backchannel: new guy nails small solar setup

Most of my backchannel comments are lamentations about bad decisions and techncal ignorance.  But this newcomer did a great job designing a small system: and it’s worth talking about.

> Just need enough power to charge a phone and occasionally a laptop, power the roof vent fan, and a small rv fridge would also be nice.

It’s nice to see someone have a reasonable expectation of what can be powered from a small setup.  No electric cooking, no A/C, etc.   Carry on.

Here are the parts he picked out, and my comments on them:

  • Keyline 140A Voltage sensing isolator –  might be oversized (see battery below), but that’s no biggie if you have the $$.
  • Renogy 30A Wanderer Li PWM controller –  a decent unit for $30, and can be user-configged in the Li/USER mode.   MPPT is not needed because of the isolator doing the heavy lifting when Vbatt is low.
  • HQST 150w poly panel – poly is a good match for PWM, due to the lower Vmp and higher Imp specs.  150w on its own would be iffy, but with the isolator it will act more like 300w of panel.
  • 100Ah Chinese AGM – I’m not in love with AGM, but it’s priced fairly, will be getting it fast/hard from the alternator the way AGM likes it, and the batt has higher voltage than many AGM.  It says up to 14.9v Vabs and 13.8v Vfloat on the label.  Not only will this keep the batt healthier but the higher the Vbatt the the greater the PWM output.  Note: AGM accepts a max of ~C/3 when discharged to 50% DoD.  That’d be ~35A max here so a 50-75A isolator would be fine.
  • Bestek 300w PSW inverter – dunno what he’s using it for, but this little thing is a jewel at the $50 price point.  The output on oscilliscope really is PSW, although a bit ugly.

optimal performance

My suggestions for optimal performance with such a system, pasted here:

  • turn off the inverter when it’s not being used
  • do any optional driving in the morning rather than the afternoon – this will get the bank voltage up where the PWM controller can make real power
  • if you choose to idle in the morning (ie, not driving that morning), experiment with very short periods (like a few minutes). All we need is to get the voltage up to where the PWM controller can find its feet. No need to idle for an hour or anything.
  • charge the phones/laptop when there is extra power (as when alternator charging or when the bank is float or late absorption)
  • pick up the “optional” battery temp sensor for that controller and tape it to the battery for most accurate charging, particularly if running at the higher end of the allowable voltages.
  • remember to set the charging profile to AGM/sealed instead of lithium, or absorption voltage will be too low. You could even run the Flooded profile, since the EQ voltage stated in the manual is still within the voltage range specified on the battery label.
  • since the controller doesn’t have a display, you may want to get one of Renogy’s doodads to plug in for phone access, or just wire in a shunt in between the battery and the rest of the system. I haven’t tried one of the “clamp on” types but I’m sure they work fine.

 

 

 

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