Unhappy with off-grid 48 volt system

[inetdog]

7, 12v batteries in series is 84V

28 batteries / 4 strings = 7 batt per string.

Something is wrong wrong wrong here ???

Mike, the OP clearly stated that he had four sets each consisting of seven batteries in parallel and that those sets were then put in series to make up the battery bank. That is one possible but not recommended way of getting 48 volts from 28 twelve volt batteries.
The more conventional way to do it would be to make up seven strings of four batteries each and put those strings in parallel, which would probably be better than the above but still unworkable in the medium to long run.
Dereck at first misread it the same way you did, but realized his mistake. The system is a 700AH system, not a 400AH system.

[Mike90250]

7, 12v batteries in series is 84V

28 batteries / 4 strings = 7 batt per string.

Something is wrong wrong wrong here ???

[Sunking]

Definitely tap on the brakes and slow down. Determine your real needs starting with how many watt hours you need in a day. You have a monster sized system, but based on what you said you use I bet all you need is 200 to 500 watt system. If that is the case you can sell off most every thing you have and start over with the right equipment and still have money left over.

In the meantime try this. Go through your batteries and find 4 or 8 good ones. Configure them for 48 volts. If you use just 4 of them use around 600 watts of panels and one of your charge controller. If you find 8 good ones use 1200 watts and one charge controller. That should get you up and running and more than satisfy your needs. In fact I think even at 600 watts with a 48 volt 100 AH battery is way more than you really need.

Good Luck

SK

[inetdog]

... looking at the posting history....

It is very refreshing to see someone who is willing and able to do some research!

Once you have a good estimate of your actual power loads, it should be possible to put together a satisfactory system which is a lot smaller and less expensive than what you have and will deliver far better reliability. If you are lucky you may be able to reuse a good part of what you have.

[thastinger]

Sounds like you'll be having a yard sale soon.

You need to start back at step 1 in designing an off-grid PV system...which is determining your loads. From what you wrote down as loads, 5 of your panels and a single string of T-105RE batteries (8 in series) would be more than enough for your needs.

Some of those small fridges are terribly inefficient so you need to determine it's actual use with a Kill-A-Watt meter.

The good news is that getting the system correct might not cost more than you've already spent as you can sell about 10 of those panels and however many of the batteries are still alive to recoupe the cost of the proper equipment. You can probably sell that inverter as well because a 5Kw inverter running a small fridge (its only full time load) is going to be very inefficient. As a side note, the more battery connections there are, the greater the chance of a fire, and you have alot of connections.

The good news is that once you buy the proper stuff, you'll only have 24 cells to water instead of the 168 you spend all weekend with now.

[Sunking]

It appears to me that Bastian is describing four parallel groups of seven batteries each making up the 48 volt bank.
That is even worse than making seven strings of four batteries each and putting those strings in parallel, although both are bad ideas.

Oops my bad you are right. Doesn't change my response though, it is just worse than I originally thought. OK 48 volt 700 AH battery bank can support up to 7 to 9 Kwh per day, and his panels have the capacity to generate more than that in a day assuming a minimum 3 Sun Hour day. From his description of usage he is not even close to using 7 to 9 Kwh per day.

Bastian like I said at a minimum replace all your batteries. Except this time DO NOT used 12 volt 100 AH batteries. If you decide you need 700 AH of capacity then use 700 AH batteries. At that lever you are looking at using 6, 4 or 2 volt batteries. No such thing as a 12 volt 700 AH battery because they would weigh too much.

To protect that large of an investment, please consider replacing your charge controllers. You cannot go wrong using a Midnite Solar Classic 150.

[Bastian Bakkes]

My word , between you 2 guys , one must surely get the right advice , looking at the posting history . Thanks again for the practical and understandable answers you have given me .

[inetdog]

It appears to me that Bastian is describing four parallel groups of seven batteries each making up the 48 volt bank.
That is even worse than making seven strings of four batteries each and putting those strings in parallel, although both are bad ideas.

If this is the actual system configuration, then he has a 48 volt 700AH battery bank and has enough panel power to handle that size battery bank, depending on the actual power usage and number of sun hours.

Bastian, if you are trying to maintain your system by replacing individual batteries as each one goes bad, you are making yet another mistake, since mixing new batteries with old in parallel configuration is a good way to cause charging imbalance and faster death of the remaining batteries.

Under some circumstances you could reconfigure your group of batteries to make a smaller bank of better matched cells, but in the fairly near term you would be better off starting from scratch with batteries chosen as Sunking recommends.

One more note: When you have highly parallel configuration of unmatched batteries you can end up with your high power dual CC sending more than the safe amount of current into only a few of the parallel batteries, aging them even faster than a simple imbalance would account for.

If you do not understand our reasoning here, keep asking questions.

[Sunking]

Wow you really screwed up in equipment selection. With 3920 watts of panel power is way overkill for your meager needs. Other than the panels you pretty much need to scrap every thing.

Lets start with the batteries. The mistake is you used 12 volt batteries and were forced to use 4 parallel strings. That is asking for dead batteries in the near future as it is nearly impossible to keep them equalized. Essentially you have a 48 volt 400 AH battery. If 400 AH was the right number to start with you should have used 6 volt 400 AH batteries. It would have taken 8 of them in series to equal what you have now. Difference is they would last around 5 years assuming you bought quality deep cycle batteries.

Nest issue is you used an inferior controllers. I assume you got the 40 amp controller which explains why you had to use two of them because with a 3920 watts @ 48 volts is roughly 80 amps. I hope you divided your panels into 2 separate arrays with 1 controller on each array serving a common battery. What you should have done is buy 1 quality 80 amp MPPT controller like Midnite Solar Classic 150.

Not even going to comment on the inverter as I have no experience with it.

At a minimum you are going to have to replace your batteries as they are toast with 3920 watts will require a minimum of a 48 volts @ 400 AH or as large as 960 AH. Except this time if you intend to have a 400 AH battery, then buy 400 AH cells. You need to get out of that 12 volt box trap you built for yourself which is the root of your problems.

Lastly I suggest you replace your 2 controllers with a single 80 Amp MPPT controller. For example a Midnite Solar Classic 150 can take as much as 5000 watts input with a 48 volt battery.

By no means consider a 24 volt system. Stay with 48 volts as that is what your panel wattage demands. Your 40 amp controller limits you to 2000 watt input at 48 volts. That is why you had to use 2 controllers. If you were to switch to 24 volts would require you to loose half of your panels down to 2000 watts. A 40 Amp MPPT controller operating @ 24 volt battery can only input 1000 watts @ 24 volt battery. Do not even consider the C60 controller as it is antiquated and pretty worthless. FWIW Midnite Solar controllers are the best on the market and run circles around all other.

[Bastian Bakkes]

I would appreciate some urgent sound advice on my 48 volt system . It consists of 16 x 245 w panels , 2 x Microcare MPPT controllers , a 5 kw Microcare inverter and 28 x 12 V Deep Cycle ( 100 Ah each ) , arranged in 4 banks of 7 batteries per bank to give you 48 volts . I am using this house say every second weekend , with only me and my wife . There is only 1 fluorescent light in kitchen , in living room are 6 LED down lighters as in bedroom are 4 down lighters . The water supply is through a .37 kw pressure pump , which switches on when tap is opened . No washing machine ( we had one some time ago and might bring it back some time ) , no kettle , no TV etc , only a PC . The only other thing that draws current , is a small bar fridge , which is supposed to be on all the time . Time and again , I have to chuck away food left in the fridge after finding the power disconnected . The batteries die off , one by one .
I made contact with another solar installer , complaining towards him about the poor performance of my system . He assured me that the components used in my system , are the main reason for failure . Firstly , he pointed out that the deep cycle batteries , will never get fully charged with this set-up , and that's the reason they die off one by one . He also pointed out that he has no faith in the Microcare Controllers and Inverter ( locally manufactured ) , he would chuck them out , replacing them with 2 x Xantrex C 60 regulators , change whole system to a 24 volt one by fitting 12 x 800 AH x 2 volt Motoma lead acid batteries and replacing the inverter with a 24 volt Victron ( 3000 watt ) .
My concern is : Is this the right decision to change to 24 volt ?. Should one not stick to the 48 volt system and replace the batteries with decent ones . Question is , how many batteries will I really need to do the job with the appliances as mentioned above , if one stays with 48 volts ?. The other option is to change to 24 volts ?
Please , which option should I take ? The system already cost me an arm and a leg , I would not like to spend another one !!!