Inverter Settings

Metz

I am very new to solar power.
I have a 24volt 3KVA Voltacon inverter and am just about to buy 2 x 12volt 210Ah Solite batteries to put together for the 24 volt system.
Three settings on the inverter I'm not sure about because I don't want to damage the batteries by overcharging or discharging too much, they are 1) 'Bulk charging voltage' default 28.2 volts.
2) 'Floating charging voltage' default 27 volts.
3) 'Low DC cut off voltage' default 21 volts.
However some say 'Bulk' charge higher, like 29.2, the Solite company say in their charging instructions something about charging at 16 volts which I assume would mean 32 volts on a linked 24 volt system. Others say don't discharge below 50% which would be 24.4 volts (12.2 is 50%) not 21 volts.
Very confused.com ..... don't want to damage anything

Photowhit

Use the battery manufactures settings for the Bulk voltage and float if they suggest a float voltage.

The low voltage disconnect will vary due to your wiring and how you use the inverter. The 50% is a good rule but a system in operation will have voltage sag when loads are running. The resting voltage of around 24.2 volts does represent 50%, but under load the voltage will be less!

I would be sure to wire the battery bank with the manufactures suggested wire gauge for the distance it will be from the battery bank. Likely 2/0 (NOT 2 gauge) for up to 5 feet and 4/0 up to 15. This will help the inverter not seeing a greater sag due to wiring loses.

Batteries differ in the amount of voltage sag under load. This will also vary dependent on the amount of load. 3000 watts is a very large load and a 24 volt 200Ah battery bank is not particularly large. Consider that batteries ah rating is based on a 20 hour discharge (normally) at 3000 watts you can pull (3000 ÷ 24 = ) 125 amps! This is would have your inverter shutting down pretty quick! Really no need to have that large of an inverter hooked up to that size battery bank.

softdown

I recommend 11.5/23 volts for the low voltage disconnect level. Some say 12/24 volts which looks good *in theory*. In the real world, a 12/24 volt disconnect level will produce a lot of shutdowns when the inverter has constant and varying loads on it.

Your inverter is much too large for this battery bank.....generally speaking. This is extremely common and keeps the forum hopping with solar newbie challenges. I suspect that most of us may have started that way.

Metz

Thanks for the info Photowhit and Softdown.

Live in Cyprus so sunny most days and only use battery power to get the fridge and a few led lights through the night.

What size battery bank would you suggest ?

softdown

Don't know enough about your loads. The majority should use more panels, larger battery bank, and smaller inverter.

#1 Costly Solar Mistake: Killing the batteries via undercharging and over using. There are losses, losses, losses everywhere.

westbranch

A fridge is a major load,in your hot area could be over 2 Kwh per day.  Depending on the age could be higher,newer fridges, inverter type compressor may be less, but definitley lower startup surge...  that is the killer number...  I have a  new fridge and can run it on a stout 1500 W inverter....ymmv

BB.

A Kill-a-Watt type meter can be a big help to better understand your AC loads:

http://www.reuk.co.uk/Kill-a-Watt.htm

But--Lets look at some guesses... First, you have a 3 kWatt AC inverter--If you want to use that full energy, I would suggest a 600 AH @ 24 volt battery bank minimum (upwards of 6 kWatt surge current for a few seconds) for reliable operation. (that is roughly ~200 AH per 1 kWatt of 24 volt AC inverter or 1 kWatt of solar panel typical maximum for a flooded cell lead acid battery bank).

For a typical (US) refrigerator (not the most energy efficient), you would need a minimum of ~1,200 to 1,500 Watt AC inverter for the fridge and a few LED lights/small household loads (laptop, LED TV, cell charger).

Next... Lets say your power usage is 2 kWH per day, and you want 2 days of storage and 50% maximum discharge (usually this is works out to be a nice / optimum lead acid battery bank design):

• 2,000 WH per day * 1/0.85 AC inverter eff * 1/24 volt battery bank * 2 days storage * 1/0.50 maximum discharge = 392 AH @ 24 volt battery bank

A ~400 AH @ 24 volt battery bank will support a 2 kWatt maximum inverter and solar array nicely.

Looking at charging your battery bank, suggest 5% to 13% rate of charge--And 10%+ for full time off grid:

• 400 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 753 Watt array minimum (weekend/seasonal system)
  
• 400 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,506 Watt array nominal
  
• 400 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,958 Watta array "cost effective" maximum
  

Based on 2 kWH per day loads and hours of sun per day:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Limassol
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 55° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
4.06	4.76	5.88	6.34	6.77	7.14
Jul	Aug	Sep	Oct	Nov	Dec
7.09	7.11	6.94	6.07	4.64	3.81

Lots of sun--So if you use December, the minimum "break even" array would be:

• 2,000 WH per day * 1/0.52 off grid system eff * 1/3.81 hours of sun per day = 1,009 Watt array minimum (break even December)

If you choose a 1,506 Watt array (recommended), your average power per day (in December) would be:

• 1,506 Watt array * 0.52 off grid system eff * 3.81 hours of average Dec sun per day = 2,984 WH per day (Dec)
  

And, of course, a lot more during non-winter days...

My guesses.

-Bill

Metz

Thanks for that Bill, understand some of it  so I need to get another 2 or even 4 batteries.

The inverter supplier recommended DC cut off voltage at 21volts, I asked him if it was way below 50% and if it may drastically shorten the battery life, he didn't reply, what's your thoughts ?

BB.

Please do not buy anything yet on my say so... I am 1/2 a world away and making a lot of guesses on what your power needs may be.

Regarding batteries--You may have limitations on what is available in your region. For example, I am a fan of reducing the number of parallel battery connections. That usually means getting lower voltage batteries that have higher amp hours.

For example, in North America, there are 12 volt @ 100 AH batteries and 6 volt @ 200 AH batteries. They weight the same and store the same about of energy (remember that Energy=Voltage*Current*Hours). So the 6 and 12 volt batteries are "the same" electrically-But physically, for a 12 volt bank, you would connect:

• 2x 12 volt @ 100 AH batteries in parallel = 12 volts @ 200 AH
• 2x 6 volt @ 200 AH batteries in series = 12 volts @ 200 AH

I would recommend the 2x 6 volt option. Fewer cells to check water on (6 vs 12 cells), and no parallel battery connections (parallel connections are more difficult to debug / monitor for faults--especially 12 volt batteries in parallel for a 12 volt bus).

In general, I prefer 1 string of batteries. With 2-3 parallel strings about the most I would suggest. However, for many folks, they do not have the ability to purchase other battery configurations. So--You have to make do with what you have.

I appreciate that this is an English based forum--And for many people, English is their second (or third) language (me--It is my only language, and I am not very good at it). Please ask questions until you are clear on what we are saying.

Solar power systems required the "balancing" of many different components (solar panels, charge controllers, battery banks, AC inverters, etc.).

I highly suggest that that you get your paper design worked out first--Then purchase the components that meet your design.

Tell us a little more about your limitations (only XXX watts of panels can fit on roof, cannot afford more than 4x 6 volt @ 200 AH batteries, need a 3 kW inverter to run a well pump, etc.). We want to make sure your system will support your needs.

-Bill

Metz

Very few if any proper experts on off grid solar in Cyprus so we more or less tried to figure it out as we went along with bits of info from mates who had a little knowledge and suppliers of the equipment. Not been easy and doing it on a tight budget also. Learning that the batteries are the Achilles heel of the solar system.

We have 8 x 285w CanadianSolar panels and a split system.
(1)
A Voltacon 12volt 1.5kva inverter charger with 2 separate feeds from 4 panels wired separately into pairs. This is hooked up to 6 x 2volt fork lift batteries that unfortunately are on their last legs and I will have to replace soon.
(2)
A Voltacon 24volt 3kva inverter charger wired from the other 4 panels. This is hooked up to 2 x Yuasa cargo 180ah batteries to make 24volt. Managed with this through the summer but now the days are shorter we've started to run out of power at night.

Also because I am a total novice I left the inverters on their default settings so the 12volt has been bulk charging at 14.1 volts, floating charging at 13.5 volts and DC cut off at 10.2 volts. The 24volt has been bulk charging at 28.2 volts, floating charging voltage at 27 volts and DC cut off voltage at 21 volts. As I understand it the DC cut off voltage is low and will have been knocking 7 bells out of my batteries at this setting.
I now have just bought 2 new Solite batteries, 12 volt 210AH, flooded calcium lead acid sealed maintenance free batteries for the 24 volt system but understand, from what you said already, I need at least 2 more if only discharging to 50%.

Big learning curve all this
Thanks for all the help.

Photowhit

Metz said:

Also because I am a total novice I left the inverters on their default settings so the 12volt has been bulk charging at 14.1 volts, floating charging at 13.5 volts and DC cut off at 10.2 volts. The 24volt has been bulk charging at 28.2 volts, floating charging voltage at 27 volts and DC cut off voltage at 21 volts. As I understand it the DC cut off voltage is low and will have been knocking 7 bells out of my batteries at this setting.
I know have just bought 2 new Solite batteries, 12 volt 210AH, flooded calcium lead acid sealed maintenance free batteries for the 24 volt system but understand, from what you said already, I need at least 2 more if only discharging to 50%.

Ideally you don't want the low voltage shut off to ever turn off your system. Much better that you realize you haven't been generating much electric and reduce the load. Cloudy days, or high demand during the day. For instance I've been running 2 dehydrators and a 900watt water heater much of the day today. Likely I started drawing from the battery bank as soon as the sun went behind the trees and partially shaded my array.

The problem with a low voltage shutoff, in a volt system voltage, 12 and 24 volt system, it that there will be voltage sag when higher wattage items are running, and starting. We don't know the demands on your system. A water pump running at close to your system's capacity will create a momentary load that may shut down your system if you set the low voltage shut off at say 11.5 volts, even with fully charged batteries! On the other hand if you never use more than lighting at 60 watts or less, a 11.5 volt low voltage shut off will mean you will draw your system down below 50% regularly.

mcgivor

The suggestions of low voltage shut off being too high are very valid, for example if your refrigerator were to start and your inverter were to shut down then auto restore when the voltage recovered, the refrigerator would try start  again and repeat the cycle. Refrigerator compressors do not like to be short cycled and is an excellent way to kill them.

BB.

Do you need two separate systems--Or can you combine all of the solar panels and the one 24 volt inverter into one system?

In general, managing two separate systems on the same single site is not my idea of fun. If many of your loads are small and you only need the large capacity inverter once in a while--You might want to look at getting a smaller inverter to run those loads continuously, and only fire up the large inverter when needed.

What AH were the 2 volt cells? How did they work out for you? It might not be a bad idea to build your 24 volt system out of a set of 12 of those cells.

Sealed/maintenance free batteries tend to not last as long as flooded cell. And they tend to be more expensive (getting distilled water, and doing cell maintenance is not always fun either).

-Bill

Metz

Hi Bill.

The smaller system runs upstairs and the larger downstairs. I bought the 2 volt cells second hand for a project that didn't work out so they were standing for 2 years, we used them on the small 12 volt system because they are a fortune to buy here in Cyprus and we only had the 6, they are old and starting to fail now.

Steve.

Metz

OK I've ordered another couple of batteries.

Need more help ...... I read the chart above and set my inverter "low voltage cut off" on the 24volt system to 23.90volt from the chart above but the inverter hardly powers anything from full batteries before it says "low voltage"....... the inverter supplier suggests it be set at 21volts..... have I got the wrong end of the stick with this chart ?

Thanks.

BB.

Those are open circuit voltages (no load). Batteries with no load are not useful to us.

The heavier the load. The more voltage drop. The less charged the battery, the more voltage drop. The hotter the battery, the more voltage drop.

Using voltage cutoff is not very accurate for lead acid batteries.

For ac inverters, the cutoff voltage is the point where the inverter will stop working as designed and/or overheat under heavy load.

Start with 23.0 volt cut off and see what happens.

In practice, you need other methods to protect the battery bank. Battery monitor, specific gravity, your experience with loads+ batteries as to how much energy you can store.

Also, check your wiring. For a 24 volt system, I would suggest no more than 1.0 volt drop from battery to inverter under maximum load.

Your wiring may be too thin and or too long. You may have poor connections or corrosion.

Or your battery(ies) may have issues.

-Bill

Photowhit

I understand that you are ignoring me, but...

What type of things are shutting the inverters down? Loads, you have had several questions about defining your loads!
Does it shut down immediately?
How is it wired? Do you have nice thick wiring for your battery connects and inverter to battery? How long is the wire run?
Please be sure to define which system you are having problems with, you have described both 12 and 24 volt systems.

Voltage will sag with introduction of loads. That is why the low voltage is set near the 'death' level for some batteries. Concentrate of the recovered level, after the voltage has recovered for the load after an hour or so. If you are going to determine the stat of charge by voltage this would be the voltage you should be concerned with...

Metz

Thanks Photowhit and BB.

Photowhit ..... sorry mate, certainly didn't want to look like I was ignoring you mate, very grateful for all the help I can get as I'm totally green to all this and trying to wade my way through all the technical jargon and get to understand it : 

Both the systems are wired in thick multistrand cable, the type used on electric welding equipment or electric fork lift trucks. The cable runs are short, the batteries are only 2 - 3 feet max from the inverters. It's all been installed in the last few months so everything is new, the connections are good with no corrosion.

On the 24 volt system, Conversol 3KVA 2400Watt MPPT 60A Inverter. 2 x Solite 210AH (20HR)/430MIN(RC)/1050A(EN) Calcium Lead Acid Maintenance Free Batteries. On this system set at bulk charge 29volts, float 27.5volts and cut off set at 23volts with only the small Bosch fridge/freezer running.... runs perfect all day, by the end of the day the inverter is saying the batteries are full but after only around 3hours the inverter is flashing low voltage, from previous advice I am going to add another two of the same batteries to this system asap, so if the fridge is say around 250 Watts an hour and each 200 Watts is taking around 10 DC Amps from the batteries then surely even on just these 2 batteries it should last at least 10 hours ..... what am I getting wrong ?

As for the 12volt system, I'm not having any luck whatsoever, Conversol 1200Watt 2xMPPT. 2 x Yuasa 180AH Lead Acid Batteries. 2 days ago the inverter sparked and smoked inside, switched it off immediately, following day switched it back on and is still working, not sure it's charging properly now though because with nothing drawn off if and sunny all day it's not showing the batteries as fully charged by the end of the day like it used to !!! Currently negotiating with the company that supplied it ( Voltacon UK) as it's still under guarantee. 

The 240volt side is all wired in 2.5 twin and earth standard UK mains wire. Lighting wired in 1.5 twin and earth from the consumer unit.