Is my MPPT CC killing my 12V AGM battery at 16V?

[boatmaster]

I have googled this problem for a couple of days now and am really in for some expert advise.

My setup:
2x 100W flex-panels with 5A output at 20V each on a sunny day with 25˚c
30A MPPT Charge Controller (with temperature sensor) set to stop charging at 13.8V
120Ah AGM battery

What makes me wonder:
The voltage measured at the battery terminals easily rises to 16V at 25˚c ambient temperature, which is way above the gassing voltage, as I belive.

Things I observed:
When the measured battery voltage rieses above 13.8V, the LCD display on the CC claims to stop loading and no current (in terms of Ampere) can be measured between the panels an the CC. Also, If I disconnect the panels, the voltage at the battery terminals immediately drops to 13.8V.

What I want to know:
Can the above do any harm to my battery, or is this just normal behaviour for an MPPT Charge Controller?
It somehow doesn't feel right to have 16V at a 12V battery terminal

If anyone has experienced the same, of even has an explanation for the phenomenon, a reply would be greatly appreciated.

---
MPPT = Maximum Power Point Tracking
CC = Charge Controller
AGM = Absorbent Glass Mat (Deep Cycle Battery)

[inetdog]

Something is wrong with your CC and it is killing your battery if it has not already.
Or something is miswired.

A PWM CC with high leakage in its series control element could behave this way.

[boatmaster]

Thank you very much for your prompt answer.

Since this is already the third CC in this Installation I have a followup question, that you might also know the answer to.
Can two 100W flex-panels (parallel) blow a 30A Controller? I know that sounds insane, but I live in Queensland, Australia where we get a fair bit of solar radiation on clear day and I am trying to rule that out as a possible reason for the failure. The first two CCs I had were 20A standard Controllers that lasted for about a month each and I just installed that last CC today...

If you were in my shoes, which CC would you go for?

[jflorey2]

Since this is already the third CC in this Installation I have a followup question, that you might also know the answer to.
Can two 100W flex-panels (parallel) blow a 30A Controller?

Of course - if they exceed the voltage or current ratings of the controller. The data sheets of the panels, the temperature of your installation and the data sheet of the CC will tell you if this is a risk.

[Mike90250]

Using a name brand (outback, morningstar, midnight, tracer) controller, you might have gotten a single dud.. but 3 now. Something else is wrong, Are those panels 90v thin film panels ? Do you ever disconnect the battery from the controller, leaving the controller hooked to the panel

Post the specs off the sticker on the back of the panel

[inetdog]

Thank you very much for your prompt answer.

Since this is already the third CC in this Installation I have a followup question, that you might also know the answer to.
Can two 100W flex-panels (parallel) blow a 30A Controller? I know that sounds insane, but I live in Queensland, Australia where we get a fair bit of solar radiation on clear day and I am trying to rule that out as a possible reason for the failure. The first two CCs I had were 20A standard Controllers that lasted for about a month each and I just installed that last CC today...

If you were in my shoes, which CC would you go for?

Unless the panels were totally inappropriate high voltage panels, which would not have given you anywhere near the current you need, it is unlikely that the panels are part of the problem.
There may be something else entirely (spikes on the battery bank, AC lines shorting to the DC lines, high voltage between panel +/- and frame ground, etc.) which is stressing the CC.
More light will produce more current, but unless you are on the International Space Station probably no more than 120% of nominal current. The voltage will hardly rise at all and may drop just because the panels get hotter.

If the panels are exposed to a cold night sky then suddenly hit by full sunlight you might get up to 125% of the nominal Voc output voltage and that might damage a CC which was not suitable for that panel voltage in the first place.

A PWM CC will usually be more vulnerable to excess input voltage than an MPPT CC.

Tell us more about the specifications of your panels and CC(s).

[boatmaster]

Thanks for your help guys, this is getting better by the minute.

@jflorey2: I dug out the specs, did the maths and think I should be safe. Well, theoretically.
@Mike90250: I don't think I ever had the battery disconnected, but I'm not sure now.
@inetdog: Thanks for pointing out the fact that 125% would be the absolute max. And although remote towns in Queensland have a lot in common with the ISS, they are not.

First I peeled off a panel and the Sticker on it reads:
Power (W): 100
Vmp (V): 17.7
Imp (A): 5.7
Voc (V): 21.7
Isc (A): 6.1
Cell efficiency: 21.3%

Then I took a look at the specs of the Controller:
Maximum Power Current: 30A
System Load loss: < 13mA
Loop Buck: < 100mV
Battery Float Voltage: 13.8V
Discharge Stop: 10.6V
Discharge Resume: 12.6V
Temperature compensation: -4mV/Cell/˚C
Solar panels maximum open circuit voltage: < 48V
Peak power of solar panel: 380W

So far so good. What struck me was the following Line in the spec:
Charge Mode: MPPT+PWM

Why two different modes and no mode switch on the CC?
So I took the controller apart and am still looking for anything, that looks remotely like an inductor, which I belive would be needed for the voltage transformation.
Am I missing something here? Apart from an inductor?

mppt-2.jpg

mppt-1.jpg

[PNjunction]

What I want to know:
Can the above do any harm to my battery, or is this just normal behaviour for an MPPT Charge Controller?
It somehow doesn't feel right to have 16V at a 12V battery terminal

Based on the limited information, it sounds like your charge controller is set to do a flooded EQ, which is usually a no-no for agm, and yes, is killing them.

Are you the one programming the controller, or is someone else like an installer just popping one in, and replacing it often for you? Have to ask.

Did you just set the controller to 13.8v and call it a day? If you are doing daily discharges, you'll need at least 14.4v absorb, not just 13.8v float. What are the values of your current setup:

Bulk / CC: ?
Absorb / CV: ?
Float V?
Are there any time limitations for each of these stages that you have set?

And from the looks of it, this unit may not last very long in a marine environment.

[littleharbor]

Looking at your naked CC I would say it isn't an MPPT controller. The 48 volt max VOC is simply the upper limit of a 24 volt nominal panel. On way to rule out MPPT would be to series wire your two panels and see if you are getting something around 11 amps into your battery bank. I'd wager a guess you are going to get something more like 2.5 amps.
Those tricky Chinese controller mfg. are doing themselves a disservice touting these low cost controllers as MPPT. Then again they seem to be selling boatloads of the bogus controllers so maybe the joke's on us.

[Mike90250]

No inductor and no big caps, you bought a label.

[inetdog]

No inductor and no big caps, you bought a label.

MPPT = Maximum Profit Point Tracking.

[Sunking]

No inductor and no big caps, you bought a label.

Ditto you bought chi-com junk.

[boatmaster]

All right guys, I get it. Thanks so much for your advise.

That leaves me with the following setup and the question which controller to buy:

2 Panels 100W each, 17.7-21.7V, 5.7-6.1A
120Ah AGM battery (hopefully not too damaged by the junk cc)
All in a marine environment with temps up to 40˚c in the cabin.

Recommendations are welcome, since I obviously don't fully understand how these CCs work.
Also I have left all of them at factory settings, which were usually:
Battery Float Voltage 13.8V
Discharge Stop 10.6V
Discharge Resume 12.6V
So if I need to change that, please let me know.

Thanks again, I really want to fix this for good.

[PNjunction]

Since we know that 3 of them shoot to 16v, don't even use them at all. I suspect there is something else going on as well.

I don't think that controller is adjustable. Or if it is, the only thing you can adjust is the float voltage, which would only be good for infrequent use of the battery.

Which brings up a few questions about the environment you are wanting to use them in:

1) Is this just a stand-alone system to power up some random things, and if so, what are they and how long do you think you'll be operating them daily / weekly?

2) Are you planning to recharge from the boat's alternator, or other source such as shore power?

3) Is this battery somehow tied into the starting system as well with an isolator, etc?

As good as we are, this will help if we need to recommend that you visit our friends over at CruisersForum Batteries/Electrical/Solar subforum for more exacting marine requirements.

[boatmaster]

@PNjunction: Thanks for your hint to CruisersForum.

I have the PV system and board battery permanently isolated from the starter batts for safety reasons.
Wasn't always that way and got me into trouble once.

I power the lighting, the pumps, fridge and laptop with it and always seem to have enough power.
I use the boat only three months of the year, but in that time I discharge the battery daily to about 70% of its capacity.
The rest of the year its sitting on a trailer in the yard, which leaves the panels exposed to the sun for most of the time.

Should I adjust the settings of the controller in those periods?

Since the CCs I had so far have proven to be crap, I am looking at an EPSolar Tracer 2215bn + MT50 right now.
It seems to be fully adjustable and is available to me within a couple of days. Yeah I know, but there's no amazon Down Under.

Mike90250 and Sunking: Any warnings? I don't want to be buying chi-com junk again.