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I have 400W solar panel connected to a 12V 60A MPPT charge controller charging a 12V 110Ah battery (Phoneix TX-1000.) I'm running a 12V DC load (300W) directly from the battery.

In the evening when there is no solar, the MPPT still remains energized from the battery.

enter image description here

I want the MPPT charge controller to be off/de-energized if there is no power from solar.

The simplest way to do so is to use a diode between the MPPT and the battery to prevent the backflow from the battery to the MPPT. The problem is that such rating (60A) diode has high voltage drop and they aren't available in the local market.

Can we use a MOSFET in this case as there will be negligible voltage drop and they're easily available in local market?

JRE
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engr.duet11
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    How much current is the MPPT controller using at night when it is deactivated? – Andy aka Jul 08 '22 at 12:48
  • Didn't measure it, however all of its indication lights are on. – engr.duet11 Jul 08 '22 at 12:53
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    You need a very low on-resistance for the voltage drop to be "negligible" across a MOSFET at 60 A. Something to think about if your local market may carry counterfeit products. – pipe Jul 08 '22 at 13:05
  • MOSFETs with low on-resistance are easily available here, in the local market. But can I use MOSFETs in this case? If yes, then how... – engr.duet11 Jul 08 '22 at 13:14
  • if you search on "mosfet reverse voltage protection" you will find plenty of ideas, but at 60A you would want to do some pretty careful device selection and heat design, along with testing and maybe using parallel devices. – danmcb Jul 08 '22 at 13:22
  • 60A is MPPT rating while MPPT output current is under 30A. For 400W solar panel on maximum Efficiency 0.85, it output power will be 400W * 0.85 = 340W. Dividing 340W by MPPT output voltage (14.2V) we get 24A. So current will be well under 30A. – engr.duet11 Jul 08 '22 at 14:01
  • Not just a MOSFET: https://electronics.stackexchange.com/questions/480237/nmos-reverse-current-protection/480238#480238 – DKNguyen Jul 08 '22 at 14:46
  • Thanks. That's quite helpful. – engr.duet11 Jul 08 '22 at 15:42
  • Please measure the back current drain. My guess is that it’s negligible and your efforts are best spent elsewhere. – winny Jul 09 '22 at 08:04

2 Answers2

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Yes, high current MOSFETS are used in our PWM charge-control 15yr-old design, on the panel side, to prevent current leakage back through the panel. The first models were only 20A, but as MOSFET designs improved, it's gone up to 80A. The 25-yr old design uses an external diode.

The reason for using MOSFETs wasn't just to save energy: it was to reduce the size of the heat sink. A normal solar system will 'almost always' have too much energy available 'almost every day', and the power lost every night is negligible. Those controllers don't use much current to run the clock and memory, and on a short day, 10mOhm/60Amp=600mW reverse leakage protection could loose just as much power.

Overnight energy loss only becomes an issue when the controller is left on for a long period with the panels not connected. We did't have a disconnect MOSFET on the battery side because then we'd need the extra complication of adding a button battery for clock and status.

david
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Perhaps you can use an "Ideal Diode controller", an active compoenent that manages a MOSFET to minimize the forward voltage drop, and prevent reverse flow:

idea diode cct

Image from engineering.com's primer on these devices: https://www.engineering.com/story/ideal-diodes-or-else-part-three-ideal-diode-controllers-and-applications-in-the-power-domain

I have used these for power supply OR-ing in the past up to 20 amps and I like them.

Bryan
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