Low battery voltage cutout part of solar charge controller?

[michaelrider5]

I have twin roof mount Renogy 100 Watt Solar panels feeding a Renogy DCC30S DC to DC charge controller with an integrated MPPT Solar solar controller charging twin Trojan 105 6V 200 AH batteries. I have on two occasions had the solar controller cut off the charge to the battery when the voltage falls below a certain value. My trailer sits a lot, especially during Covid, and I have a tendency to "set it and forget it". The last time I checked the state of charge and charge rate was months ago, and then the batteries had 12.7V in the shade and 13.8 in full sun. The MPPT controller also does a lot better job pulling a charge from any sunlight at all than the PWM controller I had in the past, so I didn't give it another thought until I went up to check the water in the batteries and found them pretty much dead at 8 volts. The solar panels were putting out 16 volts at the controller input, even with the sun low in the west this time of year. Apparently if the voltage in the battery falls below a certain value the controller cuts off the charge. If I charge the battery back to above 12V, the controller takes over and, weather permitting, will keep them charged, but for how long????
I did find a small .2A draw, which would be enough to kill the batteries over a couple of weeks if there was no charging, but when I parked the trailer, even on cloudy days there was output from the solar to the batteries. It's counterintuitive that the solar would be shut off when the batteries get low. I may be missing some subtle note in the owners manual or just clueless?? We are on the central coast of California and the weather is pretty mild, and as most of you may know, we have NOT had many cloudy days this year.
Has anyone else come across this problem? Renogy is hard to get ahold of with questions like this, but they have a ton of systems out there, so someone has had to have the same issue.
Thanks in advance for your help and Happy Motoring!

 

[Isaac-1]

How old are the batteries? It sounds like the batteries are shot and are not holding a charge

 

[nibroc]

have to agree with the bad batteries^^^^^^^^^^^^^^^^^

 

[Mark_K5LXP]

What do the status indicators on the controller show? Knowing what mode it's in might reveal why it is, or isn't charging. I do see in the manual that there is a chassis battery priority mode, where the house batteries don't charge if the chassis battery is low. That would mean the chassis battery is perpetually low which is a separate question but maybe that's part of it. I don't agree that bad house batteries would cause this. Batteries that low would be sucking up as many amps as you could give them. My guess is that for some reason the batteries aren't receiving a charge (controller, or other reason) and the parasitic load has pulled them down. Charging and testing the batteries would be one data point but per the OP there's some reason why they're not charging when they should be.

Mark B.
Albuquerque, NM

 

[Ex-Calif]

I'd back the bad battery theory barring any other inputs.

 

[elewis33]

My understanding of solar charge controllers is that they will cutoff charging if the battery voltage gets too low because it appears to the SCC that there's no battery at all. And again, my understanding is that if you have solar power coming into an SCC with no batteries connected you will fry the SCC. So bottom line is I think this is a self protection mechanism of the SCC.

 

[Mark_K5LXP]

The only cutoff voltage limits shown in the user manual are low panel voltage and high panel voltage. Nothing about low battery. It would be a poor design if the real world condition of a disconnected battery would "fry" a solar controller.

Mark B.
Albuquerque, NM

 

[elewis33]

The only cutoff voltage limits shown in the user manual are low panel voltage and high panel voltage. Nothing about low battery. It would be a poor design if the real world condition of a disconnected battery would "fry" a solar controller.

Mark B.
Albuquerque, NM

I don't disagree with your comment about the disconnected battery, but this is directly from the Renogy SCC manual.
You would hope that a charge controller is smarter than this, and I'm not saying this is applicable to all SCCs but this is what the manual says for mine. When I saw this I took careful note of it because I thought it wouldn't be a problem. But apparently for this one at least, it is.

 

[Ex-Calif]

The question remains why did the batteries discharge in the first place?

If OP comes back we need the codes if any and the state of the current batteries...

 

[Mark_K5LXP]

To be sure, not all SCC's are the same. Hard to imagine a company like Renogy wouldn't have that as a standard product feature, it's not like they haven't done this before. Maybe there's more to the story. I've been using Morningstar products and they have no such issues, one I'm reading up on actually has you check open circuit voltage from the panel before connecting the battery. Having a battery disconnection while in service can happen for a number of plausible reasons even if just basic servicing, an intentional shut off or maybe a fuse/CB opens up. If the controller can't handle that, then it's a poor fit for a system in the real world.

Mark B.
Albuquerque, NM