Username1 said:
This question came to mind when thinking about heated battery packs...
Let's say you had extremely good insulation that let out virtually no heat. You also have a heating pad that heats up to 30°c. From what I understand you can never heat an object hotter than the source, meaning the battery can't go above 30°c from the heater alone...
If the heat can't escape the insulation, and the battery can't be heated above 30°c, and the heater stays on constantly, where is the energy going?
The heater can't stay on constantly if it only reaches a specific temperature within a space that lets no heat out.
It must have a thermostat that shuts off power to the heater once it reaches that specific temperature. (whcih will then never reengage if it's that well insulated).
Username1 said:
I didn't mean a heater that electronically cycles.
I mean an electric heater where the element itself only reaches 30°c.
I'm not aware of anything that can do this, without some form of sensing and feedback control, within an insulated environment where there is no energy loss.
There are certain alloys of elements that increase resistance with temperature (PTC's) so that they automatically use less power the hotter they get because the (average, if AC) voltage on them is constant, but their resistance increases, so their current decreases, and so watts used (power) decreases, and so does power dissipation (heat created). But there is a limit to the resistance they'll create, and if current continues to flow and heat continues to be generated, temperature will continue to increase if in an insulated environment with no energy loss.
If they're in a normal environment (even well-insulated) they'll probably maintain a temperature as designed, but not in a hypothetical near-perfectly insulated environment.
It has an on/off switch and you leave it on. Eventually the air/battery inside the box reaches 30°c. The heater is still on, so where is the energy going?
There's no way for this to occur, within a system that lets no energy out of the insulated area. If the heater is still on, it's going to reach a temeprature above that point, becuase the energy isnt' going anywhere if it's that well insulated. So will everything else within the insulation.
If the heater has a thermal control that rolls pack power (like a PWM circuit, etc) rather than simply shutting off, it will cycle power to the heater that way, and could in theory (depending on the temperature sensing, and the feedback control mechanism) maintain a nearly constant temperature at the sensor itself (or sensors, if there are multiple), based on however much energy loss there is...but it is still "cycling power", just not in the usual completely on/off method.
If you monitor power usage of the heater, you would see the change in power usage as temperature is reached, regardless of method used to control power usage.
If you have a heater with no thermal control at all, and it always runs at a specific power level, and is insulated so there is effectively no energy loss, then it will continue to heat up until the point of failure of either it's electrical insulation is reached, or of the electrical connections, or eventually of the actual heating elements themselves (most likely there would be a fire in progress by this point).
You can test this yourself even with pretty imperfect insulation, as long as it is good enough to make heat loss significantly less than heat input. Just setup a thermal sensor inside the insulated space with the heater that claims to maintain only a specific temperature, while monitoring power usage of the heater, and let it run.
Either the temperature will continue to increase until sensor failure, connection failure, or fire occurs, or else power input will decrease. (or both, depending on the heater's design).
