In an effort to simplify a small solar system and to keep the cost down, I am wondering what (if any) drawbacks there are to having the Vmp (voltage at maximum power) in a solar panel setup, at 12.5% over the nominal battery bank voltage and not using any solar charge controller. For example, for a 48V battery bank, 12.5% over is 6V over which is 54V of charge voltage. This is in line with (the same ratio as) the 13.5V trickle charge voltage often used on a 12V battery.

So the idea is to select and combination of panels such that the series Vmp voltage is close to 54V (for a 48V battery bank), and NOT use a charge controller, with the assumption that the batteries will never get overcharged. However what I am wondering is as the batteries become close to fully charged and draw less current, will that "unload" the panels and allow the charge voltage to go higher, closer to the Voc (open circuit voltage) of 70V or so, and thus cause a "chain reaction" of increasing voltage at decreased load, thus possibly overcharging the batteries.

I guess one easy way to check this would be to just take a fully charged 48V bank of batteries and directly apply solar panels with a series Vmp of close to 54V and check on an ammeter how much current is flowing. If it is something small like 1A, then that will not overcharge the batteries as it will just "fight" self discharge (I have four healthy identical 12V 100Ah AGM batteries in series). However if it is something more substantial like 4A, then that may have the possibility for overcharging the battery bank if it is already fully charged. The maximum current the very small solar system can output is about 5A so no matter how strong the sun is, it wont exceed that. The panels are rated slightly under 5A (Imax).

  • \$\begingroup\$ "the assumption that the batteries will never get overcharged" is false. AS load reduces, voltage rise towards Voc until battery starts to boil acid \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 6 '18 at 5:03
  • \$\begingroup\$ Do the panels always produce max V? \$\endgroup\$ – Solar Mike Dec 6 '18 at 5:05
  • \$\begingroup\$ Is this an assumption or have you actually tested it? The idea is if the load decreases, and the solar voltage then increases, then the load would then increase, dropping the solar voltage, reaching some equilibrium. I will test this very soon (next 24 hours) and report back \$\endgroup\$ – David Dec 6 '18 at 5:06
  • \$\begingroup\$ I am using 2 different types of solar panels (Amorphous and PolyCrystalline). The Poly-Si have Pmax at 35.6V and Voc of 44.3V. The amorphous are around 18.0V Pmax and 23.67 Voc. So the idea is under load, I should see 35.6V + 18.0V = 53.6V which is a maintenance voltage for a 48V bank, meaning it should NOT overcharge the battery bank. \$\endgroup\$ – David Dec 6 '18 at 5:09
  • \$\begingroup\$ David that is only true to if you draw max power \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 6 '18 at 5:12

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