I see, I have to start a bit earlier. In this post I described all used components and linked to them:
https://diysolarforum.com/threads/lifepo4-heating-pad-for-cold-temperatures.5/post-32925
The pads does not have their own temperatur control. The two controllers are meant to handle this.
The two controllers are only modules. The temperatur controller works like a thermostat. When the temperature drops below a certain point it passes a load signal. This signal will energize the voltage-controller. It's mission is to pass a load signal, when the voltage rises into the areas of charge voltage. It detects charge voltage.
Doing so now I have a load-signal that depends on temperatur and voltage. Because the two controllers can manage up to 10A I can use this signal line to power the heating pads. They will heat as long as either the temperature rises beyond a certain point or the voltage drops below charging voltage. So these two controllers decide when to heat and when not.
Additionally I'd like to have this signal to also manage the nc-relay. The normally closed relay will open to cut off the plus line between battery-cells and battery-poles.
So yes, I'm trying to pass the charge current through the temp-controller, then through the voltage-controller and from there on to the pads and also the relay-steering.
Everything I've sketched above is meant to happen inside a battery-case and will be part of the battery. I want to be able to attach any kind of chargers to the battery and it will protect the same way.
Warning: Some charge controllers don't like to be hooked/unhooked from the load while the charge source is energized.
Hmm, I'd like to know how the builders of the BMSes with integrated low temp cut off have solved this problem.
Your first drawing differs from mine in two points:
1) You connected the voltage-controller first and than attached the temperature controller. So it's now dependent on it, I did it the other way round.
2) The two controllers in your drawing are powered only by the battery. In mine they are powered by both, battery and charger, depending on the switching position of the relay. (If relay is closed and there is no charge current, it's powered by the battery, otherwise it's powered by the charge current because the relay only opens when a charge current is available through the battery poles.)
I also abstracted the cell managing/balancing lines coming from the BMS because for me they are not important here.
When it comes to drawing 2 and 3. The charge current comes from outside the battery directly through the battery poles, not over a separate line. I am also going to pass the charge current through the controllers, so there is absolutely no need for a 2PDT to do so. That's because the controllers can pass up to 10A and they by themselves act like a relay.
In my drawing in fact there are two plus-lines going to the temp-controller, one is to feed the controller itself with power and the other one is the actual signal-line that will be passed to the voltage-controller and used later for the heating pads. I just abstracted this in the drawing, because I'll bypass the line near the controller itself. But I've seen, you'd been drawing this.
The whole thing is very simple but I'm not a native english speaking person so it's a little bit hard to explain. Especially the controller-modules make the things very easy.
P.S. When it comes to complexity of electronic components, you're right, the two controllers have additional components that may fail. But, on the other hand, the more complex your BMS gets (and it does with all the protection systems onboard), the more electronic components can also fail inside your BMS. Here I'm balancing a simpler BMS with additional controller-modules.