Is This Wrong?

BC_Warren · August 2017

I had my solar panel stolen last week and bought and installed a new one this week. (That part is immaterial, but it still bugs me).

The panel is rated at 100 watts. It claims it can generate 21.5 V on an open circuit and deliver 17.1 v to the battery. It is apparently fused at 5.8 A.

Here is what I fear may be a problem.

My battery has only about 12.9 V charge, which the voltage regulator optimistically calls 99% full.
When I put a voltmeter to the output of the solar panel, I get 19.5 V
When I put the voltmeter on the input side of the controller, I see it is getting between 12.9 and 13.5 V.

The controller shows it is providing no further charge to the battery. I would like to see the battery up above 14 V before it is considered approaching full.

Advice and insights are very welcome.

Solray · August 2017

A 12 volt battery charged to 14.4 will not remain at 14.4. Are you using the data from the controller to know what voltage it reached when the charge was considered full?

Raj174 · August 2017

@BC_Warren
If there is no load on the battery, then it is considered full at 12.7 volts.

Rick

Estragon · August 2017

I'm assuming you unplugged the panel to take the voltage reading, and it was in full sun at the time? Also that you plugged it back in and when reading the voltage at the CC input, it was still in full sun?

If these assumptions are correct, there are a few possibilities. A defective panel or connector seems most likely. When the old panel was stolen, could the wiring have been damaged during the theft?

If you have or can get a load (like an automotive light bulb), you could check whether the panel voltage collapses under load at the panel output. If it looks okay at the panel output, repeat by disconnecting the CC input test and with load there. If the voltage collapses under load there, wire or connection damage is likely.

littleharbor2 · August 2017

A battery will clamp the voltage at slightly higher than it's own voltage. If you have a pwm charge controller it will only pass whatever amperage your panel is producing. You lose the potential power between the voltage of the battery and the Vmp. of the panel. An MPPT controller, on the other hand will take all potential power, at whatever voltage is coming in from the panels and step down the voltage while stepping up the amperage, giving nearly all the wattage available from the panel.

BB. · August 2017

More or less, solar panels are current mode devices. Full sun with panel square to sun, it will output around 5.8 amps with 0-17.1 volts on the output (a little lie, really about 15 volts with hot panel on hot day...). Above 15 volts, the output current will fall to zero (over 19-20 volts or so).

So, the Vmp of the panel needs to be higher than the Vbatt+Vcontroller-drop+Vwiring-drop.

With 13.x on the Vpanel input, it would appear that your controller is "turned on" and sending current to the battery bank.

Note that the chart above is resting voltage. If the controller is charging, the battery voltage should rise to 14.75 volts (or whatever absorb voltage it is set for). And if enough sun, hold absorb voltage for 2-6 hours.

- Bill

BB. · August 2017

Also, your wiring and fuse should be 1/0.85 times larger than the Imp rating of the panel:

5.84 amps / 0.85 NEC derating = 6.9 amps (next standard fuse/breaker larger)

-Bill

BC_Warren · August 2017

Thanks, folks -- that has given me a lot of leads to check out.

Is This Wrong?

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