I recently bought some used solar panels that are about 25 years old, because I got a great deal on them. They have some odd operating values, which is confusing me as to which solar charger will work with my system.

I have 6 panels @ 230 watts each (1380w total).

Their specs are ( as far as I can tell. I needed to peice a label together because there is really no information available on them online):

Name: Mobil RA-220 8b
Peak power: 230w
Vmax: 8.27 (V)
PF Pmax: 27.77 (I)
Voc: 10.55 (V)
Isc: 30.49 (I)

I am attempting to make a 24 volt system with either 4 or 6 panels. My friend said some of those values above are labeled incorrectly (volts vs amps). However, with her math, she recommended the Victron MPPT blue solar 100/50 solar charger, which is rated at:

max 1400w
Max PV open circuit voltage: 100v
Max battery current: 50 amps

Would this charger work with this setup? If not, why? And is there another comparably priced ($300) charger that would work in the market?

I know I need to connect my panels in both series and parallel. What specifically would you recommend?

Thank you!

  • 2
    It depends if you have full sun on the panels. Depending on the configuration 1 panel in partial shade can kill the output. Also solar panels do have a life span and start to fail at 20+ years so checking each panel in full sun to check if it is producing the rated voltage would be my first step.
    – Ed Beal
    Jul 9, 2016 at 20:19
  • 1
    Measure the open-circuit voltage output with a meter. It helps to do this in the sunlight. When they specified the service life of solar panels, they really had no idea, they were wildly guessing at when something will go wrong like wire insulation rotting out, the solder crystallizing, the glass hazing out etc. They say 20-30 years because no one would believe 200, but that is well within the realm of possibility. In the 1980s when I saw Planet of the Apes and the subway station lights still worked, I laughed and said solar/LED. Jul 10, 2016 at 21:28
  • thank you both. I checked the voltage on the panels, four of which, test between 9-10 volts. The other two are missing their central junction box, so they are not showing any voltage. Knowing that these 4 panels are outputting ~9 volts, do you know of a good configuration (series / parallels) that would bring my system to 24v? Could I be slightly over (~27v)? My thinking is 920w (4 panels) / 27 = ~34 amp rating charger. Similarly, 1380w (6 panels) / 27 = ~51 amp rating charger. Can these values within range, or must they be exact? For instance, could I either buy a 30a or 50a charger?
    – swis24
    Jul 12, 2016 at 17:37

2 Answers 2


Wire the panels all in series to get the highest possible voltage to the controller. This will keep your current lower, requiring less wire, and allow the MPPT charger to work more efficiently. If you are charging a 24V battery bank, you need at LEAST 26V but it will operate much better with higher voltages. Just try not to exceed 100V. 6 panels is probably the max you'd want to run.

The Mobil ra220 panels actually have 2 modules each, and if you're measuring ~9V per panel they are configured in parallel. I'd recommend rewiring them in series for the reasons stated above.

  • thank you, FYI: I have not hooked them up yet, only tested the raw wire coming out of the panels. so with your recommendation, I am gathering my Voc would become ~54v and my amperage would remain at around 30a? Is this correct? Would the 100/50 charger work in this case? Even though my battery bank would be 24v, my MPPT charger could convert the ~54v current down? Is this the best possible configuration for my system (rather than doing 2 series), and does a large difference in voltage (54 -> 24v) put stress on the controller?
    – swis24
    Jul 25, 2016 at 15:56

I just did a math to your solar panel system.

1, As you said, the panels are about 25 years old, the efficiency must reduce about 20%, so the 230 Watt solar panels can only produce about 180W power, in the same philosophy, you can only get the optimum current about 21.76.

2, After 20% reduction, total power could be only 180W x 6 = 1080 Watt. And 50A solar controller is good for your 24V solar systems. I agreed with your friend, she did a good math on it.

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