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I want to run a 12V (65W) fridge from a battery circuit on a boat. However, I want to protect the battery from being drained and potentially damaged by the fridge if the voltage drops too low so I need a switch that will open the circuit if the voltage drops below ~11.5V and close it again once it reaches 12V. What is this sort of switch called so I can search for one?

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What you want to implement is called deep discharge protection for battery. You could find a simple cutoff relay over the internet or build such switch using a Microcontroller which can monitor 'battery's charge status with much higher accuracy. A good way to do this is by measuring both voltage of battery terminals and current. With this peice of information you could esitmate battery's internal resistance that is generally better correlated with depth of discharge.

The key challenge with estimating battery's charge status only by looking at voltage is the fact that terminal voltage is strongly varies greatly depending on the connected load. If a power hungry device draws a lot of current(e.g start of a compressor/motor), the battery's terminal voltage could instantly dip to a pretty low value and eventually recover. While the voltage dropped this does not mean the battery was fully discharged and would cause a false alarm/activation of your relay.

Additionally the device you would use to cutoff battery must implement some form of a voltage hysteresis. Think of it this way, Let's say your battery's voltage dropped to 10v and your circuit cutts off all load, as early as you cuttoff the load the voltage would jump back to 11v, the load would power on again, eventually your load would just keeps turning off/on repeatedly possibly causing more harm to the device you are powering.

Hope this helps.

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Many compressors for 12V (mobile use) already have a built-in cut-out voltage.

For the Danfoss BD35 or BD50 compressors (which are very popular 12V compressor for use on boats and RVs), you can find the instructions on setting the cut-out voltage here.

The cut-out voltage is probably already set from the factory (the value could be dependent on the manufacturer of your fridge), but it can be changed by setting a resistor value (R9 in the schematic) as seen in the document I refered to and the table below.

Danfoss BD35 Setting cut-out voltage with R9

Take care not to set it too high as the voltage at the compressor will often drop at startup when it needs the most current.

If you don't have a Danfoss compressor there will probably be another way to set the cut-out voltage.

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  • \$\begingroup\$ Very cool, thank you. I will check the appliances I was thinking of for this feature. \$\endgroup\$
    – Paul Smith
    Mar 12, 2021 at 15:20
  • \$\begingroup\$ I have this chart for my setup - AIUI the factory default is a battery-trashing 10.4v and the best you can do is merely 'damaging' at 11.3v? I fully accept that I may be missing something here though? \$\endgroup\$ Oct 23, 2022 at 18:38
  • \$\begingroup\$ What you might be missing is that these values are measured at the compressor, while the compressor is running, pulling a significant load from the battery (especially during startup) and also includes the voltage drop due to the wiring. There needs to be enough headroom in the cut-in and cut-out voltages not to cause the compressor to shut down at startup, even with a healthy battery. \$\endgroup\$
    – StarCat
    Oct 23, 2022 at 20:40
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The search terms that worked for me were "Low voltage switch" and "undervoltage protection".

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