I would like to be able to drain a car battery down from fully charged at 12.6v down to between 10.5 to 11.9 volts without having to manually check it with a multimeter periodically, yet have some obviously visible sign that the discharge process has finished. Anywhere in that voltage range is acceptable, although closer to the higher end of the range is preferable.

The car batteries typically have capacities between 50 to 100 amp hours. I was planning to use one or two 12v 60watt ( 5amp ) car headlights as the load, such that the discharge time would be both reasonable, but not so fast as to damage the battery.

Is there some circuit design / device which could meet that goal and be implemented by someone with a basic knowledge of electronics?


OK, this ICStation Battery Low/Under Voltage Protection Kit is basically exactly what I'm looking for:

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Obviously the easiest thing would simply be to mail order an item like this, however I'm not sure I could get it shipped to the country I live in. Would it be horribly difficult to build something like this?


OK, so here's a project suggestion for building something like this.

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    \$\begingroup\$ I smell an xy problem here. What are you going to do that for? And should that voltage be open circuit? after how much recovery time? "approximately 11.89V" with that much digits, approximate seems to mean rather precise. You will need some active circuitry for that. \$\endgroup\$ – PlasmaHH May 15 '17 at 11:22
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    \$\begingroup\$ It seems you want a threshold comparator, although one good to 0.08% won't be trivial. However, I agree with @Plasma. This smells like a XY problem. Pop up two or three layers and tell us what you really want to accomplish, leaving out any hair-brain schemes you dreamed up to accomplish it. \$\endgroup\$ – Olin Lathrop May 15 '17 at 11:30
  • \$\begingroup\$ Look at MAX1614 circuit, maybe? \$\endgroup\$ – Marko Buršič May 15 '17 at 11:55
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    \$\begingroup\$ Or this maybe? amazon.co.uk/12v-Battery-Discharge-Protector-Guard-x/dp/… \$\endgroup\$ – Christian May 15 '17 at 14:19
  • \$\begingroup\$ Probably you are trying to drain the battery to a certain percentage and you are trying to use voltage threshold as an indicator of percentage threshold. There are complete products that does this, and does it well. If you build one yourself, you are just re-inventing those off-the-shelf tools. Basically there are two ways you can do it: 1) use analog solution, voltage reference and comparator, 2) use digital solution, a ADC and a micro controller. \$\endgroup\$ – user3528438 May 15 '17 at 14:19

OK, after poking around I figured out what I want is called a Low Voltage Disconnect ( LVD ).

I found an article on how to build one, Simple Low Voltage Disconnect with NE555, however it's probably easier just to buy one.


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