Not charging past 65%

[B3nn]

Hey all.
I've had an off-grid office solarsystem that works great typically, undersized but works great.

I have the 40arenogy Rover charge controller. With 4 100w solar panels that are 2 in parallel and 2 in series.
I've always got about 40~ volts and 4-6amps. Recently I got a pair of extra panels that are flexible, I was planning to use them for camping and overlanding in my bronco. And when I wasn't using them I would have them on my office as a third set of panels. Since adding these to my system it seems like the batteries never charge past 65%~.
And I noticed the amps have hardly gone up with the extra panels.
If I disconnect this third pair of panels it will charge up higher but it would take longer.
Is there a better way to add this new pair of panels to get better charging?

 

[Browneye]

A second controller?
What are you using to indicate state of charge? What kind of batteries?

 

[Bud Martin]

What are the spec of the panels?
So you go from 2s2p to 2s3p wiring for your panels, correct?

 

[B3nn]

I'm looking at the charge controller mostly, to check the batter charge. But I also have a switcher that checks the voltage and a shunt that checks the battery charge.

I have renogy agm batteries. 2 200Ah

Ya 2s3p.

I'm a totally newbie FYI

 

[Browneye]

A shunt meter would need to be reset at full charge in order to show SOC accurately.
A charge controller isn't a good SOC meter.

AGM's have a fairly constant voltage drop as charge level drops:

 

[Mattb4]

If you want to understand SOC under load voltages this pdf: https://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf gives the curves for various state of load as well as other information concerning charge rates and such.

 

[B3nn]

Ahh. So you're saying I shouldn't focus on the battery percentage as much as the voltage since the percentage is way less accurate? Cause it can reduce the percentage just by having a load on the battery, even though the battery actual percentage isnt as affected??

 

[Browneye]

Yes, that's a good way of explaining it, as we don't know what the percentage is based on. Normally a shunt-meter is set to 100% when the battery is full, with a given capacity, and it counts coulombs in and out to register a percentage of charge.

A battery voltage sags under load. Lead-acid batteries are notorious for it, more so than lifepo4, although they also sag. And the voltage is higher under charge current as well. A charge controller will give a percentage of charge based on a given set of voltage parameters as state of charge.

And therein lies the problem with feeding an inverter - the more it pulls the more the battery sags, and so the more it pulls to keep up with the current, putting a strain on everything - battery, wiring, over-current protection, etc.

For charging, it also affects parallel charging sources since the bus will show a higher voltage than the actual battery.

 

[B3nn]

Got ya. That makes sense. I slightly knew that part but thought the battery percentage would be a bit more accurate. But I did go out to my office and the switched had switched to solar power so that means the voltage got pretty high, which indicates the batteries were charged up. So I'll just not worry so much about it

 

[Browneye]

12.1V for AGM is low - about 65%.
Without knowing your charge controller or settings, it's hard to say why charging current terminates before the battery is full.
675Ah is quite a bit of battery capacity. These are AGM's? Like six of them?

 

[B3nn]

Ya that was first thing in the morning, after being used through the night. I have the renogy rover 40amp. BButseems you were right and that I shouldn't be worrying about the percentage. The voltage has been going up to 12.9 which to my understanding is when it gets a full charge.

 

[Browneye]

Yes, 12.9 is full charge for lead. The only way to get an accurate SOC by voltage is with no-loads or charging for at least a few hours, preferably overnight - that will show the true battery voltage. It's called 'settling' or a 'settled voltage'. Current flowing in our out will show a higher or lower voltage than actual settled voltage.

 

[mikefitz]

Under charge the battery voltage should rise to around 14.5 volts, depending on how the conntroller is set up.
If the battery had a voltage of 12.1 in the morning that suggests about half charged. With only 6 amps ( that's low for 400 watts of solar) and 200 amp hours to recharge, you don't stand a chance of reaching full charge inthe day.
I am inclined to agree with the reading of 65% SOC given by the controller.

If this under charge situation has been inplace for some time the batteries may be damaged.

If the volts are only reaching 12.9 volts under charge this indicates the battery is not being fully charged. To reach a fully charged condition the batteries need to reach 14.5 volts and be held at that voltage for the absorption period of a few hours.

It seems there are issues with the system that need to be resolved.

Mike