Outback charge controller question

kenneth123 · July 2014

I currently have a system at my off grid house that consists of:
2-24 volt 1576ah battery banks for a total of 3152ah at 20hr rate (fork lift batteries)
I am using 2 fm80 charge controllers on a flex power 2 board
I am undersized currently on the solar which i have 18-232watt solar panels for a total of 4176watts

I am wondering how long of a absorb time and float time i should use for the batteries and any other settings I can change on the cc's to get the most out of my system?

What voltages should I input into the controller?

How undersized is my array for my battery bank?

Cariboocoot · July 2014

Re: Outback charge controller question

Welcome to the forum.

You sure are undersized on PV. 4176 Watts produces about 134 Amps maximum, yes? That's around a 4% peak charge rate. It would maintain the battery but it will be ineffectual at charging it.

Your 232 Watt panels probably have a Vmp around 30. That means you should run two in series per string, total of nine strings. Obviously four on one controller and five on the other: a 1856 Watt array producing 60 Amps and a 2320 Watt array producing 74 Amps. It would really help to get two more panels on that first controller. But not enough.

I don't suppose you could go up to 48 Volts, could you? As it is you've nearly maxed out two 80 Amp controllers and you haven't even reached the minimum 5% charge rate. You are looking at needing four controllers here for 24 Volts.

If you went with 48 Volts you'd wire the two batteries in series. 1576 Amp hours @ 48 Volts is the same amount of power as 3152 Amp hours @ 24 Volts but more power can be put through the controllers as Voltage rather than Amps. Vis:

3152 Amp hours @ 24 Volts. 10% peak current is 315 Amps requiring four 80 Amp controllers to manage 9818 Watts of array (fill out what you have now and double it).

1576 Amp hours @ 48 Volts. 10% peak current is 157 Amps requiring two 80 Amp controllers to manage the same 9818 Watts of array.

Two more charge controllers will cost you another $1200 or so. Plus four controllers will be difficult to manage (unless you buy four Classics and use the Follow Me function).

So let's ask the question: do you really need all that battery capacity? That's like 32 kW hours AC stored up there (at 50% DOD). My guess is you don't and probably the easiest thing to do is sell half the battery bank.

kenneth123 · July 2014

Re: Outback charge controller question

I am going to add 2 more cc and 4500 watts of panels soon. currently i am running 9 panels on each controller. I don't think i can go to 48volt with out changing the inverters?
Would it be a issue to run 2 more controllers?
Can the outback flex power breaker box handle that much?

I picked up two banks of batteries at a good price and wanted to eventually have a system that i would not need a generator and would last me through bad weather.

vtmaps · July 2014

Re: Outback charge controller question

kenneth123 wrote: »

I picked up two banks of batteries at a good price and wanted to eventually have a system that i would not need a generator and would last me through bad weather.

For most off grid homes, the most cost-effective balance of components is to have a battery capacity that gives two days of autonomy. Of course that means you must run a generator on the third cloudy day.

Sometimes folks want a battery bank with more than two days of autonomy. This means a big battery bank and, necessarily, a big array to charge it.

And then when you have a stretch of good weather your battery is getting charged up every day, but the cycles are very shallow. Repeated shallow cycling is bad for lead acid batteries... it causes lead dioxide to clump (rather than make a thin film) on the positive plates. Those forklift batteries are made to be discharged.

One way to deal with this to not charge the battery for a day or few days. That way you will have deeper, but fewer cycles. Your battery will live longer. Of course, you still need a very large solar array and more controllers to charge the large battery bank.... and all those panels and controllers will sit around doing nothing most of the time... got any good opportunity loads?... maybe get an electric hot water heater.

If you want to implement this charging protocol there are several european manufactures of equipment that can make charging decisions based on battery SOC. Midnite's controllers can skip bulk and absorb based on number of days... they plan to make it SOC based in a firmware update.

I'm not sure whether Outback or Schneider equipment can do "skip-a-day" charging based on SOC.

Naturally you will still need a generator because after you've skipped charging for a few days and the battery is ready for a charge, it will start to rain.

--vtMaps

Cariboocoot · July 2014

Re: Outback charge controller question

kenneth123 wrote: »

I am going to add 2 more cc and 4500 watts of panels soon. currently i am running 9 panels on each controller. I don't think i can go to 48volt with out changing the inverters?
Would it be a issue to run 2 more controllers?
Can the outback flex power breaker box handle that much?

I picked up two banks of batteries at a good price and wanted to eventually have a system that i would not need a generator and would last me through bad weather.

Yes you would need to change the inverter and over-current protection to go to 48 Volts, and rearrange the PV wiring. It may still be the cheaper option depending on what equipment you already have.

Two more controllers and another 4.5 kW of PV? Wow. That's a lot of money to spend. Why no generator? How much/often bad weather do you get?

The old formula of having enough battery capacity to last days without good sun is not favoured anymore because the system ends up being ungainly; large capacity not used during sunny weather which adds to costs and has no longer lifespan. These days you'd plan on 25% DOD daily, with 25% in reserve, and on day three start the generator if necessary to at least Bulk charge. On the whole it's a lot cheaper and adds the redundant power supply of the gen should something go wrong. If you want more PV capacity for cloudy days you do not have to expand the battery bank as well (merely limit the output current maximum). But if you expand the battery bank you must increase the PV capacity to charge it.

So if your actual daily needs are less than 32 kW hours reducing battery capacity may still be viable, even if you need to increase PV for bad weather charging (which is difficult to predict).

As for the way you are going you will need a lot of redundancy in the charging area. The 315 Amps of charge current is beyond the normal for most wiring. You will need ample bus bars to connect each of the charge controller's outputs, the batteries (with 4/0 wire), and inverter to. The lines between batteries and bus bars will be a death trap if anything is dropped across them. Put battery fuses (one each) as close to the batteries as possible to reduce this risk. I'm not sure what the limit is on the OB box but I think you are over it. Trying to fit the quantity and size of wires here into a single box would not be easy.

If I were to start out on a system that needed four controllers I would skip the FM80 and go for MidNite's Classic 150 which co-ordinate better. Don't fuss too much about trying to get four FM80's to agree on battery Voltage or charge stage. As it is you should have a MATE and HUB to connect up the controllers so they can share at least the RTS input, which is also essential.

kenneth123 · July 2014

Re: Outback charge controller question

do you think that one bank is plenty for this system? should i get rid of one bank and pick up a generator?
I currently run a 1/2 eval cooler, efficient dish washer, wash and a gas dryer. all led lights and a couple tvs.

I am wondering how long of a absorb time and float time i should use for the batteries and any other settings I can change on the cc's to get the most out of my system?

kenneth123 · July 2014

Re: Outback charge controller question

and a efficient electric fridge.

Cariboocoot · July 2014

Re: Outback charge controller question

kenneth123 wrote: »

do you think that one bank is plenty for this system? should i get rid of one bank and pick up a generator?
I currently run a 1/2 eval cooler, efficient dish washer, wash and a gas dryer. all led lights and a couple tvs.

I am wondering how long of a absorb time and float time i should use for the batteries and any other settings I can change on the cc's to get the most out of my system?

Invest in a Kill-A-Watt meter and measure the daily power consumption of everything you can under normal circumstances. I mean we can use <3kW hours per day here, but at our in-town house it's more like 16kW hours per day (more stuff, more use, more power available, more use). Without actually measuring you're guessing a lot. Where you are makes a big difference too, as in the amount of solar you can expect throughout the year. PV Watts can give you some guidance there, but don't take the results verbatim (it is meant for grid-tie systems which can utilize full PV output). When and how you use the power will make a difference too.

Battery charging parameters are specific to the battery involved and how it's used. In this case the maker won't have recommendations for off-grid use of a forklift battery. They can be a bit fussy and you will have to watch the results and fine tune your system for best performance.

Basics: 24 Volt system would have an Absorb Voltage of 28.8 to start with. Absorb time should be set to maximum, and then watch the current as it goes down during Absorb. When it drops below 2% of capacity (63 Amps) you should set the time to be the next 1/2 hour increment above that. And check it several times to make sure it is getting its charge. Buy a hydrometer and actually check the specific gravity of each and every cell. There is no better way to be sure the battery is charged. Once you have a regime that works you only need to check them periodically. End Amps can be set to override the maximum Absorb time, switching to Float when current indicates charging is done. This is why you need to watch the current, and you need to compensate for concurrent loads (usually averaged). Float Voltage the manufacturer may have a recommendation for. It is around 27.6 or less for a 24 Volt system, as low as 25.5 (usually only for storage).

With your low charge rate none of this may be possible yet unless your DOD is very shallow (which is not good).

kenneth123 · July 2014

Re: Outback charge controller question

what size generator would you recommend?

Cariboocoot · July 2014

Re: Outback charge controller question

kenneth123 wrote: »

what size generator would you recommend?

Well if you've got to charge up that much battery with a generator ... you've got a problem.
315 Amps @ 24 Volts is 7,560 Watts, not including losses and power factor. So you're looking at a 10 kW generator there. You are also looking at the need for an industrial capacity battery charger such as one that would typically be used for recharging the forklift batteries you've got. Find a charger suitable for that job and you'll see what its input power requirements are. It will no doubt be 50 Amp 240 Volt.

Vic · July 2014

Re: Outback charge controller question

kenneth123 wrote: »

I currently have a system at my off grid house ...

I am wondering how long of a absorb time and float time i should use for the batteries and any other settings I can change on the cc's to get the most out of my system?

What voltages should I input into the controller?

Hi Kenneth ..,

The customary Forklift batteries use fairly high SG electrolyte. This usually means that recharging from Renewable Energy would require an Absorb voltage of around 29.5, or even a bit higher.

You DO need a Hydrometer or two, as measuring SG is the true way of knowing State Of Charge (SOC). Would expect that your batteries should read about 1.280 SG at 77 - 80 degrees F -- 25 C, nominal.

You need to use the Outback Remote Temperature Sensor. Probably one on each battery bank, near the center of the bank. I would place these RTSes about mid-way down the outer case of the target battery.

With multiple battery banks, and CCs, the correct way to measure the temperature of each battery could be fairly subjective. If you had an IR thermometer, you might want to try to see what the variation in case temperature of each battery and bank. The Outback system allows sharing an RTS among OB devices.

Float is usually not timed. It is the state where the CCs attempt to maintain full charge.

Regarding generators, it really is a good idea for most off gird systems to have some generator power. The generator will need to supply the battery charge power, as well as any loads that would normally be supplied by the inverter/s. Recharging a large battery bank from 50 or 60 percent SOC can be a long slog -- taking a number of hours. This can be demanding on generators. This can mean that a generator should be oversized somewhat. Also if your off grid site is very much above sea-level, an additional size increase is called for -- about 3% per 1000 elevation above sea level. There is also some added capacity needed for elevated temperatures -- this depends somewhat on the exact generator. Often this is speced by the manufacturer of the generator.

If you do not have a Hydrometer (or Refractometer) it is imperative that you get one or several immediately, and measure and record in your battery logbook the SG reading of each cell.

My opinions. Good Luck, Vic

kenneth123 · July 2014

Re: Outback charge controller question

i currently have the absorb time set to four hours and have a end amp of 0. what would you recommend for me to try for these settings to get the most charge out of my system?

vtmaps · July 2014

Re: Outback charge controller question

kenneth123 wrote: »

i currently have the absorb time set to four hours and have a end amp of 0. what would you recommend for me to try for these settings to get the most charge out of my system?

I don't recommend you try anything until you get and use an hydrometer (I like the 'Hydrovolt' sold by Midnite solar).

You will ruin your batteries by sulfation if your absorb time is too short or if your absorb voltage is too low. You will shorten the life of your batteries if the voltage is too high or the time too long. It's a delicate balance and every battery bank and environment is different. Your goal should be to find the minimum voltage and the shortest time that gets the job done on your batteries in your environment. The hydrometer is your guide.

Cariboocoot mentioned using end amps to know how long the absorb charging should last. That's a good idea, but it depends upon the absorb voltage and the charge rate being high enough, and also upon the temperature, age, and capacity of the batteries. If the voltage is too low (even by .2 volts) or the charging current is too low, end amps may not be accurate. If there is any deficit charging, the internal resistance of the battery will increase and lower the end amps current. That will give you a false sense of security that you are properly charging your batteries.

Once you use your hydrometer to find your charging parameters, end amps is the best way to charge your batteries. But even then, you should periodically verify (with hydrometer) that your end amps protocol is working properly.

--vtMaps

kenneth123 · July 2014

Re: Outback charge controller question

I do have a few hydrometers. I have checked and am achieving the recommended 25.4 volts (2.12volts per cell) each day and a hydrometer reading of 1.265 each day.

Vic · July 2014

Re: Outback charge controller question

Hi Kenneth,

An SG reading of 1.265 may be a bit low for forklift batteries, but if every cell is at this level, you are close.
Did the manufacturer of the batteries provide you with the target SG for your batteries? My guess is that the target SG is about 1.280. There is a fairly small temperature compensation required for SG measurements, for temperatures above or below about 77 - 80 degrees F (about 25 C).

While terminal voltage can be used to indicate State Of Charge (SOC) of lead-acid batteries, it is most accurate when the battery has rested (no charge or discharge at all) for three, or so hours. When one becomes very familiar with their system, voltage under load can be a fairly good rough guide as to charge level. Voltage readings should be compensated for the temperature of the batteries.

FWIW, YMMV. Vic

zoneblue · July 2014

Re: Outback charge controller question

This system should be 48V, no doubt about that. Double it to 10kWp and thats exactly 4 times more reasons to make the change.

But reiterating what others have said you appear to have designed this system by picking large random numbers for key variables. Strongly urge you do a full design exercise before you spend any more money. Start with target loads and monthly insolation numbers. We can help once we have those figures.

vtmaps · July 2014

Re: Outback charge controller question

kenneth123 wrote: »

I have checked and am achieving the recommended 25.4 volts (2.12volts per cell) each day and a hydrometer reading of 1.265 each day.

Vic wrote: »

An SG reading of 1.265 may be a bit low for forklift batteries, but if every cell is at this level, you are close.

If Vic is correct (he usually is), then your SG is a bit low. Sounds like slow death by sulfation. If you are only getting to 99.9% SOC each day, then over a period of time you will be at 99.9% of 99.9% of 99.9% of 99.9%...etc. Over time you will ruin the batteries. Many forklift battery manufacturers recommend a weekly routine (not corrective) equalization as a way of breaking out of the 99.9% of 99.9% cycle.

--vtMaps

Outback charge controller question

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