Battery bank size recommendations

[Gerrit1984]

Hi all,

I am still in the learning phases with solar, as previously mentioned i run 2 5kw Mecer Axpert inverters with 24 Panels. With 2 strings of four 120AH omnipower 12V batteries.

I want to upgrade my battery backup to go full off grid!!!

My question is what size battery backup should i have (Ah) , and what type of batteries to go for!! To ensure longest lifetime on system, i assume the best would be to have enough backup to ensure only one cycle every 24hrs?

With the current setup i still uses about 40kwh of electricity in 24hrs!! This is mostly at night running aircons( rustenburg is hot), pressure pump for water and normal household activities.

Any and all recommendations are well come

Thank you in advance

[phil.g00]

Some discussion as to the merits/economics of going off-grid with batteries here:

https://powerforum.co.za/topic/4543-lead-acid-batteries-discussion/?tab=comments#comment-64537

 

 

 

[DeepBass9]

Say 2 days backup of 2x 40 kWh = 80kWh, so you need about 400Ah @ 48V to provide that so an 800Ah lead acid battery bank (50% DOD) or an 500Ah Lithium bank (80% DOD).

Expensive. It will be cheaper to make sure your usage is as low as possible first. Factor in a generator as well for weeks like this.

Edited December 9, 2019 by DeepBass9

[Wilfred]

1st you need to determine what your usage are between 16:00 and 8:00 in summertime and also how the sun will behave in wintertime, I don’t have that data yet and will only know after the next winter. 
You will need a bank to supply you with enough kWh to go through the time there is no sun. Let’s say it is not raining, then you need enough battery backup to carry you through the night.
Problem is that winter time and summer time will differ, also what you will use will differ. If you use the batteries with heavy loads it might shorten their lifetime, in other words it might cause it to turn cycles quicker. This is the most expensive portion of your system, so you will need a couple of things added to get you totally off-grid, for example gas/solar geyser, but solar geyser might want to use electricity during the early mornings. Gas oven, etc.
 

Everyone have their favorite when it come to batteries, I like batteries that have an indication of SOC, such as Narada or Pylontech. New batteries such as Dyness (which is basically Pylontech) also have this indication with LED’s on the front indicating SOC. These are the lithium-ion batteries. With gel batteries I don’t know anything except that you need a proper battery monitoring system and you can only discharge 50%, therefore you need way more batteries, also that you can’t just add on and they also don’t have the lifetime that you would expect of them. The Pylontech’s came with a 7 year warranty + 3 year extended warranty by registering them on Pylontech’s website. They say 6000 cycles for 80% DOD, and 4500 cycles for 90% DOD, my batteries never went below 20% thus far. My cycles turn between 24 hours and 47-48 hours. I am currently on cycle 30 and have used the batteries for 40 days.

I currently have 4x US3000 Pylontech 48v batteries, I believe they are 70A max charge/discharge, 3.5KW x 4 with usable 3.2KW x 4. I have not have any problems so far with these batteries, even when I recently accidentally ran the geyser/s off them. Meaning that my batteries have never given me an alarm or error and they have never shut themselves down. I would like to add 1 or 2 new batteries before next winter, will see when the time come.
The difference is that we don’t have continuous loads of more than 1000w per hour from 16:00 to 08:45. 
I designed my system to produce 42KwH in 6 hours of sunlight (without any losses), so the panels will handle most of the loads when the sun shine. My current usage is between 32-36kWh per day.

Currently we are using the grid (in this rain), so my batteries are fully charged for most of the day, this is to prevent us sitting in darkness with the current load shedding schedules that have just been changed to stage 6.

[DeepBass9]

Stage 6? Is that even a thing? How many stages are there?

[Wilfred]

No idea, to be honest, I am caught off-guard by this whole load shedding thing and struggle to decipher this stupid app lol. Because I am trying to preserve my batteries and not run heavy loads when load shedding occur in my well planned “use as much when the sun shine” mentality  Apparently it mean more areas will have load shedding at the same time as some of the power stations have flood. Also maybe longer outage periods and more times per day.

[phil.g00]

6 minutes ago, DeepBass9 said:

ay 2 days backup of 2x 40 kWh = 80kWh, so you need about 400Ah @ 48V

Your maths is a bit dodgy here.

100Ah at say 50 V = 5kWh, so 80kWh = 1600Ah usable battery power.

So battery size:

LA at 50% DOD = 3200Ah & lithiums at 80% DOD = 2000Ah.

[phil.g00]

37 minutes ago, DeepBass9 said:

How many stages are there?

There are 8 stages, each stage is a further GW (1000MW) off the system if I recall correctly.

The problem is that they are operationally in uncharted territory, and an operational mistake in the balancing act could cost the entire grid.

Then SA would be probably be without power for weeks, or more likely months.

Electricity is needed to start a power station up and the system will be like a house of cards that probably will collapse several times before it is up again.

And naturally, the expertise to do a black start no longer reside within ESKOM, and wouldn't be able to get to South Africa anyway in the event of a national blackout.

Edited December 9, 2019 by phil.g00

[phoenix752]

I think it's good to consider Lead Acid batteries when you go 100% off-grid. You can consider 2V batteries which have a big Ah rating, they go as far as 1500Ah I think. But it will need regular maintenance and cool working conditions for them to last a long time. Because at such high energy needs, Li Ion or LiFePo4 becomes unbelievably expensive

[VisN]

49 minutes ago, DeepBass9 said:

Stage 6? Is that even a thing? How many stages are there?

[Wilfred]

With this coming load shedding schedules, my system will pay for itself very quickly. My office is ran from home, I have 5 staff members and by not loosing 4,5 hours daily is worth it through and through.

[phil.g00]

4 hours ago, Wilfred said:

With this coming load shedding schedules, my system will pay for itself very quickly. My office is ran from home, I have 5 staff members and by not loosing 4,5 hours daily is worth it through and through.

Load-shedding seems to be an acute problem, not a chronic one.

It's painful, but a load-shedding bout is still relatively infrequent.

We haven't plumbed the depths of supply security enjoyed by our African neighbors to the north just yet.

If this affects you during the working day, gear up so that PV output ( and not batteries) would cover of those power cuts if the sun was shining.

Then an auto-starting standby generator is a more cost effective solution than batteries for the remaining number of power cuts a year, when the PV alone can't cope.

 

[Gerrit1984]

What about capasitors, are they any good. I know they are crazy expensive but if you look at the lifetime arent they worth it at the end?

[Richard Mackay]

31 minutes ago, Gerrit1984 said:

What about capasitors, are they any good. I know they are crazy expensive but if you look at the lifetime arent they worth it at the end?

This is even more state of the art than Li-Ion

As an electronics man my gut tells me that capacitors are good for even more rapid cycling than any battery (as opposed to storage capacity)

This is where they come into their own: https://www.economist.com/science-and-technology/2014/07/10/first-one-up-the-drive

[Gerlach]

Go Youtube Bluenova and go check the video where they open one of that Super caps systems that is selling in SA. I will not spend my money on that if it looks so bad inside.