Batteries give much less power than their capacity

vahitlafci · December 2016

Hi. We've set up a solar and wind hybrid system to our farm. There are 6 panels, each of them 260W 24V and 4x143aH agm batteries. The problem is batteries should give about 6000W but it cannot give even 3000W if there is no wind. After the sunset charge controller shows their voltage about 25.6 volts. Here is the connection diagram :

Image: http://www.imgim.com/680incip5778175.jpg

I'm saying 3000W because the load is constant about 300w and between sunset and sunrise is 10 hours. Also system voltage drops linearly until about 23v. After that it rapidly drops and system shutdowns below 21.6v. During this the voltage of batteries which are near negative terminal(0V), rapidly decreases down to about 9V and the voltage of batteries near to positive terminal(24V) stays around 12V. What might the problem in the system?

Edit: Cables are thick enough. (9awg)
Edit2: We've send the batteries which are near to negative terminal to the technical service and they said batteries are ok.

Estragon · December 2016

Although the total capacity of your bank is about 6000 watt hours, the usable capacity is less that that. For best battery life vs cost the rule of thumb is to aim to not disharge batteries more than 50%. As you have seen, voltage drops off quickly as the bank gets very deeply discharged.

If left in the deeply discharged state for very long sulfate will harden on the plates causing loss of capacity and eventual failure.

vahitlafci · December 2016

Estragon said:

Although the total capacity of your bank is about 6000 watt hours, the usable capacity is less that that. For best battery life vs cost the rule of thumb is to aim to not disharge batteries more than 50%. As you have seen, voltage drops off quickly as the bank gets very deeply discharged.

If left in the deeply discharged state for very long sulfate will harden on the plates causing loss of capacity and eventual failure.

How much it can provide about? Voltage drops quickly but not the voltage of all batteries. Only 2 of them (at the bottom) drops quickly while others(on the top) stay almost the same.

softdown · December 2016

You need really thick battery cables. Think 2/0 or so....with good connections. Otherwise....recipe for disaster.

Estragon · December 2016

9awg isn't really beefy enough for your battery cables. 2400w at 24v is potentially 100a. What is the rating on the breaker or fuse protecting the wire to the inverter?

vahitlafci · December 2016

Estragon said:

9awg isn't really beefy enough for your battery cables. 2400w at 24v is potentially 100a. What is the rating on the breaker or fuse protecting the wire to the inverter?

There is no fuse or breaker. Although the inverter is 2400w, max. momentary power we use is 400w

Estragon · December 2016

If there really is no fuse or breaker protecting the wire (could be inside the inverter?) I would recommend shutting the system down and disconnecting the batteries immediately.

vahitlafci · December 2016

Estragon said:

If there really is no fuse or breaker protecting the wire (could be inside the inverter?) I would recommend shutting the system down and disconnecting the batteries immediately.

I'm sure that inverter has it. It's a hybrid inverter has lots of features.

Estragon · December 2016

I'm not sure what's in that box labelled 2400w 24v. There may be breakers protecting wires to generator, batteries, and maybe 120v out.

westbranch · December 2016

The best you can expect is 50% of the batteries capacity,~ 3000Whrs, that is 1000W for 3 hrs for an example.

9 awg wire is suitable for the power (Watts) from the panels to the charge controller after that it needs to be much larger.as said above...

vahitlafci · December 2016

Estragon said:

I'm not sure what's in that box labelled 2400w 24v. There may be breakers protecting wires to generator, batteries, and maybe 120v out.

Inverter has 24v battery/pv input. It also has a total(Pv + AC input) current limitation to charge the batteries which is about 30A and it cuts AC output if load is more than 2400W.

vahitlafci · December 2016

westbranch said:

The best you can expect is 50% of the batteries capacity,~ 3000Whrs, that is 1000W for 3 hrs for an example.

9 awg wire is suitable for the power (Watts) from the panels to the charge controller after that it needs to be much larger.as said above...

If I increase wire's radius, will this might significantly affect the total power we receive from batteries? For ex. an hour extra before total discharge.

Estragon · December 2016

If there is a breaker in the inverter box, please check the size.

Photowhit · December 2016

Lot's of stuff, first "... sunset and sunrise is 10 hours..." I trust you are in the southern hemisphere? It's more like 14 hours here nearing the shortest day of the year. Central USA.

Your load is 300 watts (at the inverter?) or is this 30 - 10 watt lights? do we know if there is a power factor? Did you include the inverer's losses (most are @85% efficient other than a narrow area where they peak at 90-94%)

How far is the inverter from the battery bank? There will be losses in the wiring from the battery bank to the load.

You say only "...Only 2 of them (at the bottom) drops quickly..." are these in the same string? Did you wire your battery bank correctly so they have equal resistance for each string? Multiple strings of batteries can be trick, 2 strings are usually manageable, but I suspect this is the crux of your problem! If they are 1 string are they the string with the shortest leads from the inverter?

The simplest way to ensure that they have equal resistance is to wire each string to a common bus bar with equally sized wires.

Most AGM's can be run down to 20% of capacity, there will also be a a bit of voltage sag from the load. Curious what the recovered voltage is after they disconnect and have sat without a load for a while?

300 watts (or 350 watts if inverter wasn't included as part of the load) represents 12.5-15 amp at 24 volt load on a battery bank of 286 ah or about 1/20th of capacity, as the voltage falls, it will represent a larger portion of the remaining battery capacity, hence will discharge at a more rapid rate. As the voltage falls more amperage is drawn for the same wattage and the battery has less amperage in the "bank".

I hope we have established that the batteries are in the southern hemisphere, but I'll ask to cover all the bases, are the batteries warm? 50-80 degrees? Lead acid batteries have diminished capacity as they get colder.

Estragon · December 2016

Increasing wire size should help. Small wire can cause loss in voltage. As voltage drops more amps are drawn for the load, and more amps through a given wire size makes the voltage drop even worse. As the batteries are drawn down, voltage drops as well, even more amps drawn and so on.

bill von novak · December 2016

vahitlafci said:What might the problem in the system?

Here are your problems:

1) Your bank is undersized. If you are trying to get 6000 watt-hours (not watts) then you need at least 12KWhrs of battery storage.
2) Your wiring is undersized. 9awg is insufficient for 100 amps. You need 2awg at least.
3) Your bank is unbalanced. Repetitive deep discharges have likely amplified the (originally small) differences between "top" and "bottom" batteries. There is no way to restore the capacity to the lower batteries; the best you can do is monitor them closely (with a voltmeter) until they are replaced, and keep them in balance as best you can via manual means - individual charging or equalization charging.
4) You have insufficient protection. You need a single fuse or breaker either at the + terminal of the battery bank or at the end of a single wire run from the battery bank.

vahitlafci · December 2016

Photowhit said:

Lot's of stuff, first "... sunset and sunrise is 10 hours..." I trust you are in the southern hemisphere? It's more like 14 hours here nearing the shortest day of the year. Central USA.

Your load is 300 watts (at the inverter?) or is this 30 - 10 watt lights? do we know if there is a power factor? Did you include the inverer's losses (most are @85% efficient other than a narrow area where they peak at 90-94%)

How far is the inverter from the battery bank? There will be losses in the wiring from the battery bank to the load.

You say only "...Only 2 of them (at the bottom) drops quickly..." are these in the same string? Did you wire your battery bank correctly so they have equal resistance for each string? Multiple strings of batteries can be trick, 2 strings are usually manageable, but I suspect this is the crux of your problem! If they are 1 string are they the string with the shortest leads from the inverter?

The simplest way to ensure that they have equal resistance is to wire each string to a common bus bar with equally sized wires.

Most AGM's can be run down to 20% of capacity, there will also be a a bit of voltage sag from the load. Curious what the recovered voltage is after they disconnect and have sat without a load for a while?

300 watts (or 350 watts if inverter wasn't included as part of the load) represents 12.5-15 amp at 24 volt load on a battery bank of 286 ah or about 1/20th of capacity, as the voltage falls, it will represent a larger portion of the remaining battery capacity, hence will discharge at a more rapid rate. As the voltage falls more amperage is drawn for the same wattage and the battery has less amperage in the "bank".

I hope we have established that the batteries are in the southern hemisphere, but I'll ask to cover all the bases, are the batteries warm? 50-80 degrees? Lead acid batteries have diminished capacity as they get colder.

1- I'm in Turkey. When I say 10 hours, sun almost set at 8PM and rise at 6AM.
2- Yes at the inverter. Inverter shows the W and VA which are the same which means no power factor.
3- Wires between inverter and battery bank are less than 20 inches.
4- Here is the connection diagram. Blue circled batteries' voltage drops quickly. After we sent them to the technical service, I've connected the remaining 2 batteries in series and the voltage of the battery at the bottom of new connection ,dropped quickly as well.

Image: http://www.imgim.com/6870incic5713980.jpg

5- I will ensure that the resistance is equal when I go back to my homecity(I'm in a student in different city)
6- Never tried

7- Voltage drop to 21.6v from 23v in almost 5 minutes. It shouldn't be that fast.
8- Batteries are in average 30 degrees but there is a strafor layer between the batteries and the ground.

Estragon said:

Increasing wire size should help. Small wire can cause loss in voltage. As voltage drops more amps are drawn for the load, and more amps through a given wire size makes the voltage drop even worse. As the batteries are drawn down, voltage drops as well, even more amps drawn and so on.

I will try to increase the size.

bill von novak said:

vahitlafci said:What might the problem in the system?

Here are your problems:

1) Your bank is undersized. If you are trying to get 6000 watt-hours (not watts) then you need at least 12KWhrs of battery storage.
2) Your wiring is undersized. 9awg is insufficient for 100 amps. You need 2awg at least.
3) Your bank is unbalanced. Repetitive deep discharges have likely amplified the (originally small) differences between "top" and "bottom" batteries. There is no way to restore the capacity to the lower batteries; the best you can do is monitor them closely (with a voltmeter) until they are replaced, and keep them in balance as best you can via manual means - individual charging or equalization charging.
4) You have insufficient protection. You need a single fuse or breaker either at the + terminal of the battery bank or at the end of a single wire run from the battery bank.

1- I'm trying to get 3000 watt-hours at maximum.
2- I will increase it
3- As I said before at this comment, after sending that 2 battery to the technical service, connected the remaining two in series and again the battery at the bottom dropped quickly aswell.
4- I will connect a fuse but I didn't get that how might it help to increase the power we receive ?

Thanks so much.

Estragon · December 2016

Connecting the fuse won't do anything to increase the power. It is to protect the wire. Without it you have a serious fire risk in the event of (eg.) a short in your inverter. You also need fuses or breakers on the wire from the charge controllers, generator, and probably the panels.

bill von novak · December 2016

vahitlafci said:

1- I'm trying to get 3000 watt-hours at maximum.
2- I will increase it
3- As I said before at this comment, after sending that 2 battery to the technical service, connected the remaining two in series and again the battery at the bottom dropped quickly aswell.
4- I will connect a fuse but I didn't get that how might it help to increase the power we receive ?

Thanks so much.

1 - OK. As long as 3000 watt-hours (from the battery, not the inverter) is the MAXIMUM (not the average) that battery size should be OK. It's marginal but should be OK.

3 - Any time you discharge a pack heavily - even a good pack - one cell/battery is going to die before the other; the voltage will drop precipitously until the pack voltage gets low enough to trigger the LVD of the inverter (20-22 volts.) The pack will then sit there at a low voltage until the sun comes up again. The "high" batteries (sitting at about 12 volts, or 2 volts per cell) won't like being at a low state of charge but won't be damaged much if it's a short time. The "low" batteries (sitting at 9 volts, or 1.5 volts per cell) are being damaged badly by being at such a low state of charge.

It's even worse if it is just one cell. To get that battery down to 9 volts you might have 5 cells at 1.8 volts and one cell at 0V (i.e. almost reverse biased) - and that cell will die VERY quickly.

The next day the sun comes up and all the batteries charge up again. That night they are drawn down again. The low battery, after suffering some damage the night before, is now even lower capacity, and so hits 9 volts sooner - and the system shuts down again. It then sits at 9 volts for even longer, damaging it even further.

That's why one battery will tend to "go south" if the pack is heavily (and regularly) discharged.

To get around this:
a) Always charge to 100% (or as close as you can reasonably come.)
b) Never discharge more than 50%.
c) Run equalizing charges when practical (only on flooded batteries.)
d) Monitor cell voltages rather than pack voltages and then shut down when any one cell goes below a threshold (1.9 volts or so)
e) Never let any battery sit for any length of time below about 11.5 volts.
f) Do regular measurements of SG (for flooded cells) to monitor state of charge; all cells should be roughly equal.

4 - A fuse will not increase energy received, but could save your life (or your house.)

Photowhit · December 2016

It's odd that only the batteries on the negative side of 2 battery strings are having issues. I must ask, you don't have any 12 volt loads that are connected just to one of those batteries? How did you measure them when you figured out they had a problem? Did you remove the connections before measuring?

The only wiring issue I see, is that the wind turbine is connected incorrectly. If you have 4 batteries in series and parallel, they should only have one connection point for positive and one for negative. If connected with equal gauge and length wire, it works out even to pull from the positive of one string and the negative from the other string. Like your solar is connected, or like this;

I see no problem and can't really think of one where one battery would be drawn down more than the other in a string, Though I have seen a single battery have issues, to have 2 at the same time would raise flags. a lead acid battery is 2 volts nominal, a 12 volt 'battery' is actually a bank of 6 - 2 volt cells, by creating a 24 volt battery bank, you are just creating a string of 12 cells.

mcgivor · December 2016

As @Photowhit said it is odd that one battery in a string of two would be drawn down unevenly, unless it was down on capacity, but since when the good ones when put in series displayed the same result, indicates something else. The problem it seems would be in the battery string as all losses outside the battery, only contribute to the load total and would have equal draw from both, not favoring one. If the connections were not diagonally opposed, and cable lengths unequal in length, one string may be favored not one battery in each string.This leaves only one thing that I can think of and that is a center tapped 12V load, but off both strings? If there any other wires on the jumpers between the batteries, they should not be there and while you are at it check for any volt drop across the + & - battery terminals, not the lugs, at the jumper under load. Otherwise can't think of anything else, for now.
Did you build the system?

tabbycat · December 2016

Are these panels grid-tie (~30 volt VMP) or off-grid panels (~36 volts VMP) ? Thanks.

vahitlafci · December 2016

I guess I explained the situation a little bit complex. Sorry for that, let me try again

System was built in the beginning of august and there wasn’t anyone living in the farm before the October because cows weren’t bought and farm was empty. So we were turning the lights on to act like there Is a person in there to prevent theft and we were visiting there every day to feed our dogs.

System was working continuous if we use 5 florescent lamps + 2 LED’s and the refrigerator only. If we turn all of the projectors, system would be down when we come back. One day I went to check at 6AM and it wasn’t up. That’s why I said 10 hours. Replaced all the lamps with LED lamps later.

After I built the surveillance system, I’d looked at to Inverter’s LCD in order to check the power usage while all of the lamps, projectors and the refrigerator’s motor was working. It was about 300W (Both Watts and Volt amperes were equal). That’s why I said 300W because it is the maximum momentary power we need.

After we bought the animals, system was cutting the power in about 5 hours if we use all the devices. If we don’t use projectors and surveillance system, it was cutting the power at 3 AM whereas load was less than 70W. So we needed to use generator for an hour every day in order to use only 5 led lamps.

Because the problem is big, an electricity technician came from the manufacturer of the system (Both panels, batteries and the inverter), removed old wires, rebuilt the system with thicker and shorter cables.

System is working better now according to my father. He said that system works continuous without using the generator even if there is no wind at all. If he uses projectors system was down in the morning.

Also he said that when he woke up at 7AM and checked the battery voltage, it was 24.6V while projectors were off.

To be honest, I haven’t checked the system after the technician. I will check the system and measure everything again and post the results in here.

Photowhit · December 2016

Not knowing how much wind you've had makes the morning voltage mean very little.

I'm glad you are having better luck, perhaps earlier you had a few cloudy days?

A clamp meter might help with determining the actual load when running. The inverter will use energy by it's self and the reading you are getting from it is likely what it's outputting and not how much energy it is drawing from the battery.

westbranch · December 2016

If there was a refrigerator running al the time that will likely be the source of the problem! It should draw somewhere around 1 Kwh overnight (in the summer) and in winter it will be a lot higher depletion from the batteries as you now have to power it for ~ 2 times as long... BTW if it is a small fridge the draw will be up to 2 x higher than a newer full size (18 cu ft) unit... they are not very efficient even though small in cu ft... If you can afford one an Inverter type fridge is the way to go, especially when you have a small battery bank.

hth

Batteries give much less power than their capacity

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