When a thermoelectric generator's burner lights off, the generator does not immediately deliver power. The hot shoe has to heat up, the temperature difference across the thermopile has to build, and only when that gradient reaches its operating range does the generator produce useful watts into the battery. That climb takes minutes, not seconds, and during it the site's electrical load has to come from somewhere else. The battery bridges the gap, carrying the RTU, radio, and instruments from the moment power is needed until the TEG is fully warmed and self-sufficient. Sizing that bridge, and understanding when the gap occurs, is a practical commissioning concern that decides whether a site rides through a restart or briefly goes dark.
TEG Startup and Warmup Gap in one line: TEG startup time is the several-minute interval between burner light-off and the generator reaching full rated output, during which its DC contribution ramps up from zero as the thermopile's temperature difference builds. The warmup power gap is the energy the site consumes during that ramp, which the battery must supply because the TEG cannot yet carry the load. You size a bridge battery to hold the RTU through the longest expected warmup plus a margin, so a restart never causes an RTU brownout or a dropped communication link.
A thermoelectric generator produces power from a temperature difference across its thermopile, and that difference does not exist at the instant the burner lights. At light-off the whole assembly is at ambient temperature, top to bottom, so the delta-T is zero and the output is zero. The burner then begins pouring heat into the hot shoe, and the hot face starts to climb while the finned cold side stays near ambient. As the gap between the two faces widens, a Seebeck voltage appears and grows, and the generator begins pushing current into the battery. Full output arrives only when the hot shoe reaches its operating temperature and the whole thermal path has stabilized, which is a matter of minutes because there is real thermal mass to heat.
The shape of that ramp is a curve, not a step. Early in the warmup the delta-T is small and the output is a trickle. As the hot shoe approaches its target the delta-T widens quickly and the output climbs toward rated. This means the generator is contributing something part way through the warmup, but it is not contributing enough to carry the full site load until near the end. For the purpose of sizing the bridge, the conservative view is that the load has to be supplied from the battery through most of the ramp, with the TEG's early trickle treated as a bonus rather than something to rely on.
Ambient conditions stretch the warmup exactly when they hurt most. A cold-start in freezing winter air begins from a lower initial temperature and fights a cold-side heat sink that sheds warmth aggressively, so the hot shoe takes longer to reach its target and the ramp is slower. This is the same season when the battery is already stressed by cold and by short solar days if the site is hybrid, so the longest warmup gap and the weakest battery tend to coincide. Designing the bridge around a mild-weather startup time is a mistake; the number that matters is the warmup time on the coldest morning the site will see.
The bridge battery is simply the site battery doing its ride-through job, but the warmup gap gives it a specific requirement that is easy to overlook. The battery must hold the full site load, at a voltage above the RTU's brownout threshold, for the entire duration of the longest warmup plus a margin. If the battery is deeply discharged when the TEG restarts, its voltage sags under load, and a long cold-weather warmup can pull it below the point where the RTU resets or the radio drops its link, which is the exact failure the whole scheme is meant to prevent. The bridge requirement therefore couples two numbers: the warmup time and the battery's ability to hold voltage under load through that time.
Sizing the gap is arithmetic once you know the pieces. Take the site's load in watts, convert to the current the battery must supply, and multiply by the warmup duration to get the amp-hours the bridge has to deliver during a restart. That energy has to be available on top of whatever the battery is already asked to do, and it has to be deliverable while keeping terminal voltage above the RTU's minimum. In practice this rarely dominates the overall battery sizing because a warmup is only minutes long, but it does set a floor: the battery can never be allowed to sit so depleted that a normal restart drags it into brownout. That floor is why a TEG site usually still keeps a healthy reserve rather than running its battery to the edge.
The startup gap also argues for controlling how often the TEG cycles. A generator that lights off, runs, shuts down, and relights repeatedly imposes a warmup gap on the battery each time, and each gap is an energy withdrawal and a voltage dip. Where the burner can be left running steadily rather than cycled, the site avoids repeated cold-starts and the associated ride-through demands entirely. Where cycling is unavoidable, the bridge battery has to be sized for the worst case of a cold relight, and the control logic should avoid rapid restart loops that would hammer the battery with back-to-back warmup gaps and never let it recover.
The warmup gap is invisible on paper but obvious in data, which makes a monitoring platform the natural place to characterize it during commissioning. When the TEG is first lit at a site, trending the battery voltage and charge current through the startup shows the real shape of the ramp: the voltage dips as the load pulls from the battery, then recovers as the TEG's output climbs and finally takes over the load and begins recharging. Capturing that curve on the actual unit, in the actual weather, gives a truer warmup time than any datasheet figure and confirms the bridge battery held voltage above the RTU threshold the whole way.
Once the site is running, a cloud SCADA system such as Merobix turns every subsequent restart into a check on the power system's health. A restart that used to recover in a few minutes but now drags on longer signals a burner that is not reaching its target temperature or a thermopile losing thermal contact, both of which stretch the warmup and deepen the gap. A restart where the battery voltage dips lower than it used to before the TEG catches up signals a battery losing capacity. Because these events are logged and trended, the slow drift shows up as a pattern across many restarts rather than as a single mysterious outage.
The operational payoff is that the site's most vulnerable moment, the transition from battery to generator, is observable from a distance. An operator watching restart behavior can catch a lengthening warmup or a deepening voltage dip and schedule a burner service or a battery replacement before the day a cold-morning relight finally fails to hold the RTU through the gap. At a site nobody visits routinely, that difference between a trended warning and a surprise brownout is the difference between planned maintenance and an emergency truck roll to a dead station, which is why the startup gap deserves to be treated as a monitored parameter rather than a one-time commissioning note.
A thermoelectric generator typically takes several minutes to climb from zero to full rated output after its burner lights, because the hot shoe and thermopile have real thermal mass that must heat up before the temperature difference reaches its operating range. The output follows a curve rather than a step, trickling early and rising toward rated as the hot face approaches its target temperature. Cold ambient conditions lengthen the warmup because the assembly starts colder and the cold side sheds heat more aggressively.
During the warmup after burner light-off, the TEG cannot yet carry the site load, so something else must power the RTU, radio, and instruments through the gap. The battery bridges that interval, supplying the full load at a voltage above the RTU's brownout threshold until the generator warms up and takes over. Without an adequately sized and charged bridge battery, a long cold-weather warmup can sag the voltage low enough to reset the RTU or drop the communication link.
Multiply the site load current by the longest expected warmup time, including a cold-weather margin, to find the amp-hours the battery must deliver during a restart. That energy must be available while keeping terminal voltage above the RTU minimum, so the battery is never allowed to sit so depleted that a normal restart drags it into brownout. The warmup gap rarely dominates total battery sizing, but it sets a reserve floor the site must always keep.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.