# Solar and Batteries... understanding runtime



## CrackPot

I am having a hard time understanding my actual runtimes on my solar system & batteries.

I have (4) Power Maxx 29DC batteries, supposed to be 120Ah each, 480Ah in the set
I have a Go Power GP-SW3000 115v inverter

With the system fully charged (12.1v) using the solar panels I set up the dehydrator (600 watts) to make some jerky, and within 2.5 hours the low-voltage alarm was going off and the batteries where at 11.7v

Using this chart:








That shows I had used 70% of my reserve power in the batteries... burning only (600 x 2.5 hrs) 1500 watt-hours @ 120v? Just 12.5Ah from a 480Ah battery bank and I'm in the yellow?

So my $400 worth of batteries only give me 1500 watts at 120v? That doesn't make any sense to me. Can someone help me understand this?


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## Maine-Marine

Did the dehydrator stop working???


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## CrackPot

No, when the low-voltage started beeping I moved the plug over to grid power. I wanted to check on why it was reporting low voltage at just 11.7v. it turns out that model inverter does that. However, I found the chart I posted and was concerned. I don't want to damage brand new batteries by running them too low, hence my concern. Now I question the whole system. If $3,000 in batteries, inverter, charge controller and panels only gives me 1500 watts per day (21-cents of power "on-grid"), I don't see how the expense can ever be justified.


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## paraquack

First off, I cut my teeth on the big forklift batteries for electric forklift. They are very expensive deep cycle batteries and run 5 day a week for years. 
I prefer 6 volt golf cart batteries. They are designed of deep discharge and full recharge over and over again. RV batteries, marine batteries, car/truck batteries are not and will probably go down hill quickly. Other batteries than deep cycle (golf cart batteries) will not have a total life span (full recharge/discharge cycle) by comparison. My golf cart batteries in my RV (not for starting, lighting, TV, fridge, water pump lasted me for almost 15 years. Any 12 volt battery system has to put out 10 times the 12 volt amperage to give you 120 VAC at the same amperage.

You needed 600 watts at 120 VAC = 5 amps But your battery has to put out 50 amps 12 VDC into the inverter (plus a little is wasted in heat). *In Theory* if everything were perfect, the batteries would give you 9 hours. But since the batteries are not designed to pump out that kind of power constantly ( no recharging during use) you're not going to get 9 hours.

Does you battery give you a "Reserve amps" rating? Might say 2-6 amps for 20 hours.
I found the EverStart Maxx Group Size 29 Marine Battery and people are saying it's rated at 1 amp for 20 hours. That sound ridiculously low to me.
Let's it says 10 amps for 20 hours. that means exactly what in sound like. It will put out 10 amps for 20 hours. 5 amps will last 40 hours plus. 20 amps should last 10 but don't count on it. Problably only 8 hours. One posting on a marine battery sight says the EverStart Maxx Group Size 29 Marine Battery will only give you 65-70 full charge/discharge cycles.

Considering all, did you fully charge your batteries before use? Even though a voltmeter says they are at 13.2 volts they might have only a surface charge. Disconnect all your batteries and charge them individually and then test them. With four batteries tied together, you might have one that is a little "iffy" and dragging down the others.

*stolen from* Marine Battery Maintenance
In an ideal world, you should deplete only about 20 percent of the capacity of your batteries prior to recharging. Such practices will greatly prolong the life of the batteries and shorten charge times. You'll need to size the batteries for your boat's daily hunger for amp hours and add extra amp hours to the bank to avoid discharges that exceed 20 percent of total capacity, if practical. Add up the draw in amps for each appliance aboard based on the time it is actually in use in a given day, and that will provide you with a general idea of the total amp hours you use in a 24-hour period. To achieve the best results, select batteries in a size that together in the bank can deliver at least three or four times the total amp hours you require in a given day.

try these links for RV/Marine batteries 
Battery Basics: A Layman's Guide to Batte | BatteryStuff Articles
Battery Care by Don Casey - BoatTECH - BoatUS

This link is for deep cycle batteries
Battery School | Batteriesnorthwest.com | General deep cycle battery care procedures


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## HuntingHawk

12.1 volts should not be full charge.


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## HuntingHawk

What you are missing is you are figuring 600 watt output from the inverter. That would be 600 divided by 120 equals 5amps. 10X draw from the batteries would be atleast 50amps draw from the batteries.

Your batteries aren't getting to full charge. They should get up to 12.5-12.7volts.


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## Maine-Marine

and... consider using a load disconnect 

Morningstar SS-10L-12V


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## HuntingHawk

Something is wrong somewhere. You should be able to operate the dehydrator for atleast four hours.

Did you use the 12VDC wiring chart for panels to controller, controller to batteries, & batteries to invertor?


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## Ripon

.21 cents a day is a low rate. In some staes 1.5kw is three times that. A $180 yr savings would be 6% on $3k and not needing the power co is priceless.

What size panel array do you have? Your panel total rated watts x 5.5 x .80 is the typical daily production in most sun drenched zones.



CrackPot said:


> No, when the low-voltage started beeping I moved the plug over to grid power. I wanted to check on why it was reporting low voltage at just 11.7v. it turns out that model inverter does that. However, I found the chart I posted and was concerned. I don't want to damage brand new batteries by running them too low, hence my concern. Now I question the whole system. If $3,000 in batteries, inverter, charge controller and panels only gives me 1500 watts per day (21-cents of power "on-grid"), I don't see how the expense can ever be justified.


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## tinkerhell

CrackPot said:


> I am having a hard time understanding my actual runtimes on my solar system & batteries.
> 
> I have (4) Power Maxx 29DC batteries, supposed to be 120Ah each, 480Ah in the set
> I have a Go Power GP-SW3000 115v inverter
> 
> With the system fully charged (12.1v) using the solar panels I set up the dehydrator (600 watts) to make some jerky, and within 2.5 hours the low-voltage alarm was going off and the batteries where at 11.7v
> 
> Using this chart:
> View attachment 9146
> 
> 
> That shows I had used 70% of my reserve power in the batteries... burning only (600 x 2.5 hrs) 1500 watt-hours @ 120v? Just 12.5Ah from a 480Ah battery bank and I'm in the yellow?
> 
> So my $400 worth of batteries only give me 1500 watts at 120v? That doesn't make any sense to me. Can someone help me understand this?


In order to calculate the Ah drained from the batteries, you need to calculate the amps from your batteries, and you already know the hours.

I believe you mistake, and it's an easy one to make, you have a 600 watt load, you could have worked that number back to figure out your amps at your batteries. Instead, you took a 12.5Ah figure ( which can only be relevant at 120 volts) and you applied it to math on the battery side of the system.

Please allow me to crunch new numbers for you:

You have a 600 watt load that ran for 2.5 hrs.

- 600 watts at 120volts is a I = P/V = 600/120 = 5 amp load. That is you dehydrater (at 120V) ran at 5A per hour for 2.5 hours. It consumed 12.5Ah ( at 120volts)

- on your battery side, the batteries delivered approximately 600 watts of power to your inverter, the battery current was I= P/V = 600 / 12.1 volts = 49.58 Amps. This was a constant drain for 2.5 hours, so the Ah drained from the batteries was 49.58 x 2.5 = 123.97 Ah

Conclusion

You wrote "Just 12.5Ah from a 480Ah battery bank and I'm in the yellow?"

This is incorrect, you drained a 123 Ah from a battery bank that was initially sitting at 12.1v ( please reference your chart, 12.1v is about 50% of your max capacity of 480Ah).

Edit: imo, that sounds like way too deep of a discharge. I'm not sure I would allow the low voltage alarm to watch dog your system, you would be better off to monitor it yourself, and always keep the system in the top half of the green in your chart.


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## rickkyw1720pf

Voltage is not the best way to tell a batteries state of charge, a battery hydrometer is. There are a lot of things that can affect the battery voltage.


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## CrackPot

Thank you all - you're really good!

Yes, I applied a misunderstanding of the conversions. What I have for the panels is (4) 140 watt panels I bought just for such testing... and I made some ASSumptions. You have made it clear where I was off, now I can move forward (as money permits) to get where I want to be. I also found that HuntingHawk was right, the batteries were not fully changed to begin with... I was leaving the inverter on all the time, a draw of 24 watts * 24 hours = 576 watts a day... but was only averaging 616 watts a day off the panels (I don't have them mounted on the roof yet... there's partial shade), so the inverter was drawing more than I was generating (with the charging overhead) and not letting the batteries fully charge.

Now I can mount the panels, get a few more panels, use the energy-saving option on the inverter and see where I am in a month or so. I just wanted to start now where we have the least amount of sunshine per day to get a winter-comparison. It also looks like I need to look at more/different batteries. You've been really helpful!

Thanks!


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## CrackPot

Oh, BTW, for the recent posts, we pay $0.13 per KWH here where I am. I was saying 1.5KWH * $0.13 = $0.19 (not $0.21 like I said, bad math... go sit in a corner).

I keep getting distracted from the real purpose here, to have SOME power in a grid-down situation... Not really lowering the bill or giving the power company the finger... Just to have running water and a refrigerator to preserve food would be good enough!


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## Ripon

And that's the best reason to have your presented set up. But saving a few Bucks pays for more preps.



CrackPot said:


> Oh, BTW, for the recent posts, we pay $0.13 per KWH here where I am. I was saying 1.5KWH * $0.13 = $0.19 (not $0.21 like I said, bad math... go sit in a corner).
> 
> I keep getting distracted from the real purpose here, to have SOME power in a grid-down situation... Not really lowering the bill or giving the power company the finger... Just to have running water and a refrigerator to preserve food would be good enough!


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## HuntingHawk

Make sure to use this wiring chart to reduce power loss which is high with DC amperage.

101 Wire Loss Chart


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## HuntingHawk

You want one of these to be able to properly set the angle of your panels.

Angle Finder w/ Dial Gauge


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## HuntingHawk

My solar system is 520watts & operating for three years now. So about the same size as yours. To get alittle payback I operate a 5cuft chest freezer year round with it.


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## HuntingHawk

Give serious thought to being able to adjust the angle of the panels. I'm at 30 degrees & threw the year my panels need adjusted monthly by 5 degrees & range from 15 to 45. Keep in mind also that there will be times you need to clear the panels so they do need semi accessible.


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## HuntingHawk

I made steps for my panels to make it easy for altering panel angles.


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## HuntingHawk

Glad you got some of it figured out Crackpot.

12 to 120 is a 10X factor. So any time you take 12VDC battery power & run threw an invertor it requires minimum 10X more power then the invertor output needs to be. Due to construction of cheaper invertors that might be as much as 12X.

Now, if you are doing something like setting up a storm shelter it is well worth looking at getting items that operate on 12VDC. Items like a TV will cost more but that will be forgotten when you are drawing 1/10 the power.


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## CrackPot

Thanks HuntingHawk. Yes, I am working to the wire chart you linked as that can make a big drop in current from roof to basement using just #10 (i have 4/0 for that run). I also designed a roof mount that can be adjusted for angle since here it goes from 26º to 79º between winter and summer! I have two angle gauges, digital and mechanical and will adjust at least bi-monthly. I'd like to keep them off the roof, but just too many trees. But, if they become firewood then it won't be a problem! We're in New Jersey now but have eyes on Idaho in a few years if we make it that long. Yes, the solar goes with us!!


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## HuntingHawk

Not mine, but ground pole is a good option versus roof mounted.


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## CrackPot

I wish I had that kind of view! I'm surrounded by 50' poplar and elm. The roof is the only option. But... in Idaho...


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## 8301

CrackPot said:


> I wish I had that kind of view! I'm surrounded by 50' poplar and elm. The roof is the only option. But... in Idaho...


With that kind of view the sunsets are incredible!


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