Rule of thumb re battery bank to pannel array size

[EDh]

Hello

I sometimes wonder if a solar panel array should be matched to the size of the battery bank.  Are more panels better?  so that it charges the bank quicker?  Or should solar panel array watts & battery amps be matched?

Thanks for any info

Regards

Edh

[ghurd]

Good question.  I'm sure there will be a lot of opinions.  This one is mine.

Only so many amps can be pushed into a battery before the voltage comes up artificially due to surface charge.

Too much battery (without enough PV) is a waste, because they rarely get a good charge.  And self discharge makes it worse.

Extreme example: A 1W PV and a 220AH battery bank.  The PV can't keep up, but every drop of solar power is going into the battery.

Too much PV (without enough battery) is a waste, because the battery surface charge comes up, the controller begins to regulate the amps, and much of the solar potential is not being used.

Extreme example: A 220W PV and a 1AH battery.  The battery is always at a high voltage when the sun is shining, so the controller has the PV near completely shut off.

I believe a good match is with solar amps equal to battery AH / 20 or 25.

A lot will depend on how the power is used, the solar conditions of the area, if the PVs have a tracking system, etc.

The 1350AH 12V bank with 42A of solar is pretty close.  

In the morning, if the batteries are usually staying above 70% , almost always above 50%, and in the evening if they are often up to 100% for 2 consecutive days, then all is good.

"Usually, almost and always".  If I get too specific, let me know.  LOL.

Are the batteries chronically low or chronically high?  Have the 3 x 130W been added?

G-

[EDh]

Batteries are usually good providing I have full sun each day.  When I go 2 full days & no wind to even turn my wind mills I have to fire up my Generator to top up before evening.  If I don't I then drag down to about 65% of battery.  

Re the 3 130's, I am rethinking the solar setup.  After Flux suggested a MX60 I purchased one.  Seems like Outback & some other forums don't recommend using mono crystal (my 5 GE Panels) & poly crystal (my 3 130Kyocers's) together.  I was going to series the KC 130 & the 110 GE to make 72 volts.  I would then have 2 GE 100's left over which I thought I would then buy 3 more mono crystal panels & a 2nd MX60 & then the 2 systems would parallel at the battery.  So hence I thought maybe I was getting carried away & would burn up my batteries.  I am probably going to take my wind gens out of the system & supplement my battery power to provide more days without runing the gas gen.  I found a fellow across the peninsula here that has GE solar panels 110 W for sale, but I see some of the solar dealers say the GE 110 is discontinued.  So I may just buy 3 Kyocera add them to my existing 3 KC 130's & have a separate 6 KC 130 for 780 watts which is withing the MX60 specs for 12 volts.  I think Sunwise maybe the only other company that has a similar panel to my mono GE's.

Since we live on an island & have to drag the 5 gal can in & out it gets to be a smelly process running the generator.

Thanks for your interest in my endeavor.  I will probably pour barrels of money into this project before I am done.  I just don't want to cause injury to me or the equipment.

Regards

Ed

[seanchan00]

I thought the amount of power needed daily should be considered too. I always assume that deep cycle battery has more cycles in it's life if discharged say 5% before recharged compared to being discharged 20% before recharge. So If I needed average of 10 amps hour per day, 2 amps of solar power for 5 hrs will top it back up and a 70 ah battery will suffice but a 220 ah battery will definitely last longer years plus some insurance against days of poor sunlight. That's also why most people off grid will use both solar and wind power so one will be putting juice in when the other is not.

SeanChan.

[ghurd]

Wow.  So many options.

With a decent controller the batteries will not burn up.

Mono and poly should be fine.  The issue is more amps / amps, instead of mono / poly.

I really don't like the idea of seriesing 6.6A with 7.4A PVs.  It would work.  The 6.6A PVs would drag down the 7.4A PVs.

Another option could be maxing out the MX-60 MPPT for voltage, amps and efficiency,

Then using the extra PV with a 12V controller.  

Gosh, maybe even the stray PV or 2 with NO controller?  A battery that size should handle a couple amps without a burp.

"some of the solar dealers say the GE 110 is discontinued"

You want 50K pieces of 110W PVs with your name on them, I CAN arrange it.

GE is an import company.  I obviously do not believe the TV commercials.

I believe if GE was actually controlling the factories that were making PVs for them (in China BTW), then they could decide if they wanted 36, 40 or 42 cells per PV.

GE seems to sell whatever decent PV they can get in quantity.  Nice PVs either way.

Sunwize is also an import company.  Just conjecture, but a strange coincidence the factory can't keep up with GE, but can keep up with them?

Confused.  

"I am probably going to take my wind gens out of the system & supplement my battery power to provide more days without running the gas gen."  

Uh, What?  

Wind is cheap and usually works when solar isn't at its best.

There are ways to control windmills without interfering with the solar charger's algorithms (very much).  Do you know which end of a soldering iron gets hot?

Gas is still going up.

G-

[ghurd]

Very Excellent!  Very pertinent!

I hear that argument often.

But if drained only 5%, will the battery last 4X longer than if drained 20%?  

The batteries must be 4X larger, or the charging system must be OVER 4X times larger. Too many variables.

The other side of the same argument says batteries will supply more amp-hours in their lifetime if the batteries are drained down to only 5~10% charge.  It would cost less money for batteries, per watt hour extracted.

I do expect this would be correct IF the battery could be Properly recharged in a quick time period and every time it became that low.

However, no one knows when the sun will shine or the wind will blow.  I expect in this situation sulfation would ruin the batteries in short order.

You are correct.  The amount of power needed daily was considered when I asked about the battery state of charge in the morning.

If the battery is usually 95% SOC in the morning, the owner is not using enough loads or spent too much money on the system.

With your examples, a 70AH battery losses 0.7AH per day to self discharge, and a 220AH battery losses 2.2AH per day.  This difference is only 1.5AH per day, or 5W of PV.

The real situation, and big difference, is in the rate the battery can absorb the charge from a PV.  The surface charge and regulation becomes a major factor.

If you can obtain a 7AH battery, a 250ma PV, a PB137ACV (or a regulated VW PV), then on a sunny day drain the battery to 12.0V. and measure how long it takes the battery to reach regulation voltage of 13.75V.  It does not take a long time!

While the battery is above 13.7V, but below 13.8V, the PV does nothing to increase the charge.  The situation occurs at a very high percentage of the time about solar noon, when a majority of the AH per day is available.  So if the battery is too small, the PV amps are wasted because the controller stops them from boiling the battery to death.

Simply because the PV can supply 2A for 5 hours does not mean the controller will let it.

A wonderful example of too much solar for too few batteries is the man in Mexico with home-brew PVs on his roof.  I can not find the postings. (Mexico City? "sandovalch"?)

His batteries were dead every morning, even though his PVs could make enough AH every day, because his batteries could not hold the AH charge required daily.

The battery needs to be big enough, but not too big.

G-

[scottsAI]

Hello Edh,

Batteries once bought will die. How you get there is up to you.

Books are written to answer your question. Lots of different believes about what is best.

I go for the balanced approach. The battery will die someday, the battery's purpose is to store energy.

Over sizing a battery today is NO guarantee it will last forever.

Only takes one bad error to cause damage, like go on vacation, controller dies, batteries die.

Once you may recover, twice... you have to go with how lucky you are. Me... NOT lucky.

SOC state of charge. This can be a very misleading term.

A charge controller will stop charging the battery once a voltage is hit.

SOC is assumed to be 100%... Not true, until a finishing charge is done it's not 100%

Most likely mid 90's, so lets say 95%.

The finishing charge will take 5 times or more energy to charge than from 80-85%.

Charging 80-85% is the most efficient area on the battery, very close to 1ah in/out.

A battery operated at 90% to 95% is an inefficient use of the battery.

The energy to charge the battery from 90 to 95% is 2-3 times more than 80-85%.

Battery discharged to 50% would not cause me any sleep. 65% I would not start the generator unless over night I would expect it to go much below 50%. I would not spend more than a day below 50%.

The deep cycle battery is rated to go to 80% discharge. I would try not to go there, but would not hurt myself trying to prevent it. If it happened more than once or twice I would look at what your doing...

Once a quarter you must do a finishing charge. It's one of the good things to do, but do not do it every day, it's bad thing too.

Sulfating = sum of the time * %discharged.

Finishing undoes some of the sulfating, not all, becomes an accumulative effect.

Under charging, over charging both cause damage.

A battery is made to be used. Be nice.

If your running your generator more than once a week you should look at your power source. Solar may not be the way to go. Got any geothermal, hydro, wave, tide?

Someone that operates at 10% discharge, will have a battery 5 times bigger than mine. I will buy a cheap battery and have it last 1/4 the time the other does. While that is going on, my money is invested growing, batteries are getting better, then I will buy another cheap battery and smile.

Have fun,

Scott.

[Flux]

You can't win with batteries, they are dyeing from the day you get them.

Put them on float charge, never use them and they will die mainly from plate corrosion.

Work them hard to greater than 50% discharge each day and they die by shedding plate material ( wear out). Between these extremes the life depends largely on the number of cycles and the depth of discharge.

There are suggestions in some places that working at less than 10% discharge is not good for long term life. Similarly there is indisputable evidence that working with low discharge cycles gives a dreadfully low charge efficiency.

In the few cases where the charge and discharge cycles are regular and controlled then you are more or less  able to know where you are going and the algorithms for charging work .

With renewable power there are too many variables to be able to take into account so it becomes much of a compromise.

I tend to agree with Ghurd, small battery banks will have low charge efficiency but they achieve near the float life of cells on standby ( not as long as you may like or expect depending on the type of cell selected).

Too large a battery bank and you will not get it fully charged at regular intervals and unless you use the generator regularly to equalise then you will sulphate them.

You have to decide if you prefer to use the generator to equalise a large bank or to supply part of the load occasionally with a small one.

I tend to agree with Scott that batteries are there to be used and perhaps aim for 20% discharge but don't panic at the occasional 60%. If you can't get them up quickly after a big discharge then run the generator. With both wind and sun together there will almost certainly be times when you will need back up ( depends on your local conditions).

The other factor is not all batteries are not created equal. Wet cells have a relatively high rate of self discharge so large banks are difficult to keep charged.

VRLA are available in two grades those meant to be used ( expensive) and those designed to float and never be used ( cheaper). with luck these may only see 2 significant discharge cycles in their entire life. Probably changed at 5 years whether they have cycled or not. Those type cells will not be much use if used for RE purposes and would need to be small capacity in relation to the charging available and the load so they mostly float.

Flux

[EDh]

Here is a bit more info on my system usage.  Daily I seem to use 275 to 300 amp hours.        This info comes from my E meter etc.  

I have 3 wind generators 12v, which produce various amps on my amp meter vs my e meter.  Lets suppose I keep them for the free power.  Sometimes the winds do get extreme & then they kick out 50 to 60 amps, however the diversion load controller kicks in.  

I have a couple of C60 Xantrex Charge controllers in my inventory along with the DLC thats in the windmill system.

Question

If I buy 1 more mono panel (GE Or Sunwise) & 3 more KC 130's  I would have 6 KC 130's & 6 GE's, can I series the 6GE's @ 72 volts  then series the 6 KC130's for 72 volts then parallel these 2 arrays at the MX60 I just bought or should I purchase 1 more MX60 & run each array thru their own MX60?

That fellow that has the GE's has enough to supply me with 4 GE's if I need them.

Any opinions on the GE vs the KC's.

Re: the soldering iron, I am pretty sure its the pointy end that gets hot.

Thanks for your patience on my situation.  I forgot to mention that I was not wanting to add any more batteries to the system,  these guys are expensive.

Thanks again

Edh

[ghurd]

I'm really not qualified to say much about the MX60.  Thinking out loud.

The only problem I can see is the KC PVs have Vmp at 17.6, while the GE PVs have it at 16.7V.  The strings of 6 of each would be off by 5.4V or about 5%.  I don't know how that would affect the MX60.

Not sure how the dump load would mess with the MX60 doing what it does, but I think it could confuse it.

Might be possible to modify a simple charge controller to short the windmills, before a diode, so the windmills would stop making power instead of dumping it.  Probably depends on the windmills.  They would only do any charging when the batteries were low.

I think that would let the MX60 keep control of the situation.

KC vs. GE.  Both should be good panels.

[wpowokal]

Here is a link to an artical that may help....

http://www.sandia.gov/pv/docs/PDF/fieldinvest.pdf

allan down under

[coldspot]

Hello

Great info by everybody !!!!!!!!



"You can't win with batteries, they are dyeing from the day you get them."

I thought that was only for "Acid type", ?

The very reason that they are made and sold,

re-pete sales.

I'm still not up to actually playing with my

"Edison Cells" yet, but someday soon this spring

I'll get them all together,(two sets of 25 each) and hooked up as the dump load from the "Acid type" battery bank.

Still wondering about a system to use for this???

One controller for first bank to dump to second bank,

but then what to use for this bank??

Anyway

Thanks