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First off, I am aware that premade devices like this exist however I have been unable to find any that fit my exact needs which are as follows.

  1. 0 power draw when in the off state
  2. Minimal voltage drop when in the on state
  3. Being able to set it to trigger at 10.5V
  4. Simple design
  5. Manually reset

When I attempted to design a circuit to fit those needs myself I came up with the following. (R1 just represents the load) Battery protection circuit

The idea is that when I connect a battery with a voltage above 10.5V and S1 is pushed to jump start the circuit the normally open relay will then start keeping it's self closed. I would then have R2 tuned so that it had just enough resistance so that when the voltage on the main circuit dropped to 10.5V the voltage to the relay coil would become too low to keep the contacts closed. The contacts would then come open and cut power to the whole circuit.

In theory this is a nice simple elegant solution to the problem for a layman like me. The issue that I'm running into is that there isn't an "exact" voltage where a relay shuts off so it would be nearly impossible to get it to shut off when the main circuit is at exactly 10.5V. I can't use a transistor or similar device because even a small voltage drop on a 12V nominal circuit like this makes a huge difference. I need 10.5V reaching the end device when the battery is at 10.5V.

I am hoping to keep this circuit as simple as possible so that I can have it packed up nicely as a single rugged unit. Currently the only application I have for it draws a constant 5 amps but I would like to be able to modify the circuit to accommodate larger loads if needed in the future so an electromechanical component seems ideal for switching the main circuit.

My question is, is there a component or simple circuit that I could put in the place of R2 that would only allow voltages over 10.5V through and completely cut off below that? If I can just get that relatively low amperage (under 200mA) circuit to completely cut off at a designated voltage then I can switch the main circuit with whatever relay or contactor I want with minimal modification.

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    \$\begingroup\$ An electromechanical relay will drain your battery. There are MOSFET high side switches that have very low voltage drop, they consume almost nothing. \$\endgroup\$ – Marko Buršič Mar 5 at 19:38
  • \$\begingroup\$ problem with powering the relay with slowly dropping voltage is that the relay may open contacts slowly ... that may cause a sustained arc to develop across the contacts \$\endgroup\$ – jsotola Mar 5 at 19:45
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You identified the problem very precisely: a relay hardly has a specific "turn-off voltage". Your idea with the transistor is great: You can build circuits with a very precisely defined voltage-dependent behaviour from semiconductor components like that.

Things get even better: one of the very big downsides of your relay approach is that keeping the relay closed will inevitably consume quite a bit of current (to keep the coil energized), which will drain your battery (and hence work against the purpose of the battery).

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is cool in that it changes the resistance of its junction (the connection between the contacts called "drain" and "source") purely based on the voltage applied to its gate contact – and potentially, quite drastically so!

Even a single P-channel MOSFET in place where your relay's switch used to be, together with a voltage divider made from very large resistor values (to avoid draining the battery) could solve this. Better, yet, there's voltage reference-comparators like TL431LI that are meant for exactly these kinds of things, switching a transistor at exactly the right input voltage on or off. I'd frankly look up "high-side switch MOSFET" circuits, and marry one to a "TL431 undervoltage" detector.

However, the solution gets even easier: there's undervoltage protection circuits against over-discharging batteries. Sounds like exactly the job you have. Look up TI's BQ296100 and chips in the same category if this spikes your interest.

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  • \$\begingroup\$ The TL431 is exactly what I was looking for. Thank you. This whole thing right now is for a 20 hour deep cycle battery capacity tester so I needed a sharp on/off point to get an accurate test which is why I didn't go with a MOSFET initially. That's also why I wasn't concerned with the additional load of keeping the relay closed because it would be a consistent load so I could just tune the main load slightly to get the desired total amperage draw on the circuit. However the MOSFET WITH the TL431 fits all of those requirements even better. \$\endgroup\$ – Aaron Engler Mar 5 at 20:17

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