Dilemmas of Off-Grid Battery System...

creosote · January 2016

Looking for some advice from the experienced folks on this forum...

I bought a home 3 years ago where the original owner had an Off Grid Solar system put in during new construction (3 years before
I bought the home). At the time there was no local power at the home location so they started off grid. Then 3 months later the power
company ran AC power into the neighborhood.

The system is a Xantrex 6048 Inverter with MPPT60 & MPPT80 Charge controllers. Around 6000W of solar panels total feeding into the
two controllers. It has a 980AH AGM battery bank (16 Deka batteries - 4 per string - 48V total). Since we have full-time power from the Utility company the batteries don't get used much anymore. Just in the past weeks I've noticed the Inverter sending a lot of power to the battery bank all the time. It seems to be stuck in BULK charge but the voltage is never getting above 52V. Also, some of the batteries have started producing a terrible burnt sulfur smell. One of the is bubbling and some acid is coming out the top plugs. I checked the voltage on all the batteries and they range from 9V - 15V. The hottest battery that is bubbling is at 15V+.

I talked with Xantrex/Schneider support and they said to call the battery manufacture. The battery specs are: AGM 8A8D/T975 1450 CCA 245AH @ 20 Hrs.

I'm trying to determine there are several bad batteries that are messing up the entire bank? Since we don't use off grid power much could I just reduce the battery bank down to one string of 4 good batteries (240AH total)? Any other troubleshooting recommendations?

Also, Is there any way to remove all the batteries from this setup and have the Inverter run as a grid tie system? I don't believe that's possible but thought I'd ask.

Thanks in advance..

vtmaps · January 2016

You are in danger! Turn the system off before you have a fire or explosion. AGM batteries should not be bubbling or hot or releasing acid.

If you want power for the rare occasions when the grid is down, buy a generator. Battery based power systems are not cost effective if the grid is available.

With 6000 watts of solar panels, it might make sense to buy a grid-tie inverter. Whether it make financial sense depends upon the net metering laws in your location. Without good net metering laws, it probably doesn't make sense to go grid tie unless you have large daytime loads. It does NOT make financial sense to store daytime power in batteries for night time use.

--vtMaps

softdown · January 2016

I would suggest trying to find a reputable solar pro to fix this situation. This board is an excellent source of very helpful information. However...it doesn't sound like you have the luxury of spending the needed time to competently address this scenario.

On second thought...you write like a person with significant solar experience. Not sure what to think of that. Batteries possess a lot of energy. Like vtmaps said, unplug it until the problem can be figured out.

BB. · January 2016

Yes, turn off the system now. Those batteries (at least some of them) are failing. And nothing good will come from keeping the batteries charging/inverter-charger running at this moment.

Yes, you may salvage some of the batteries, but the reality is they may have a few months to 1 year of life left (6-7 years is not bad for AGM batteries).

-Bill

Photowhit · January 2016

If you are in the US, Look into doing a grid tied system, It should be reasonably simple for someone to string together the array to produce the correct voltage for a grid tied system, since your panels already exist I can't imagine it would not be worthwhile investing in a grid tied inverter (a couple thousand dollars) and a qualified person to rewire/reconfigure your array.

There is a chance that your array would not fall into the parameters of some grid tied inverters, so you will want to learn more about the panels and configuration before you continue. It might even be beneficial to add a panel or 2 or take a couple out of the string or 2 you will be left with... Basically don't buy a grid tied inverter before you know what you need.

Most like you will have reasonable Net Metering Laws, a few places are having challenges, Arizona/New Mexico, and some electric companies are exempt, mostly municipally run companies, and often the pressure is high for them to comply.

After being setup as a grid tied system, sell off the inverter and charge controllers and recoup some of your investment.

creosote · January 2016

Thank you everyone for your input... It sounds like I need to turn off the inverter and batteries and just run off the utility company's power until I can get this fixed. I'll try to provide more information below so you can understand my situation better...

Regarding my experience. I'm a computer engineer (not an electrician) but know enough about electrical and solar systems to do most of the work myself. I installed the HVMPPT80 and the additional 14 panels, wiring, etc to add ~4000W to the original system (that had only ~2000W solar and a wind turbine that we later disabled.)

I live in FL and I'm already approved and participating in Net Metering. We use around 700-1000kw per month and produce between
500-900kw per month from the solar. So the system definitely works well and offsets our power bill by 50-90% each month.

My preference at this point would be to just run system with a small battery bank so I could maximize the solar power to offset our power use and sell the rest back to the power company. I'd much rather keep the existing system if possible and I'm trying to understand if there's a way I could do that...even if had to buy some new batteries? Why couldn't I just use a low AH bank that would stay fully charged most of the time? If the power goes out I may only get a couple minutes of battery runtime but that would be fine with me. If I need longer runtime (hurricane) I can hookup up a generator (I already have the Xantrex generator kit addon).

OR the other option is to buy a new GridTie inverter that can handle 6000W+ . I'm guessing that would be around several thousand dollars minimum even if I did all the work myself. I'm trying to avoid this option if at all possible. The ROI on just the inverter alone may be 5+ years.

Hope this helps explain it better... your advise is greatly appreciated.

WaterWheel · January 2016

It sounds like you've already got grid power supplying most of your home's needs. With a 6048 you've already got an inverter that can sell to the grid so assuming the local utility will buy from you get the proper meter installed and make some power; the system is already paid for, you may as well make some money with it.

If you want a small battery bank for power outages put some new batteries in (less than you have now) and use grid power for most of your power needs only using the batteries during power outages. To help keep your batteries healthy it's good to run the house off battery power at least one night a month to cycle them.

westbranch · January 2016

If you are going to use those batteries minimally, andf it also sounds like sporadically, I would be reading up about LiFePO4 cells, they can sit unused for a fairly long time and can handle 80% discharge (max if you want to recover them) and do not need to be up to 100% as do FLA and SLA batts...

creosote · January 2016

foolami said:

It sounds like you've already got grid power supplying most of your home's needs. With a 6048 you've already got an inverter that can sell to the grid so assuming the local utility will buy from you get the proper meter installed and make some power; the system is already paid for, you may as well make some money with it.

I already have the proper meter installed and have been selling back to the power company since we bought the home. My concern is how small of a battery bank can I run? Is there a minimum number of amp hours needed? What would be some fair priced replacement batteries I should look at? My goal is not to rely on the batteries at all..just to have them their to regulate the under/over voltage spikes and keep the stove clocks from resetting at every power blip.

thanks,

mike95490 · January 2016

The specs for the 6048 call for 100ah for each Kw the inverter will be selling. The battery bank acts as a giant filter capacitor, Too small, and you affect the AC generation. The smallest I'd dare go is about 500ah (when it should be about 600ah)
Use a cheap flooded bank, and wait at least 2 more years before you jump to the LFP battery, they are still pretty young in their lifespan.

Raj174 · January 2016

Hi Creosote,
The deal with the battery bank is that it has to be able to handle the charging amps generated by the PV array. At 6000 watts that you need a 600 AH AGM bank, or about a 1100 AH FLA bank, or a 400 AH LiFePO4 (Lithium Iron Phosphate) Bank. All of these battery banks are sized differently because of their chemistry and/or construction to be able to accept the possible max 120 charging amps generated by your 6000 watt PV array. All of these battery banks cost between 6 and 9 thousand dollars. The only way to have a much smaller battery bank is to limit your power production, i.e. switch off panel strings or limit charging amps at the controller. Either way, If you produce less, you sell less back to the grid. As westbranch said LiFePO4 Prismatics cells may be the way to go. They don't require frequent charging and can sit for months at 50% DOD with no problem. However, they are expensive. You would need 16 batteries at about 520 dollars each. That is why some forum members are advising you to get a grid tie inverter. You will easily spend 2 to 3 times more money on a battery bank than you would a grid tie inverter. You might also consider splitting your array. 3000 watts on the Xantrex system, this would need a battery bank of 300AH AGMs, and 3000 watts on a grid tie inverter. Both could sell to the grid and you would have emergency power available if needed.

Brock · January 2016

FYI our last bank was 8 Deka 12v agm's in 2 strings. When we finally had one fail I picked the best 4 and kept those running our XW6048 for about 12 months. I then had another fail swapped that one for the best of the remaining 3 and then 4 months later had another one fail, then just replaced that one with the next best one that lasted about 6 weeks, then replaced the bank with 8 flooded L-16's.

The funny thing is even with only 4 agm's it had better surge capacity then the 8 L-16's we have now, but obviously with far less capacity. And the L-16's require work

So bottom line I would think you could easily run grid interactive with only 4 and I am guessing you could pick out the best 8 to use as a bank. Like you said the way I noticed them was the system taking longer to hit float than usual and that lead me to check each one to find one of the batteries getting warmer than the others and then checking resting voltage and it was significantly lower than the rest as well. If you can just separate them all and charge then with individual 12v chargers, let them sit a day or so and then see what they do, I think it will be easy to spot the bad ones. They may not be in the best shape but you have them and they are paid for.

I understand wanting a larger bank, but if all your using it for is to balance the PV and your selling to the grid I think 4 should do the trick and again I think you should be able to find at least 8 good ones.

creosote · January 2016

Thanks everyone.. Now I'm starting to understand the situation better regarding the battery requirements. I was hoping I could use a smaller 200-300 AH bank but it sounds like I will need at least 500-600AH. I really don't want to invest the money in new batteries since I really don't need an off grid solution. I'll probably have to start looking at Grid Tie inverters. Does anyone know if Xantrex/Schneider makes a grid tie inverter that would work with my existing charge controllers, Combox, DC Panel, etc?

thanks,

softdown · January 2016

I would think you could salvage enough good batteries, in your existing array, to "limp" along for awhile. These are really just emergency batteries anyway. Plus you have the generator. Cheapest solar batteries are golf cart batteries. These should last another ~6 years with your usage...unless they are kept in a "hot" Florida environment. By then.....we will know a lot about the newer lithium batteries real world performance and longevity.

A good battery will show a resting charge over 12.5 volts a day after the charge has been disconnected.

Lithium may well be the future of solar storage. Hopefully there will be plenty of bold investors to give it the legs that it needs. Lithium batteries are much, much better than NiMH, NiCD, etc. I suspect they have a bright future in solar storage.

Side note....bought a ton of lithium mining penny stocks years ago. Stock tanked. Better check my brokerage for a rebound. I can't remember the name of the stock.

Raj174 · January 2016

Softdown, don't you have a spare SMA SB7000TL-US-22 grid tie inverter? I read that they can handle 7200 watts and are capable of 1500 watts emergency power with secure supply.

zoneblue · January 2016

My concern is how small of a battery bank can I run? Is there a minimum number of amp hours needed? What would be some fair priced replacement batteries I should look at? My goal is not to rely on the batteries at all..just to have them their to regulate the under/over voltage spikes and keep the stove clocks from resetting at every power blip.

Those were never ideal batteries, and they were never in an ideal configuration. 4S4P allows for very uneven current sharing, and it is no surprise that some are over 15V. Those ones are now toast. What i would do:

To get you going, break the whole bank apart, bin the vented ones, then load test the balance individually. Take the best half (or 3/4), and wire them up. 6000Wp into 490Ah/48V is about 0.2C, which is fine as a charge rate goes for AGM. Discharge rate for 6000W is 0.3C, also fine.

For the future, you have two options:

a) for the small / hybrid battery, you cant beat lithium. LFP is good for 1C charge and 3C discharge rate. eg: 6000Wp into 48V at 1C is 100Ah. 3C for the inverter is 15kW+.
Or if can pick up something like a wrecked EV battery and they go to even higher rates/smaller batterys. Break up the battery and sell the cells you dont need.

b) sell the 6048 and get a GTI.

softdown · January 2016

Yes I do. It would appear to be an excellent unit for his needs.

Mine is new. Creosotes equipment is six years old. I would require something additional as a result.

Raj174 said:

Softdown, don't you have a spare SMA SB7000TL-US-22 grid tie inverter? I read that they can handle 7200 watts and are capable of 1500 watts emergency power with secure supply.

Dilemmas of Off-Grid Battery System...

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