Over Paneling (for battery bank)

[Photowhit]

Well I'm getting ready to add the last array to my 24v battery system, I'm already somewhat over paneled with 4000 watts in 2 arrays through 2 classic lites. But I run little risk of charging past the 15% max recommended rate.

I hope to add a 3rd array, about 2200watts, primarily to run more opportunity loads. I acquired the new panels due to a warehouse accident at very low cost. I had originally intended to run them as an independent system for my electric hot water heater, Now I want to do it through the waste not feature.

As the Classic with the whiz bang Jr appears fully mature, I will have my 3 classic lites connected and use the current across the whiz bang jr to limit the charging wattage. I didn't feel there would be a problem limiting the charging Current to 120 amps. 'Coot over on the NAWS forum doesn't feel this will work. Having seen the lag time with the Outback CC.

He stated;

This is exactly the kind of situation that can get you in trouble.
Batteries fully charged before opportunity load is active? No problem.
Adding a large amount of PV to supply a big opportunity load? Possible problem.

You don't want to limit the controller output or else you will also limit the power available to the opportunity load (and thus wouldn't gain anything). The Outback FNDC has the potential to work around this (never programmed one so I don't know if it actually can) because it has three shunts and theoretically at least can tell the difference between current from the controller and current to the battery, thus it should be possible to limit the latter without affecting the former (power going to loads only).

If you just pile on the PV then expect the full current potential to be available to the batteries at some point. Thermostatically controlled loads will turn on/off as they see fit, and there may be some lag in the reaction of the controller to the reduced load.

That is providing you don't get into a situation where (too) large amounts of current go to the battery during Bulk and/or early Absorb even without the load cycling. Too short a Bulk stage is not good just as too short an Absorb stage isn't.

There are a lot of variables to look at, including what the maximum current will be as a charge rate (too high for FLA's? Consider using AGM's) and how the charge controller will handle the current (limiting how?) and fluctuating load demand (will it react fast enough?). I've not tried this with a Classic, but suspect it would be the best save for the current 'dividing' between charge and loads. I suspect Bob & Robin looked at the shortcomings of their old MX and made good on them. The MX, btw, has quite noticeable lag in reaction times as well as needing to sweep periodically (programmable) to obtain the best power point for conditions. They are also not good at limiting current (will rise above programmed max).

Now in some cases this momentary whack with too high current will be brief and rare enough that the only result will be a small amount of time shaved off the batteries' lifespan. In the worst case it will be a lot of time shaved off. All a matter of how much too high the current is, how long it is applied for, and how often it happens.

I respect 'Coot's opinion, but feel the classics system will handle this. I hope that by over paneling, I will charge the batteries faster and more likely to fully charge on overcast days, and be able to run the water heater in the winter, and a mini split in the main room as well during the summer.

Since the battery charging is a electrochemical reaction, I figure a second or 2, at worse, won't create a problem. When the A/C pops off or toaster oven, I might get a small surge of current.

How do the engineers think about this?

Though we had a long winter last year, I didn't have a huge problem keeping hot/warm water, off the 4000 array, and I'm still running the water heating element at 120v (3600 watt 240v, about 900 watt on 120v, might switch to a 4400 watt 240v or about 1100watt at 120v) but we had minimal long periods of over cast days. I suspect during normal winters I will have 5 days with over cast, and hoping to have some water heating during that time. I have minimal winter time loads,suspect less than 2kwh a day.

Any and all suggestions welcome, Don't send the men in white coats, I just want a warm shower...

[vtmaps]

A charge controller's job is to protect the battery from being overcharged... as controller processors (and software/firmware) get more and more complex I worry about them making some sort of digital mistake.  Controllers do get hung up from time to time... fortunately most reports are failure to charge rather than battery fires.

My concerns are escalated when the system gets more complex (multiple networked controllers, waste not, SSR, etc), and when there is over paneling of the battery (more danger if things do go wrong).

I'm impressed by the rules for wind/hydro diversion controllers (dump loads)... There must be a back-up... you can't rely on just one diversion controller.

I wonder if you should be thinking about some sort of independent failsafe mechanism to protect your batteries... a diversion controller or maybe a high voltage disconnect (disconnect PV from controller when battery voltage too high).  Either should be independent... not run off the aux of any of your controllers.  Make sure they will work at the high battery voltage they are trying to prevent.

--vtMaps

[zoneblue]

Correct me if im wrong but the current limiting mechanism  in the present firmware is still total controller output? I thought that battey current limiting was on a list of WBJr related features that are still on the drawing board.

Ok, vts point, yeah. Lets say you put enough pv on a flooded bank to potentially produce 0.5C or something. Would you trust a charge controller to take care of that? My thoughts would be sort of.

Provisos. 

- Like any software development. features that were engineered into the code base early on tend to be better resolved. Hastily added after thought items can often fail to properly take care of edge cases.

- the classic has proven itself stable, and has in built mechanisms for self healing the inevitable crashs and resets. If the current limiting feature was implemented successfully, it should work on the same basis, no?.

- i think what Coot was talking about is the way the mppt sweeps and the like can sometimes result in short term output current spikes. Whether this is an outback phenomonon or is common to all mppt controllers i do not know. I remember threads ages ago where Tall girl related clients with big grid tie systems and small backup agm banks.

- vts idea of some redundancy sounds pretty sensible to me. I would never run a wind turbine into a hot water cyllinder with out a another seperate dump load to fall back on. How that would work on lare solar arrays im not sure.

[Photowhit]

Correct me if im wrong but the current limiting mechanism  in the present firmware is still total controller output? I thought that battey current limiting was on a list of WBJr related features that are still on the drawing board.

Thanks for pointing this out, I haven't updated my firmware since Dec? of last year. I guess I had read something in the forums that lead me to that conclusion. I'll check into it.

At worse, I could likely charge at 22-25% of the batteries capacity, which is done from time to time on these lift batteries. I'd hate to stress it out that bad, but mostly it should be reduced for other reasons as the battery becomes charged. Perhaps in an ideal messed up situation I could boil off the batteries a bit. long fall front and I've drawn down the bank to 50% then the clouds clear at 11am and I could run full out for a couple hours... though if the voltage was held, I guess it wouldn't boil... just stress and heat?

[zoneblue]

The thing about solar cc/cv chargers is that current is inherently limited:
a) early in the day (and late)
b) post bulk

>Perhaps in an ideal messed up situation I could boil off the batteries a bit. long fall front and I've drawn down the bank to 50% then the clouds clear at 11am and I could run full out for a couple hours...

PW, we have a bit of overpanel here too. In 20 months, our system has never ever managed to acheive that scenario, even though the odds suggest its entirely possible. The pack is either through or well into absorb by noon, OR, on wet winter days, charges slowly all day.  I have never seen the full array output, not once. Kinda sad!

That 'perfect storm', of sun coming out suddenly at solar noon, seems to be incompatible with our local weather system.

Having said that we do run AGM, which can take 0.3C no worrys and it was one of the reasons we chose AGM, knowing i was going to do oppurtunity hot water.

AGM, can actually take much more that that even, up to 1.5C or more, and some sources say that so long as you only do this during bulk it might even increase the cycle life. eg:
http://www.powerstream.com/SLA-fast-charge.htm

Lithium, yeah well, that too.

[Photowhit]

Yeah, I don't think it's likely to happen, but would feel more comfortable with the charging amperage judged through the Whiz Bang Jr. That said, I could set each CC to limit amperage and still have all the advantages, with out the nerves or problems. My opportunity loads are under 1500watts. I do expect in the future to be able to run higher than this, so perhaps by then I can unleash the current! and have the Whiz Bang monitor my charging.

FWIW, it's not uncommon to torture the forklift batteries, I just don't have a huge liquid reserve so running dry is more a threat than too high a charge rate.

[philb]

Are your batteries in a location where you could set up an automatic watering system? That might help relieve your concerns about running them dry. Mine only need about a gallon of water once per month. I'm hoping to do that when I get a battery shed constructed.

The battery engineers told me to quit treating them so easy. They are made to work hard. Most of their customers work the snot out of them. To my surprise, the battery's capacity went up from treating them rougher than the EverReady GC-2 batteries in my second system. I still don't go below 25.0 volts and try to float them at 27.2. EQ is 34.0. They want them to boil at least every month or two. 

I'm more concerned about overworking my Classics instead of my lift batteries. On a sunny day, they get 240 amps at midday from 3 Classics. That ends up being about 24 kWh's a day during sunny summer weather. 

I will watch my over paneling closely during the sunny winter days though. I plan to turn a few panels off until I find out how much power is actually being produced. I don't want to let the magic blue smoke out of the Classics.   

[zoneblue]

Id go along with that. Coot himself said that the odd 20% C charge wasnt going to hurt them. But point taken about the water issue. FOr me summer is the issue, the bank is lucky to cycle to 90% SOC, and the pointless daily absorbs are just wearing the catylyst out.

Days between bulk might be some help to you there. That was introduced a couple firmwares back. It works but isnt real smart. Basically every nth day it will bulk and absorb. If it fails, then it wont try for another N days. Bob also plans to add a bulk by SOC, so it only goes through absorb when teh SOC drops below a certain point.  These will all help us out there for sure.

[CDN-VT]

I also need to seriously OVER panel !! Just for Nov-March / mid April  Wet coast of Canada , Rain is our Liquid sunshine !!!


VT