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I am a rank amateur in electronics, so please assist.

I have a solar system (PV) charging 2 x 12v / 100Ah Lead Acid batteries for emergency lighting in my house (Max charge 13.8v). I also have an electric gate which has a 12v 7Ah battery with a built-in charger connected to municipal power.

I wish to disconnect the municipal power and charge the gate battery through the PV system. However, I find that if connected directly, and we have a couple of days overcast, the PV lighting system draws the current out of my gate motor battery as it depletes.

I would like the gate motor battery to not return any charge even if the home batteries are discharged to 10.4v.

Can I prevent this from happening by adding a diode?

What diode should I use? where is the best place to add the diode?

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    \$\begingroup\$ I think you meant ### Ah (amp-hour) batteries, not ### A (amp) batteries. \$\endgroup\$
    – DKNguyen
    Aug 13 at 14:02
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A diode will do it but has problems. The voltage drop means your gate battery will never charge full, and your application likely requires higher currents which tends to rules out Schotky diodes which have unusually low voltage drop (350 to 500 mV as opposed to 0.7 to 1.5 V) are difficult to find in large sizes. If you can find one it would be far simpler in exchange for reduced performance.

You cannot parallel diodes.

schematic

simulate this circuit – Schematic created using CircuitLab

If not then you need a PMOS ideal diode circuit but NOT the simple circuit you find around the internet. They aren't diodes so don't work exactly like diodes, the key difference a PMOS can only block reverse current from flowing when it originates from the input, not the output. Extra circuitry is required to handle that (and you definitely need it with a battery on both sides), then you need extra extra circuitry because the extra circuitry has voltage limits well below 12V

nmos reverse current protection

This mentions but does NOT cover the "extra extra" circuitry

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  • \$\begingroup\$ Thank you, that is what I suspected. Would it work to use a day-on / night off photaic light sensor (a reverse street light on "daylight sensor"? That way, if correctly installed, it would only allow charge through during bright light conditions? \$\endgroup\$ Aug 16 at 6:42
  • \$\begingroup\$ I suspect that the easiest and most convenient way would be for me (as a non-electronics engineer) to just add a small 14W solar panel dedicated to keep the gate motor charged. \$\endgroup\$ Aug 16 at 7:06
  • \$\begingroup\$ @RayJohnRhodes Daylight sensor: There are complications due to the switch you use. MOSFETs can only block current in one direction so extra stuff is needed if you make it out of MOSFETs. You would need to use a solid state relay or something and select one that uses MOSFETs because if you select one that uses something else, depending on whatyou use you might as well just use diodes since they will have a similar voltage drop \$\endgroup\$
    – DKNguyen
    Aug 16 at 14:58
  • \$\begingroup\$ You could use a schottky diode and a small pv to top off the voltage drop from it. \$\endgroup\$
    – DKNguyen
    Aug 16 at 14:59
  • \$\begingroup\$ Thanks for your input. One last thought, Would it solve the problem if I put a second MPPT Charge controller onto the system? My PV panel is 390W and I have a 50A MPPT charge controller. I have a spare 35A MPPT charge controller which I can fit in parallel direct from the PV to the gate battery, leaving the current system unaffected. Would this work or are there any unforeseen problems? \$\endgroup\$ Aug 23 at 9:21

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