Whats wrong with this system??

[joerossjr]

Hey you can't get better info on battery systems, than from Sunking. Once you get used to the style, you may find him
very entertaining.

What bothers me a lot more, is all the people who don't know the difference between TO and TOO, including a couple just above. Bruce Roe

Not in my post WOOHOO!!

[Amy@altE]

I did a similar conversion turning a chest freezer into a solar kegerator. You may have a different size freezer than I used, but I ran mine on a KillaWatt meter for about 4 days, and averaged 11.1W, so it used 267Wh a day. If you are interested in how I built it and the math behind the system (which the guys have already covered here) you can search on YouTube for "Solar Kegerator" I might have had a sample or two during the filming of the video

[Mike90250]

...... I think I will also replace the inverter as it does not like the startup current - I will buy a larger one just to stop the annoying screams every time the fridge starts. ......

WHOA !!!! Your initial post indicated your inverter complained at the surge, Now it's "annoying screams". That indicates an undervoltage condition at the DC input terminals of the inverter, not an overload. Your battery voltage is sagging under the starting load, and then recovers as the compressor spins up,
Either the CABLES are too thin or too long, and/or the battery voltage collapses because they are undercharged.

My 6Kw inverter "grunts" when the pump, toaster or microwave kicks in. The windings in the transformer "sing" when the starting surges hit the coils. Sort of like a complaint, but it's not the undervoltage alarm.

[Sunking]

WHOA !!!! Your initial post indicated your inverter complained at the surge, Now it's "annoying screams". That indicates an undervoltage condition

Yeah I noticed that to and started to point it out. Then I realized there is no help for this guy.

[msalari]

Thanks guys - and you too Sunking.
You are correct that the "LVP" error (low voltage) was showing at least sometimes the fridge was starting up, but I had assumed that this was just some weird error state that the inverter was being thrown into during to that surge, because the batteries were reading full and I didn't think they could possibly be as low as it was showing.
The cables are 6 gauge and the distance from battery to inverter is only a couple of feet. Now that you have explained about the sulfating batteries, this must be another symptom of my problem. My next plan will be (after I try the KillAWatt) add 2 more panels (so 345W total), replace the batteries with 2 new ones and see what I see when the fridge starts up.

But one more question, if I may (you too Sunking, now that I am prepared for your responses)... and yes I have searched through the forum to get an answer on this, but it is not that easy to find stuff: Why do people refer to a system only being able to "support" an inverter of a certain size? I mean, I always thought that an inverter could be as big as you wanted, notwithstanding a larger trickle of watts when its not in use. But does an inverter size make any other difference to the system thats behind it ? Or are people just referring to supporting the total size 'potential' of the inverter?

To put another way, wouldn't it be OK for someone buy an inverter that was a little oversized than they think they need from the start so they can ramp up other parts of the system as they expand, and not have to invest in larger inverters later on ? - again, as long as they could live with a little more trickle.

thanks again!!

[Sunking]

OK now that I got your attention, I will explain what is going on. Short story is current, resistance and voltage losses.

Batteries have Internal Resistance we will call Ri. The value of Ri is proportional to the size of the battery. The larger the battery, the lower the resistance and vice versa the smaller the battery, the higher the resistance. Resistance is called Ohm's and expressed as "R"

In electrical there are Physical Laws. One of the very important laws is Ohm's Law which consist of 12 equations. Ohm's Law has 4 varibles:

P = Watts which is electrical power or heat energy
I = Current measured in Amps expressed as eith I or A
E = Voltage which can be expressed as E or V
R = Resistance measured in Ohm's.

Here is Ohm's Law as taught in any school.



As you can see it is just simple 5th grade math of 12 equations that show the relationship of the four variables of Power, Voltage, Current, and Resistance.

So let's get back to the battery, Ri and Current. Take note Voltage = R x I. It is key to understanding.

So we have a 12 volt 100 AH battery, it is fully charged up with an Ri = .02 ohms. It Open Circuit Voltage (OCV) = 12.6 volts. OCV just means nothing connected to the battery, thus not charging or discharging with a open circuit of infinite Resistance. We connect say a 25 (C/4) amp load to the battery and now are drawing 25 amps from the battery.

As mentioned the battery has internal resistance of .02 Ohms. That resistance now has 25 amps flowing through it. That means the voltage has to drop on the battery it is going to drop from 12.6 volts to OCV - [Ri x A) = 12.6 volt - [.02 x 25 amps] = 12.1 volts. You lost 0.5 volts or 4% of your voltage and power.

The voltage loss is not limited to just the resistance in the battery. In fact it is just a small percentage. All wiring and connections has resistance. With wire/cable the resistance is determined by the size and length of the wire. Th esmaller the wire, the higher the resistance.. The longer the wire is, the higher the resistance.

In a design it becomes extremely important to control Voltage Losses and limit them to 2% or less. Well 2% on a 12 volt system is a mere 0.25 volt or 1/4 a volt. No put it together and the light should go off in your head. In the above example with battery resistance. We lost 4% just with the battery. You are already busted at 100% more losses than you can afford, and we have not even calculated wire and connection losses.

So how do we control battery losses? Real simple we have silly little rules that cover our butts. We limit maximum current from the battery to C/8. Doing so on average we loose 1% of our voltage, just leaving us 1% more for wire and connection losses. This applies to Flooded Lead Acid batteries. AGM has lower internal resistance and we can up the current to C/4.

Now tie it all together, if we have a 12 volt 100 AH FLA battery, we limit maximum current to 100 AH / 8 Hours = 12.5 amps. That means the largest Inverter you can use going back to Ohm's Law is 12 volts x 12.5 amps = 150 Watts.

Now tell me what is the largest Inverter you should be using with a 12 volt 210 AH battery?















Did you come up with 315 watts? Be real careful with wiring and connection losses and you can go up to 500 watts. But here is the deal, you are fighting two physical laws.

1. An Inverter is a Constant Power Load device. As the battery voltage is pulled lower by current, it still demands say 300 watts by the load you are running. 300 watts at 12.6 volts = 23.8 amps. But at say 11 volts is 27.2 amps. As as you pull voltage down at the Inverter, it demands more current to keep power constant. More current looses more voltage, which causes more current, which lowers voltage which demands more current. Get the picture?

2. Your Inverter turns itself off when voltage falls below 11 to 10.5 volts.

Does that help?

I promise you no one on this forum will go to as much trouble to explain and help people than I will. Have a look around. No one even comes close.

[Amy@altE]

Beautiful explanation. I'm saving this for future reference, thanks!

[SunEagle]

Thanks guys - and you too Sunking.
You are correct that the "LVP" error (low voltage) was showing at least sometimes the fridge was starting up, but I had assumed that this was just some weird error state that the inverter was being thrown into during to that surge, because the batteries were reading full and I didn't think they could possibly be as low as it was showing.
The cables are 6 gauge and the distance from battery to inverter is only a couple of feet. Now that you have explained about the sulfating batteries, this must be another symptom of my problem. My next plan will be (after I try the KillAWatt) add 2 more panels (so 345W total), replace the batteries with 2 new ones and see what I see when the fridge starts up.

But one more question, if I may (you too Sunking, now that I am prepared for your responses)... and yes I have searched through the forum to get an answer on this, but it is not that easy to find stuff: Why do people refer to a system only being able to "support" an inverter of a certain size? I mean, I always thought that an inverter could be as big as you wanted, notwithstanding a larger trickle of watts when its not in use. But does an inverter size make any other difference to the system thats behind it ? Or are people just referring to supporting the total size 'potential' of the inverter?

To put another way, wouldn't it be OK for someone buy an inverter that was a little oversized than they think they need from the start so they can ramp up other parts of the system as they expand, and not have to invest in larger inverters later on ? - again, as long as they could live with a little more trickle.

thanks again!!

A simple answer is that very large inverters are not efficient. The consume power just being turned on without any load.

The losses could be up to 5% of the watt rating on the real cheap units so a 2500 watt could use over 100 watts (2500w x 5% = 125watts) just sitting there warm and happy slowly draining your battery without running any of your loads.

[Mike90250]

Thank you St. Sunking. I never would have done that math out long.

[Sunking]

Thank you St. Sunking. I never would have done that math out long.

Wished my puter could read my mind and do the typing. I can do the math in my head, writing it down in a format that can be understood is what takes time. Also helps when you use the same example over and over again.

[msalari]

Thanks guys

[joerossjr]

So do we have another convert to the Forum?? (Just gotta get over that initial hump!!)

[msalari]

Yup. But wow does that initial hump really sting

[msalari]

Next cabin trip if I brought these batteries home and wanted to give them a real test to see if we (you) have sorted out that the issue is in fact sulfated batteries: I could charge them with our known good 10/15A charger, and run some drainage tests on each/both. I was thinking something simple like a 100W bulb on the inverter - and my KillaWatt meter is being delivered tomorrow so I can see some real numbers over time. Any other advice on how some simple tests could be done to see if their AmpHrs are in fact totally out to lunch?

Finally, can I assume that the tricks I am seeing online about recovering these batteries is not worth the energy and my only real fix is new batteries, correct?
thanks

[Mike90250]

Once a battery is damaged, it's really hard to recover it. Plate material sheds and is sitting in the debris well in the bottom of the jug. If they are sulfated, no reliable good way to recover from that. Sometimes experts can do it, but it's rarely worth the effort, to end up with a battery that works ok for another month. Daily solar cycling is really rough on batteries and what may work for starting a car, won't work to keep grandma's Iron Lung working overnight,

[msalari]

Thanks Mike
Just so its clear what we are using it for.. its 4 day weekends a few times a year. Rarely up there for a week at a time. Probably 25 days/cycles per year.
Just to put things in perspective.
Point taken the plan is to run KillAWatt on the fridge to get some real numbers and I will use two new 100Ahr batteries, add 2 new panels for a total of 345W. And I'll see if I can get the usage to less than 50% DOD

Thanks

[MetricAmerica]

If you what to take battery to 50%, look to lithium batteries, though I don't know of solar chargers larger than pocket USB unit....😂
Be careful of chest refrigerators if you are in humid climates, they collect tons of water and mold...you want a 'dry' storage for food...but since using only 'weekends', it should be OK...
Even though you have 24 volt panel, I am assuming you did covert it to output 12 volts, you never said... Only that you had 'two batteries in parallel'...personally I would make it a complete 24 v system...
And add this nice little meter to measure solar collected, it works well for me...

http://www.amazon.com/bayite-6-5-100V-Display-Multimeter-Voltmeter/dp/B013PKYILS/

[Sunking]

Next cabin trip if I brought these batteries home and wanted to give them a real test to see if we (you) have sorted out that the issue is in fact sulfated batteries: I could charge them with our known good 10/15A charger, and run some drainage tests on each/both. I was thinking something simple like a 100W bulb on the inverter - and my KillaWatt meter is being delivered tomorrow so I can see some real numbers over time. Any other advice on how some simple tests could be done to see if their AmpHrs are in fact totally out to lunch?

It will give you some Ball Park numbers and a fair idea.

One clear sign of a sulfated battery is you put it in a charger, start the charge process, and in short period of time the charger kicks off indicating the battery is fully charged up. However when you put a load on it, the batter crashes much sooner than you expect, orr just crashes almost immediately.

Why you ask?

Because the hardened lead sulfate crystals have significantly raised the battery Internal resistance. The battery can no longer take or give a charge. Go back to Voltage = Current x Resistance. Your charger now applies a charger current, and immediately the voltage shoots up and fools the charge into thinking the battery is charged up. You measure the voltage and it looks OK. Put a load on it and try to draw current, and the battery voltage crashes. Ever go out to start your car, you hear a ERRRR, Click Click. Your battery voltage crashed

OK a Battery Capacity Test or AH Test has to be done under controls. You have to be able to monitor a few things and take action. Us pros use some automated equipment like a Constant Load Current load, very accurate Current and Voltage measurement, and time. You are not going to be able to do that, but you can use a MacGyver method but it will require your full attention. Real issue is it is time consuming.

So yes you can use your Inverter, Kill-A-Watt meter, voltmeter. Problem is if you use the Watt Meter between the Inverter and Light Bulb, you are not accounting conversion losses. No big deal because if your battery is bad, the test will end much sooner than expected.

To get a really good idea, you will need to take the 12 volt battery down to 10.5 volts. If you go to sleep and miss it, you can do some damage. How long will it take. Beats me. Depends on what condition the battery is in and how much load you apply. Assuming the batteries are OK with rated capacity 12 volts x 210 AH = 2520 watt hours. Apply a 100 watt load and it would take 2520 Watt Hours / 100 watts = 25.20 Hours. In practice not quite that long because you are taking more than 100 Watts from the battery, so more like 22 to 24 hours if the battery is in good shape.

Now if you charge the battery up, do the test, and 4 hours later your Inverter shuts off at 10.5 volts, well you got bad batteries.