I have 24v / 8a (400w) of solar running into a Renogy Wanderer Li 30A 12V PWM. I noticed that when at peak input capacity there was a chance for the in line fuse (20a) to trigger. I've added another 200w of solar with a different charger (MPPT) to the system because the new panels are at slightly a higher voltage (37v) than the first set. Since then the the inline fuse in the first system will trip consistently when the batteries are in the top 80% of their charge.

My question is what causes this behavior? Is it a function of the PWM interval on the charger nearing 0ms in load length and acting as a short? I ask because I once thought to test my solar setup with a 400w bench power supply so there are subtleties here I may be missing.

The system inputs are 400w (2 x 24v) + 200w (1 x 37v) to a 12v battery bank. Both chargers support 12v.

  • I think you are in the wrong forum
    – Traveler
    Aug 22, 2023 at 18:44
  • 2
    Which "forum" would you recommend, @Ruskes? We've had plenty of solar and tripping breaker/fuse questions here...
    – FreeMan
    Aug 22, 2023 at 18:47
  • @FreeMan if so, go ahead answer the question
    – Traveler
    Aug 22, 2023 at 18:52
  • 400W/12v = 33.3A, which is far more than 20A on the fuse.
    – dandavis
    Aug 22, 2023 at 19:31
  • 3
    As usual, Ruskes has not the remotest idea what's on topic here, or there.
    – Ecnerwal
    Aug 22, 2023 at 21:09

2 Answers 2


Presumably the 20A fuse is in the output line.

Since the PWM converts the 24V (nominal - usually quite a bit higher in practice) 8A down to suitable charging voltage for a 12V (nominal - 12.5-14.5 or as much as 15-16 for an equalize charge) battery, the amperage is increased. Though how a 24V 8A (192W) panel claims to be 400W is curious, even allowing for 35-36V actual (still only 288W) So I begin to wonder if you have reported only half your actual panels when you say 24V 8A and also say 400W. That appears to be supported by your updated that there are 2x 24V inputs.

But 288W would be slightly over 20.5A at 14V, which is a typical voltage in the range of nearing the usual end of charging for lead-acid chemistry. If you actually have 400W of panels connected, it's not hard to see that overloading with a 12V battery voltage.

  • On the load side I have a Voltage meter and it does hit 14 at the end of charge. I believe the fuse (on input + from the panel) is set to the short voltage of the PV panel on that circuit. The #10 cable can go up to 30A. I'm also guessing my MPPT controller on the second circuit will end up doing the same when I connect a second panel to it too as the amp increase on voltage drop is physics. Are there any other options besides going to 24v batteries? Aug 24, 2023 at 16:06
  • More charge controllers with less PV per controller. But that illustrates why most people end up going to 24V or 48V batteries, since the PWM/MPPT charge controllers are current-limited, but most will do several different voltages, with more PV power handled per controller the higher the battery voltage is.
    – Ecnerwal
    Aug 24, 2023 at 16:11

Anytime I have a fuse blowing, my first inclination is to put an ammeter in series with the fuse, so I can casually and routinely observe actual amps going through the fuse. Then I can get a sense of whether the amps correspond to my expectations and the design basis of the system. If expecting 8 amps and regularly blowing 20A fuses, I would want to know if it's 8A normally and 20A spikes, or 20A all the time due to a design error.

  • I like this idea, and I have a version of the bayite DC 6.5-100V 0-100A (Amazon) on the load side of the system. Funny too that the picture in the "example" setups on the charge side seems to use the same charger I have too. I guess for chargers cheep = popular. Aug 24, 2023 at 15:55

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