits
Cell charge0 to +45 °CUsually the binding limit in the Gulf
Storage, 1 month-20 to +45 °CShort warehouse holds
Storage, 6 months-20 to +35 °CStock held over a summer

Li-ion capacity left after storage at full charge

25 °C, after 1 year80 %
40 °C, after 1 year65 %
60 °C, after only 3 months60 %

Battery University, BU-808: How to Prolong Lithium-based Batteries

Figures and sources

Questions

Why can my device run in heat that stops it charging?

Because charging and discharging have separate limits on the same cell. The FMC920 battery table permits discharge from -20 to +60 °C but charge only from 0 to +45 °C. Forcing current into a hot cell drives side reactions that plate lithium and consume electrolyte, while drawing current out is a gentler process.

Is a -40 to +85 °C device safe for a Gulf rooftop?

Check whether that rating includes the battery. The FMC920 wiki prints -40 to +85 °C without battery and -20 to +40 °C with it, and the row label for the wide figure says "Operating temperature (without battery)". If the unit ships with a cell, the narrower number is the one that applies.

Does an IP67 rating protect the battery from heat?

No. Under IEC 60529 the first numeral grades solids from 0 to 6 and the second grades liquids from 0 to 9, and temperature appears in neither. Sealing can make the thermal problem worse, because it removes convection as a way for heat to leave the enclosure.

What is the best way to avoid the charge window entirely?

Specify a device with no internal cell and give it external power. With no cell there is no charge ceiling, no storage ageing curve, and no chemistry narrowing the host's published range, which is why the Eurosens Dominator holds -40 to +85 °C on a 9 to 32 V external supply.

Sources

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