euls1200 avatar image
euls1200 asked

Charge controller switching to float too soon

I have a Smart solar MPPT 100V 50 amp charger controller.

I have 3x 100W panels mounted on the roof of my camper, and another 3x 100W panels sitting on the ground with PVC stands holding them up at an angle. All 6 panels are wired in parallel, and both strings (3 on the roof and 3 on the ground each connect to a 3 way connector) are then wired to a busbar that connects to the charge controller input. All 6 panels are same make and model.

I normally get 500W +/- during the day. Noticed today I was only generating about 70W. My batteries (2x 200ah lithium connected in parallel) were only 70% charged. It wasn't overcast, so I figured there was an issue. Went outside to unplug the panels that were sitting on the ground, then checked the controller. I was only getting about 80W from the roof. 2 of the panels were in the shadow of my roof AC, so I was expecting it to be lower than normal. Charge controller was in bulk mode.

I then threw a circuit breaker to disconnect the roof mounted panels and plugged the 3 on the ground back in. Was getting about 250W. Controller was in bulk mode.

I threw the circuit breaker for the roof mounted panels back on, then the controller switched itself to float mode and was only pushing about 50W to my batteries. Which were still only about 70% charged.

Whatever's causing it is new. Again, I've been generating up to and over 500W during peak hours over the last 5-6 months. It was my understanding that shadows on any panel would only drop the total system wattage if all of the panels are wired in series. None of them are.

Any idea what to check? Why would the controller only switch to float mode when all 6 panels were connected???

Any help will be fully appreciated. Thanks.

smart solar charging behaviour
2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

3 Answers
mvas avatar image
mvas answered ·

When the circuit breaker, for the shaded roof mounted solar panels, is energized, what happens to the input voltage? Does the voltage drop?

2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

euls1200 avatar image euls1200 commented ·
It don't think it did.... I'm pretty sure the voltage was still around 17-18v. Forgot to screenshot it, though, so I'll verify tomorrow.
0 Likes 0 ·
euls1200 avatar image euls1200 commented ·

Yeah. Verified it today. It switched to float mode before the batteries were full. Was over 18v in, but only pushing 3-4a out. Batteries were 80% full.

I disconnected the roof mounted panels and it stayed in float mode. I had to disconnect all panels and the batteries from the controller to power it off to get it to go back into bulk mode. Had to leave the roof mounted panels disconnected to get it to stay in bulk mode. Connecting the roof panels, too, it went back to float.

Any ideas? Seems like it only happens if there's a shadow on any of the roof mounted panels. Which SHOULDN'T be an issue with everything in parallel, right?

0 Likes 0 ·
JohnC avatar image
JohnC answered ·

Hi @Euls1200

With Li batteries you should be using a Fixed Absorb time. The Tail (Amps) setting can override this, so try setting that very low, like close to zero. The Tail dropping it to Float is consistent with your shade observations.

You could also look at the Re-bulk setting, and the reliability of your SOC reading.

2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

euls1200 avatar image euls1200 commented ·

Thanks for your response. Kinda new to solar, so I'm still learning. Appreciate the info.

I didn't have expert mode on. Was working fine without it until this week. Never had shading issues over the winter, roof mounted panels were always facing south.

Switching it on:

Absorb time is already fixed. But it's only at 2 hours.

Tail setting isn't on. Were you recommending turning it on?

Re-bulk is at 0.1v.

0 Likes 0 ·
JohnC avatar image JohnC ♦ euls1200 commented ·


Ok, if the Tail is off it's not that. Leave it off, but I'd put something like 0.1A into that box anyway (zeros aren't always handled well by coded logic).

Very generally, LiFePO4 batts are usually considered fully charged when they reach Absorb V under a realistic charge rate. Then given an hour or so at that V to balance the cells. This is why I mentioned the SOC, which is always a calculated figure and may be misleading if it's incorrect. I've already presumed your batts are LiFePO4, but making more assumptions isn't good.

Again in general, a battery can't accept much current if it's fully charged. Keep this in mind when assessing the validity of your SOC.

1 Like 1 ·
euls1200 avatar image
euls1200 answered ·

LiFePo4, yes. Sorry.

They weren't fully charged. Once I reset the charger (disconnected all panels and the battery from it; batteries remained connected to the rest of the system), then connected just the ground based panels that were in the sun, it charged back up to 98% by the time the sun went down. It didn't take an hour after plugging the panels back in to get a decent charge out of them, either. Less than a minute.

I'll try setting the Trail to 0.1 tomorrow. Thanks.

Just out of curiosity, is it possible there's something wrong with the charge controller? I wired the panels in parallel to *avoid* this kind of thing. Was under the impression shadows would only do this with series wiring.

Also curious if it could be due to having both roof and ground strings wired to a busbar instead of an actual combiner box.


2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Related Resources

Additional resources still need to be added for this topic