Baffling voltage drop

Hello everyone. This is my first post! I am having an odd problem where the open circuit voltage of my two solar panels is 21 V, but as soon as I connect it to my charge controller (a Renogy Wanderer Li 30A PWM controller), the panel voltage drops to about 13.4V. The photos show the voltage of the solar inputs, and the battery outputs. But when I disconnect the solar inputs from the controller, the voltage there reads 21V, which is normal. Any ideas what is happening? Is the charge controller busted?

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That is normal for PWM SCC, as the battery is charged up the Voltage will go up.
BTW, what is the full spec of your panels and how many and how they are arranged?

I'm afraid that's what PWM controllers do.

They simply connect your panel to the battery and let the current flow. As the battery approaches fully charged the controller actually does its "PWM" thing and chops up the current and eventually turns it off so that you don't over charge your pack.

If you want maximum power from your panel you need an MPPT controller which actually does power conversion.

To elaborate a little, PWM works by shorting the panel to the battery when in bulk mode.

Once absorption voltage is reached, it begins cycling the connection 100s or 1000s of times per second with decreasing duty cycle to regulate the voltage.

EDIT: @crossy beat me to it...

What's the Vmp (voltage for max power) of your panel (it will be on the rating plate)?

Many smaller panels have Vmp somewhere around 14-18V so it will actually be close to best rate anyway.

Bud Martin said:

That is normal for PWM SCC, as the battery is charged up the Voltage will go up.
BTW, what is the full spec of your panels and how many and how they are arranged?

The Definitive Guide to Solar Charge Controllers: MPPT and PWM Charge Controllers in Off-Grid Solar Power Systems

The Definitive Guide to MPPT and PWM Solar Charge Controllers in Off-Grid Solar Power Systems. Read Now to Discover How to Select The Best Controller!

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Click to expand...

Thanks Bud. I have two Renogy 100W panels with an open circuit voltage of 24.3V and a optimum operating voltage of 20.4V, in parallel, connected through the SCC to a 12.8V 200Ah LiFePo4 battery. My concern is that if the SCC is cutting voltage at 13.3V, the battery is never getting fully charged, at 13.6V. But it sounds like that's normal with PWM SCC's. Is that right?

I do have another concern - the SCC does not seem to know what mode it's in. If you check the screen shot above from the Renogy DC Home app, there is a "---" under the image of the SCC. That is where the charging mode (boost, float, etc) is supposed to be. Renogy says that this is "---" because the input voltage is too low.

You probably need more sun. At the moment (in winter), sun is not sufficient enough to provide any reasonable charge, dus at the slightest load, voltage and amps drops.

It will probably display something usefull if the solar is getting stronger. Might be an error in the app as well, since there are more obvious errors in the app: Charging amps 1.58 Volt??


Anyway, apart from that I would highly prefer a proper MPPT, especially with a LiFePO4.