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Low voltage shutoff

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  • Low voltage shutoff

    So im looking for one of two things hopefully someone can point me towards an existing product. Either a power inverter with a low power shutoff that is higher than 10v (that seems to be the standard) or a device meant for 12v in and 12 out and shuts down around 12. Im trying to setup a on board dvr on a buddies food truck that will run on constant power so that there is no issues with remembering to turn it off and I need to make sure that it doesnt leave them with a dead battery in the morning after sitting over night or for a couple days at a time. Both the dvr and camera can be run on straight 12 volts or with an ac adapter so either option would be fine. Or if anyone has a better idea Im all ears, I have considered a dedicated battery and battery isolator but there is very little space on this thing.


  • #2
    10V? That IMO is too low. Usually I see 10.5V and even that can be a bit low, especially if it's a relatively low discharge current.
    Even with UPS systems that I used to deal with (eg, I'd devise specs and evaluate tenders), though we could go as low as 9.6V we usually opted for 10.5V, and that assumed the batteries could be recharged ASAP. And of course, no engine cranking was involved! And that was with a 10 or 15 minute discharge (from fully charged to its endpoint of eg 10.5V); it's endpoint voltage would be higher for slower discharge rates.

    You may be better off with a PC etc type PSU like the Carnetix, DSATX etc as found on this site. (Maybe not the M2 & M4 series etc due to their reported noise problems.)
    They have programmable shutdown voltages as well as ride-thru features (ie, to survive cranking voltage dips etc).

    Otherwise maybe a convertor from places like, and if they don't have a suitable shutdown, maybe a battery protector like the ~$20 MW728.
    (Funny, I just replied in another thread about the MW728! Elsewhere I have explained how the MW728 can be tricked into higher cut-off voltages than its standard 11.5V and a way to negate that proportional increase to its turn on voltage of 12.5V else have a different increase for that. And as I recall, those mods each require an extra relay plus a diode or few, and can also prevent an on & off cycle of at least 15 second cycle periods that can occur, though a single relay can also prevent the same.)

    The above is irrespective or in addition to any isolated two battery setup. (A UIBI system I hope!)


    • #3
      Thanks for the recommendation. I ordered the carnetix p2140 and it works perfect


      • #4
        A second deep cycle battery for running such accessories would be the safest bet if you want to make sure you can start up in the morning.

        Cheap and dirty way - You can use a relay running off the ignition wire to do the isolation between the batteries when the ignition is off.

        More elegant way - An Anderson Power Products battery isolator - it only connects the batteries together when it sees the primary battery terminal rise up to over 13.5 volts, which only happens when the engine is running and the alternator is charging.


        • #5
          Or the most elegant and cheapest way - the UIBI - a relay energised by the alternator's (regulator's) charge light. (Assuming it exists, and that it can drive the relay else an intermediate smaller relay or a transistor or MOSFET buffer.)

          It's the same cost as the IGN +12V relay.
          It has the same number of wires and connections except the coil+ (#86) wire goes to the alternator's D+ or L terminal/wire (maybe thru a lower current buffer) instead of IGN +12V.
          And best of all, it is fully automated and has none of the disadvantages of smart or intelligent aka voltage-controlled battery isolators. (No drop outs, no delays, no hold-on after the engine stops, etc.)

          But if getting Anderson Power Products battery isolator or similar, DO NOT get the diode isolated or MOSFET types - only use battery isolators that use relays.

          And don't forget the fuse or circuit breakers at EACH battery end of the inter-battery link!


          • #6
            ditto on what OldSpark said. The Anderson 100 amp relay isolator is the way to go.

            Good suggestion on taking the coil signal from the alternator idiot light output too. Not all alternators have them still, but it would do better than taking it from the ignition circuit.

            Also ditto on fusing both battery feeds. The ATO Maxi-fuse is a good choice here.


            • #7
              Actually I hate MAXI fuses for some reason... Though granted, they can be bluddy hard to remove, and if that indicates a good connection, then I should prefer them to my preferred ATS fuses else plastic fuse links for higher currents.

              And for dual batteries powering stuff like fridges, I recommend self resetting circuit breakers (each end) which are small & cheap up to 50A, or 30A in ATS, and under $10 each.

              The TWO fuses are often missed - especially when there is a fault either side of the isolator LOL! Flamed vehicles, or exploding batteries or terminals, or melted wires and smoke....

              The UIBI (Ultimate Intelligence Battery Isolator) is an oft overlooked yet obvious and almost universal solution to battery isolation. It's been used for decades.

              And since almost every alternator has a D+ or L (charge light) terminal... (Many alternators won't initiate charge without a bulb to that circuit).
              Some modern EMS-controlled alternators (eg. DP types) may not have the D+ or L, but they usually have a "charging" or "not charging" signal that can be used.

              And BTW "charge light" wrt the UIBI etc means an is charging or is NOT charging light or signal - they are logically the same, only the fixed connection to the relay's coil might need altering (ie, to +12V instead of GND).

              Of course a smart or intelligent isolator with a relay is a UIBI with added artificial circuitry to ascertain of the vehicle is charging. That's required for stator systems (marine, some RTVs & bikes etc) but adds cost and unfortunately adds much undesirable behavior.