4 Replies Latest reply on Jul 31, 2019 9:18 AM by ldjohn

    Lead acid protection circuit

    zagene

      Hi everyone,

      I'm always looking for alternatives for a DC supply, as we have a constant problem with mains supply failure.

      Add to this that I am a Ham operator who like to keep things up and running, even on DC power if required.

      For DC supply I have the normal AC to DC switch mode supplies but the problem is with a suitable protection for my (very expensive according to me) AGM 12VDC batteries.

      If you fail to constantly monitor the AGM battery output voltage or with swapping batteries accidentally (and it do happens if you are in a hurry at night) connect it reverse polarity you have a problem.

      I have read many articles on battery protection especially with RPP and current flow back prevention as well as a battery low voltage cut-off so I'm starting a project that is compact and cost efficient enough to add to all my SLA's and AGM (and also all flooded cells for that matter) 12V batteries.

      So my circuit is more a combination of several other circuits combining an efficient RPP without to many heat losses as well as combining it with a ATTiny ADC to read the voltage to cut off the battery at a preset low voltage, nothing very extravagant or to large or difficult to populate on a single sided PCB and fit it on the battery output.

      My requirements would be to feed a radio or other load with DC at a maximum of 30A @ 13.8VDC (as required by my radio when transmitting.

      The circuit area I'm not very sure of is the cut-off feed from the ATTiny45 via the 2N7000 that will cut the ground from the main P-ch MOSFET as a high side switch with a maximum current of 55A, Maximum gate voltage of 20V and a RDS(on) of 20mOhm at Vgs of -10VDC.

      The second problem may be an Inrush current needed by the load which I do not totally want to limit as in certain instances my radio may require but I have to protect the MOSFET.

      The other thing maybe to place a uni directional TVS at the battery poles of the circuit e.g. 1N6377 to clamp high voltage peaks etc.

      Please advise me on the following circuit: (its available on PDF format here https://drive.google.com/drive/folders/1jG2ML2XMi52CiVaMMP-QXK-w0ViSZoYC?usp=sharing ) as well as the IRF4905 datasheet

        • Re: Lead acid protection circuit
          jw0752

          Hi Eugene,

           

          You may want to check and verify the polarity of the IRF4905 as in its current config the internal Schottky will conduct current battery to load. As far as the rest of the design I will defer to some of the other guys with more design experience than me.

           

          John

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          • Re: Lead acid protection circuit
            dougw

            Interesting application.

            You could also use something like an LTC4352 to create an ideal diode which also has under voltage protection.

            An alternative is LM74700.

            3 of 3 people found this helpful
            • Re: Lead acid protection circuit
              jw0752

              Hi Eugene,

               

              After reviewing the polarity protection application I believe I was wrong to question the polarity of the IRF 4905. It is correct as you have it. Sorry for the confusion but I have a little difficulty analyzing the current flow and logic in the P Channel MOSFETS. If it wasn't correct as you have it the internal Schottky would allow reversed polarity current to flow.

               

              John

              • Re: Lead acid protection circuit
                ldjohn

                Hi there Eugene, one scenario is when you try to switch off the load, the internal diode will conduct inside the mosfet, thereby continuing to conduct despite your efforts to stop it.

                1) consider placing back to back mosfets in your design to isolate the load, because of the diodes.

                2) consider adding another circuit in to allow a PSU to be connected as well as managed.

                3) in certain scenarios where you have equipment that fails ( radio, charger, PSU) it may be good to allow under and over voltage protection. Sometiems a charger gets stuck and takes the battery over 16V. In your scenario, simply add another channel to the A/D to read input voltage for PSU and another set of back to back mosfets to control the output.

                4) during high transient cut over currents, it may be pertinent to investigate limiting of inrush current by adapting the resistive load between Pchan gate and transistor pulling the circuit to ground for activation. This will create a small ramp time. Once engaged, no more ramp needed so thats good.

                5) make sure that you put protection on the inputs of the A/D converters. i usually place a 5v1 Zener Diode (assuming 5v logic) between the A/D input and ground so that it limits maximum input to the A/D. This is also a great trick as if things go really wrong, the A/D pin is protected from over-voltage as well as limits negative voltage to around -0.5V which should be in the micro's safe area.

                Keep going. You have the makings of a great project.

                 

                On choice of mosfet.... under full conduction at 30A, the 0.02R resistance will add about 18W worth  of dissipation and what looks like 0.6V drop over the device. Not bad at all

                1) put more in parallel, making it a great deal cooler ( by putting in parallel, you can reduce power dissipation

                2) put more in parallel, making the total on resistance better

                2) put a decent strip of heat-sink on the devices. Aluminium U-channel / whatever you have will be great. If you can use an aluminium enclosure, that will work as well. Remember to put a small fan on it to keep it cool. Don't waste too much energy.

                 

                Regards

                John

                2 of 2 people found this helpful