solar back online

what happened to the old MPPT

The old controller, a 4215BN, put up with a lot of abuse but finally died in a perfect storm of my own choices and environmental factors:

  • I was aggressively overpaneling the 40A controller with 750w of panel since I already owned it
  • I ran 3x 30.3v Vmp panels in series
  • in 100F ambient temps
  • at 10,000ft elevation

I suspect if any one of these factors weren’t present the controller would have survived.

pwm spare saved my bacon

I had a 20A HQST PWM controller (<$20) on the shelf, having bought it for experiments involving the portable panels. The mounted panels are Imp==8.27A and Imax=8.55A. I judged I could run two panels (~16.5A) at least, and maybe all three (25A) under poor insolation (6.7A each).

I reconnected the panels into parallel configuration and plugged in the HQST. I was surprised to see current often exceeded Imax; I think these are the causes:

  • PWM controllers can apparently see Imax under some conditions
  • 10k feet elevation provided insolation in excess of 1kw/m2.

I ended up running only two panels most of the time and even then there were times when throughput hit 20A and sent the controller in to error and a temporary timeout.

Max harvest from 2x nominal 20v panels running at ~14.7v would be 297w just before the controller tripped. That’s respectable output, but the bigger issue was low harvest in less-than-perfect sun and battery voltage scenarios. Being able to bring the third panel online in a PWM backup scenario would be an improvement.

a new backup PWM

Since 2 panels were sometimes tripping the 20A controller, I’d need more than 30A to bring the third online. I picked up a cheap 40A GCSOAR PWM with big wire terminals for less than $40. That should get around 450w if I end up in trouble again.

installing the new controller

I usually don’t set alarms, but I set one for early today so I could do work on the solar config before the panels started making real power. I made coffee and starting moving the big items outside to make working room.

I yanked the PWM, wired in a new breaker (for shutting off panels), and attached the 50A 5415An controller to the battery bank. Before turning the circuit breaker on I configured the controller in line with best practices charging for this bank:

  • 15.0v EQ - low end because I absorb and float on the higher end
  • 14.7v abs - high end because I deep cycle off-grid every day
  • 14.1v float - high end to make up for epever’s dumb 180 max absorption

I closed the breaker at 7:20A and it started making power in earnest. It’s good to be back to real harvest:

  • 0720 - 91.6w
  • 0730 - 153.2w
  • 0810 - 250w
  • 0830 - Vabs==14.7v reached, output dropping
  • 0930 - 450w (recovery after starting crockpot on HI/150w, settled down to ~350w)

further thoughts

Later in the day after clouds passed the system recovered by making 656w. It was such a short recovery that I am not convinced the extra capacity over 40A is meaningful, particularly given my “insurance for poor insolation” reason for overpaneling. It’s more useful to reduce stress on the controller in Perfect Storm conditions.

I am happy with this controller for the excess capacity and voltage-sensing features. If I didn’t need those I think a $120 40A 4210A/AN (with parallel panels) would suffice. I really do prefer the heavy brass terminal screws on the $139 4215BN. If the 50A model had those brass terminals (and longer absorption duration) it’d be just about perfect.


twitter comment thread for this post