Low watt density silicone heaters shaped to a cell block or module face, for holding a lithium pack above its charging floor in cold weather. Battery Pack Silicone Heaters are used for putting a controlled watt density onto a curved, irregular or large surface anywhere between -50 °C and +200 °C, where the heater has to take the shape of the part rather than the part being redesigned around the heater. Nichrome wire wound on a glass carrier, or an etched Inconel foil circuit, vulcanised between two sheets of fibreglass-reinforced silicone rubber. It is 1.2 to 2 mm thick, tear resistant, unaffected by moisture and ozone, and conformable enough to be wrapped around a drum or pressed onto a compound curve without cracking the circuit. This page sets out the industries and assemblies where the construction is specified, what has to be stated on the drawing and the order, how it is installed, and the duties where a different construction is the right answer. This construction is built up to 500 x 1000 mm (one piece), 1.2 mm thick to your outline; the full capability table is further down the page. It is rated at 0.3 W/cm² in free air and 2 W/cm² bonded full-area to metal, and the difference between those two figures is the difference between a heater that lasts and one that does not.
Typical Applications for Battery Pack Silicone Heaters
The assemblies and sectors where this construction is specified:
- Industrial process plates and platens: press platens, vacuum tables, laminating beds and mould tools, zoned so the edges of the plate can be driven harder than the middle to compensate for edge losses.
- Outdoor enclosures and telecom cabinets: anti-condensation heating that keeps electronics above the dew point through a night cycle, at a few watts rather than at a rating.
- Food service and appliance equipment: coffee machine group heads, water dispensers, warming plates and dispensing valves, where the shape is awkward and the temperature is modest.
- Aerospace and defence: de-icing on sensors, probes and small aerodynamic surfaces, and keeping avionics and hydraulic assemblies within their operating range at altitude.
- Semiconductor and vacuum equipment: gas line and valve heating on the atmospheric side of a tool, where the assembly is round, hot and inaccessible.
How to Specify Battery Pack Silicone Heaters for Your Application
- Outline drawing: There is no catalogue size to order from: this part is made to your outline. Send a drawing or a DXF showing the heated area, every hole and cutout, which face is bonded, and where the leads leave the part — the process builds up to 500 x 1000 mm (one piece), 1.2 mm thick. Everything else on this list is quoted against that drawing.
- The charging floor is the design point: A lithium cell must not be charged below 0 °C, so the duty is not "warm the pack" but "hold every cell above a threshold before the charger is enabled". State the coldest ambient, the time you have available, and the cell temperature the BMS is looking for — the wattage falls out of those three.
- What the heater is bonded to: This is the first question on the quotation, and it is not a formality: it decides which of the two watt density figures applies. Aluminium or steel with full-area contact carries the bonded figure. Plastic, foam, glass, a partial contact patch or free air do not, and the heater has to be sized at the free-air figure instead.
- Wire-wound or etched foil: Wire is the standard, cheapest and most tolerant construction, and it puts the heat in lines at the wire pitch. Etched foil is a wide, flat conductor: it spreads the heat over a larger fraction of the area, so it holds a tighter surface uniformity and safely runs at a higher watt density in the same insulation.
- Outline, cutouts and lead exit: Send a drawing or a DXF. State the outline, every hole and cutout, which face is the bonded face, where the leads exit and in which direction they run — a lead exiting on the wrong edge turns a flat installation into a 90-degree fold at the exit, which is where flexible heaters actually break.
- Supply voltage: State the supply voltage: this construction is wound or printed for 5 V DC, 12 V DC, 24 V DC, 36 V DC or 48 V DC, and it is built for the one voltage ordered rather than being switchable between them. At low voltage the current is the problem — a 120 W heater at 12 V draws 10 A, and the lead gauge, the connector and the switching device all have to carry it.
- Wattage and watt density: This construction is rated at 0.3 W/cm² in free air and 2 W/cm² bonded full-area to metal. Those are not two options: they are the same heater under two cooling conditions, and the second figure is only true while the heat has somewhere to go. Design at the bonded figure and then leave the heater unbonded, air-gapped, or stuck to plastic or foam, and the element runs at roughly 7 times the density its own surface can shed — the insulation reaches its limit in minutes and the failure is permanent. Give the wattage you need and what the heater is bonded to in the same sentence, and the watt density can be checked against the construction before anything is built.
- Temperature limit: The continuous limit is 200 °C, and it belongs to fibreglass-reinforced silicone rubber rather than to the element. Nichrome, Inconel and fired resistor pastes all survive far beyond it; what fails first is the material holding them, and no derating of the wattage moves that number. State the maximum temperature the assembly can reach under fault as well as the working set point, because it is the fault case that has to stay inside this number.
- Control and protection: This construction does not limit itself in any way. At rated voltage into a surface that has lost contact, run dry or lost its coolant, it goes on taking full power until the insulation fails, so a thermostat or a thermal cutoff sized to the assembly — not to the heater — belongs in the design and in the schematic. Where a fault must not be allowed to repeat itself, a one-shot thermal cutoff goes behind the resettable device, because a bimetal thermostat cycling on a genuine fault will cycle for as long as the power is on.
- Attachment method: State how the heater is held against its surface: free (mechanically clamped or strapped), supplied with a pressure sensitive adhesive backing and a liner, or vulcanised or bonded to a part you send in. Adhesive is convenient and it is also a thermal layer and a temperature limit of its own, so it is a design decision rather than a packaging option.
- Sensor and where it sits: A sensor can be built into the part rather than fitted next to it: NTC, PT100 or PT1000, a thermocouple, or a bimetal device. Say which, and mark its position on the drawing — a sensor under the densest part of the circuit, a sensor at the coldest corner and a sensor next to the load are three different control schemes, and the drawing is where that gets decided.
- Certification of the construction: The constructions behind this product are built and tested to UL 499, IEC/EN 60335, UL 94 V-0, RoHS & REACH, CE Marking, IATF 16949. That is a statement about a construction — its insulation system, its creepage and clearances, its earthing, its marking and its tests — and not a claim that a particular made-to-drawing outline carries an approval of its own. Say at the enquiry which standard your end product is certified to, because it constrains the build rather than being added to it afterwards.
- Leads and termination: State the wire type, gauge, length and exit direction, and the termination — bare, ferrule, ring, spade or a specific connector housing. The lead exit is a mechanical design decision: it is where a flexible heater is most often folded during assembly, and the joint behind it cannot be inspected once the part is built.
Control, Protection and What Happens at a Fault
This construction does not limit itself in any way. At rated voltage into a surface that has lost contact, run dry or lost its coolant, it goes on taking full power until the insulation fails, so a thermostat or a thermal cutoff sized to the assembly — not to the heater — belongs in the design and in the schematic. Where a fault must not be allowed to repeat itself, a one-shot thermal cutoff goes behind the resettable device, because a bimetal thermostat cycling on a genuine fault will cycle for as long as the power is on.
- The control device: A sensor on the load and a controller — on/off where the load has thermal mass and a few degrees of swing is acceptable, PID where the set point has to be reached without overshoot. This is what holds the temperature; it is not what protects the assembly.
- The limit device: A bimetal thermostat or a thermal cutoff, sensing the heater or the hottest point of the assembly, wired so that it interrupts the heater regardless of what the controller is doing. It has to be independent of the control sensor: a broken sensor wire that the controller reads as cold is one of the commonest ways a heating assembly ends up at full power.
- The fault case to design against: Ask what happens if the heater loses contact with its load, if the fluid stops, if the fan stops, or if the controller output sticks on. In every one of those the heater goes on delivering full power into a surface that is no longer carrying it away, and the answer has to be a device that acts, not a calculation showing it would take a long time.
- Where the sensor goes: On the surface whose temperature is in the specification, in direct thermal contact with it. A sensor reading the air near a heated part reads a temperature that part passed through seconds earlier, and a loop closed on the lagging measurement overshoots the leading one every time it starts.
Bonding & Installation Practice
- Prepare the surface before bonding: degrease with isopropanol, remove machining oil and release agent, and knock the peaks off a rough surface. Pressure sensitive adhesive bonds to what it touches, and an oily or a 3.2 Ra surface gives it a fraction of the contact area it needs.
- Roll the heater down from the centre outwards and apply firm, even pressure across the whole area. A trapped air bubble is a local free-air region inside a heater that has been sized as a bonded one, and it is the most common single cause of a burn mark in the middle of an otherwise good installation.
- A 3M acrylic PSA reaches only part of its final strength on contact and continues to build for 24 to 72 hours. Where the surface is vertical, curved or warm, hold the heater with tape, a clamp or a strap while it cures rather than trusting the initial tack.
- Do not fold the heater over an edge or crease it to make it fit. The minimum bend radius is a property of the element inside, not of the rubber; a crease breaks wire or cracks foil, and the break shows up as an open circuit weeks later rather than immediately.
- Strain-relieve the leads at the exit. The lead-to-element joint is under the insulation and cannot be repaired, so any pulling, flexing or vibration at the exit has to be taken by a clamp, a P-clip or a service loop before it reaches that joint.
- Never trim a heater to fit. The element is routed with a deliberate edge margin, and cutting the outline cuts either that margin or the circuit itself.
Common Specification Mistakes with Battery Pack Silicone Heaters
- Heating the pack case rather than the cells. The heater has to be in contact with the cell block or the module wall in the thermal path to the cells; a heater on the outside of an insulated enclosure warms the enclosure and satisfies a sensor that is measuring the wrong thing.
- Sizing the heater from the bonded watt density and then mounting it against plastic, foam or an air gap. The figures differ by several times for a reason, and the free-air condition is the one the heater actually fails in.
- Sizing the wattage from the mass to be heated and forgetting the steady-state losses. A pack that needs 200 W to warm up in twenty minutes may need only 40 W to hold temperature, and a heater sized for the transient will spend its life cycling hard against a thermostat.
- Leaving the protective device out because the controller "will handle it". A controller that fails on, a sensor that falls off its surface or a wire that shorts is exactly the case the thermostat exists for, and it is the only case where it matters.
- Specifying a low wattage at 230 V on a small outline. High voltage and low power means a very fine, very long element, and there is a practical limit below which the wire cannot be routed reliably — a 24 V version of the same heater is often the manufacturable one.
- Quoting only one of the two watt density figures in an internal specification. They differ by roughly 7 times on this construction, and a figure copied without the bonding condition attached to it is the single most reliable way to destroy one of these heaters.
- Bonding a self-adhesive heater to an unprepared surface. A pressure sensitive adhesive bonds to what it can touch: machining oil, release agent, a rough finish or a cold surface all leave it with a fraction of its contact area, and the heater then runs partly unbonded while being powered as though it were fully bonded.
Where Battery Pack Silicone Heaters Are Not the Right Choice
- Not above 200 °C continuous. Silicone runs to 200 °C continuously and around 240 °C for short excursions, and beyond that it hardens, embrittles and loses its dielectric strength. That is a thick film or mica-insulated duty, not a silicone one.
- Not where the assembly cannot give the heater full-area contact. A silicone heater standing off its surface, bridging a gap or bonded over a weld bead is being asked to run in free air at whatever density it was sized for.
- Not as a chemical barrier. Silicone is permeable to water vapour and is attacked by concentrated acids and several solvents; where the heater sits in the chemistry rather than beside it, an encapsulated plate is the right construction.
- Not where 1.2 to 2 mm of thickness will not fit. Between a pouch cell and a module wall, or under an optic, the answer is a 0.25 mm polyimide heater rather than a thinner silicone one.
- Not as its own protection. This construction has no self-limiting behaviour, so it is never the right answer for a design that has nowhere to put a thermostat or a thermal cutoff.