Battery power frustration

Jeromeds · November 2011

Off Grid Western Montana cabin with occasional use.
Batteries not recovering. I have 130 watt pv with 55’ 6ga wire to tri-star 45 and 80 watt pv to mark 15 then both to four Interstate 210 ah in series for 12 volt system with Sunforce 2500 pure sinewave inverter. Max use is 1000 watts per day with 4-5 hours of variable sun. second day of use I am at 11.8 volts for 13.4 when I panic and shut power off or run generator for two hours. Your forum answers make it appear I need at least one more 130 watt panel. Would increasing battery bank from four batteries to (6) or (8) be a benefit? Would an inverter with built in charger be advise over the automotive type charger?
Jerry

Cariboocoot · November 2011

Re: Battery power frustration

Welcome to the forum.

It's a pretty safe bet that increasing the battery bank is exactly the wrong thing to do.

Let's look at what you've got:
Four Interstate 210 Amp hour batteries. You said "in series for 12 Volts". Unless they are the weirdest batteries on Earth (3 Volts) I think you mean in parallel: (+) to (+) and (-) to (-) or they are 6 Volts and you've got two in series for 12 Volt strings paralleled. So the possibilities here are four 210 Amp hour 12 Volt batteries in parallel, for 840 Amp hours @ 12 Volts total or the two x two configuration which would be 420 Amp hours @ 12 Volts.

Either way one 130 Watt panel plus one 80 Watt panel adds up to 210 Watts which is not enough to charge either configuration. You might see about 13 Amps from those panels, which would strain to recharge 210 Amp hours @ 12 Volts, much less any more.

"Maximum use is 100 Watts per day". I'm going to assume you mean Watt hours here and look at that compared to the panels: 210 Watts * 5 hours equivalent good sun = 1050 - without any derating for panels and conversion. In fact you could only expect the panels to harvest about 500 Watt hours AC daily, and you're using twice that.

In short it's chronic deficit charging coupled with greater use than harvest. It happens a lot. Here's what you do:
1). Recharge the batteries by generator. Let them rest. Test the specific gravity (assuming they are flooded type). Don't be surprised if they are ruined and need to be replaced.
2). Reconfigure battery bank to supply the needed 1 kW hour per day. This would be at minimum 210 Amp hours (50% DOD). Frankly, four golf cart batteries giving you 440 Amp hours @ 12 Volts would be better.
3). Get more panel. You want to try for 10% peak charge current: 21 Amps @ 12 Volts = 252 Watts, less typical efficiency derating (77%) = 327 Watt array.

Depending on what the specs are on your existing panels, it may be time to ditch the small unit and charge controller and put more panel on the Tristar 45. It can certainly handle more.

How are we doing so far? My battery configuration assumptions may be wrong. Please be as accurate as possible because that is the critical center of your system.

icarus · November 2011

Re: Battery power frustration

Pretty simple answer: too much load, too much battery,, not enough charge capacity.

All design and hardware considerations start with the loads. Assuming you use 1kwh (Note kilowatHOUR) per day, you will need a minimum of ~ 500 watts of panel.)

As I have so often written (I should archive it on my machine so I don't have to write it again and again!) The basic, easy math for off grid is this: Take the name plate rating of the panel, divide it in half to account for all cumulative system loses, then multiply that number by 4 to account for the average number of hours of good sun, per day that one might reasonably expect, on average over the course of the year.

So,, 500 watts of panel might look like this: 500/2=250*4=1000 wh/day,, or 1 kwh.

Once again, as an FYI, we live off grid, with 400 watts of PV, into 450 ah of battery (12 vdc) and routinely consume 5-800 wh of power. We generally put in just about what we take out, so you can see the equation, while not engineeringly correct is pretty close.

Welcome to the forum. There are some pretty smart folks here who have forgotten more about PV than most of us will ever know,, and are more than willing to help.

Tony

niel · November 2011

Re: Battery power frustration

i'm sorry to say that this is a classic example of undercharging the batteries and on top of that your loads exceed what you could put into the batteries from solar. both result in low batteries and the only thing saving the battery bank is putting the generator on, but you didn't say if that is to recharge the batteries or run the loads. if it's to charge the batteries, now by how much the generator may have brought them up is in question too for we don't know the current being delivered to the batteries for that 2 hours or so.

first order of business is to be sure you can get a charging source to the batteries to bring them to a full charge and 2nd is to at least double your pv capability.

if any batteries wind up being bought it's because these ones got ruined so acting fast would be a good thing.

icarus · November 2011

Re: Battery power frustration

Additionally, ditch the automotive charger asap, and get a proper 3 or 4 stage charger, like a Xantrex or Iota. Don't buy any bigger battery bank,, go back to to your loads and adjust the charging regime. (And get them properly charged right away to try to save them)

For everyone, these are a must read:

http://www.batteryfaq.org/

http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

Take solace in the fact that you have come to the right place for good un biased advice, you have come early enough. Also realize that most new off gridders destroy one (or at least one!) set of "starter" batteries as they travel the learning curve. That is why we all suggest starting out with cheap golf cart type batteries right out of the gate instead of fancy, expensive batteries.

Tony

hillbilly · November 2011

Re: Battery power frustration

This is all good advice you're getting here. All I have to say is don't feel bad, many of us have been there and done that ourselves. At least you've not lost a lot of cash on a big bank of expensive batteries, so chock this up as part of your education. You may also want to look into some additional resources for learning more too. Solar Energy International http://www.solarenergy.org has some excellent classes, and a very handy book that covers a ton of key details to PV systems.

Keep at it, as others have said:
-tally your loads
-make sure that your battery bank is adequate to cover those with reasonable reserve
-and VERY important... make sure that your PV can handle your loads and charge the batteries.

Most of us off grid also use/need a generator for additional charging, while it's technically possible to do without one the overall cost of the system and the complexities of managing your loads get way over most folks capabilities (or they just end up killing their batteries in short order). In other words don't forget to think about the generator and the generator charger as just a key ingredient in the whole system.

Seven · November 2011

Re: Battery power frustration

Just in case you are confused by the math, or some of the terms, like I was when I first joined the site, what they are telling you is that you don't have enough panel to charge just one of your batteries. You have 4 batteries so that means you have made the problem 4 times as bad.

210w of panel, multiplied by a derating factor of 77% gives you about 162w of usable panel.
If you divide that 162 watts by a typical charging voltage of 14.4 volts it gives you 11.2 amps of charge current.
11.2 amps of current is a good charging amount for a 110ah battery, which is about half the size of one of your current batteries.

Jeromeds · November 2011

Re: Battery power frustration

Thanks for the imput and money saving advise. I put a 2/10/50 amp gen on the bank in 10 amp mode overnight. After resting a few hours the digital meter read about 6.25 volt per battery or 12.50 per all four but the hygrometer only 1.20 or 1.194 after temp adjustment which is in the red. This look like a year and half old dying? Or get them to bubble with a 50 amp shot?

BB. · November 2011

Re: Battery power frustration

Charge the batteries at a ~5% - 13% until you think they are Charged (should be around 12.7 volts resting votlage--after several hours of no charge/discharge current).

Then at ~5% rate of charge (i.e., 100 AH bank, charge at 5 amps)... This will be around 15-15.5 volts (for a 12 volt bank). And check the specific gravity of each cell every 1/2 hour.

When the SG stops rising in all cells between 1/2 hour readings, then the battery is fully charged and equalized.

If the battery is sulfated, then you will not get the specific gravity to "full charge" (whatever may be "right" for that battery).

Batteries will only gas (bubble) when they are nearly fully charged (instead of converting lead/lead sulfates, you are electrolyzing water).

Where your batteries "fizzing" pretty well?

If you have a "small" charger, you may need to only charge/equalize one battery (or battery string) at a time.

-Bill

Cariboocoot · November 2011

Re: Battery power frustration

One of the problems with the automotive chargers: won't do a proper charge sequence for a deep cycle battery. Until you can bring them up to Absorb Voltage and keep them there for a couple of hours, they aren't really being charged. As for equalization ... no chance without the proper charger.

Even so, SG of 1.190 to 1.200 does not sound hopeful.

icarus · November 2011

Re: Battery power frustration

If you have been treating these batteries as you suggest for a year and a half,, my guess is that they are near death. You may be able to nurse a few KWH out of them, but their days are numbered.

A classic sign of sulfated batteries is they charge quickly, but then they discharge quickly as well. They may not be "dead", but they are never going to be healthy either.

I would be curious to know who did the installation and or who sold you the system and what (if any advice) they gave you. It sounds like they may not have had a very good handle on the nuance of off grid batteries. I suggest that perhaps you have a conversation with the vendor, as it would appear that you have a system that is not very well designed for the load and the charge regimen.

Tony

Jeromeds · November 2011

Re: Battery power frustration

I am looking at IOTA 30, 45 and 55 amp chargers but don't know how to determine the SIZE to get. Please advise.
Thx

Seven · November 2011

Re: Battery power frustration

Find one that puts out around 10% of the amp hour of your battery bank. That is the rough rule of thumb.

Edit
After terracing the thread it seems like you have 4, six volt batteries, series paralleled for 420 ah bank. I would go with a 45a charger. If I misread something about your bank please clarify.

Battery power frustration

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