Battery Bank too big for 900W?

Feenix

Hi All!

I am fairly new to solar, and have a small system set up at my house that was basically there when I bought the place. I have 900W of PV, and eight GC2 batteries that barely pass the SG test after I equalize them.

I was hoping to purchase a new battery bank, and have my eye on the Rolls/Surrette S-530s. I originally wanted to get 12 of these batteries in three strings but I have a few questions.

-Some people suggest not having more than two strings.
-I live in Southern Arizon and get a ton of sun. I also only use around 1,000-1,500 watts per day. Would I find that I would have trouble keeping the battery bank charged with 900W of PV?

Thanks so much!

-Kristen

ggunn

Re: Battery Bank too big for 900W?

Feenix wrote: »

I also only use around 1,000-1,500 watts per day.

Watts is a measure of power consumption rate, it can't show an energy consumption amount. Do you mean that you use 1-1.5 kilowatt-hours per day?

Feenix

Re: Battery Bank too big for 900W?

That's exactly what I mean.

icarus

Re: Battery Bank too big for 900W?

Don't know what voltge you bank is wired to , but at 12 vdc, those 4 Gc batteries are around 900 ah at 12 vdc or 450 ah at 24 vdc. The 900 watts of PV is going to struggle to put ut more than about 55 Amps into 12 volts, runs 27 into 24. Ideally those batteries would me to see ~ 10% of ah capacity for charge current, or about 90 amp into 12, 45 into 24.

Since you only use 1-1.5 kwh/day, I think the PV size is about right, but the battery bank is too big. Given a choice, if you need to replace the batteries. (probably because of chronic undercharging!) I would build a battery bank of ~ 450 ah, (4 Gc batteries in series parallel for 12 vdc or 4 seriesed for 24.

Just as a comparison, we live off grid, use 5-800 WH/day. We have 450 ah of battery (4 T-105s) into 12 vdc, charged with 400 watts of PV. We use between 10-20% of battery capacity and easily put that back in on a good day, even though we re somewhat under paneled.

The bottom line is, people kill battery banks most often because the charge regimen is too small, so either increase your PV or decrease your battery. Increasing your battery is a recipe for failure IMHO.

Good luck and keep in touch,

Tony

Feenix

Re: Battery Bank too big for 900W?

Hi!

Thanks for the response. My system is 24v.

Right now my bank is definitely not undercharging (I have the eight GC, 6v, two strings of 220ah, total 440 ah). The reason why I was thinking about increasing is because in the winter I noticed I did not have enough charge, due to a combination of cold temperatures and some cloudy days? My bank was also reaching fully charged in the dead of winter at around 11AM, which means I am "wasting" sunlight.

Perhaps my cheap-o Costco GC batteries are bunk. But thats why I'm reluctant to go smaller.

Thanks!

Kristen

NorthGuy

Re: Battery Bank too big for 900W?

Feenix wrote: »

My bank was also reaching fully charged in the dead of winter at around 11AM, which means I am "wasting" sunlight.

If your batteries were full a day before, then to make them full again, you need to put back the exact amount of energy that you have taken (multiplied by some efficiency coefficient). This doesn't depend on the size of your battery bank, only on the amount of energy you used during the night. If you want to put more into your batteries, you need to make more room where this extra energy is going to go. And the only way to do it is to use more during the night.

Thar's a good problem to have ...

vtmaps

Re: Battery Bank too big for 900W?

If I understand, you have 440 ah at 24 volts and you want to increase capacity to 1200 ah at 24 volt? (3 strings of surrette 530s)

Feenix wrote: »

The reason why I was thinking about increasing is because in the winter I noticed I did not have enough charge, due to a combination of cold temperatures and some cloudy days?

So your array can't adequately charge 440 ah and you think that it will be able to charge 1200 ah? No way! Perhaps I am misunderstanding the situation?

Feenix wrote: »

My bank was also reaching fully charged in the dead of winter at around 11AM, which means I am "wasting" sunlight.

If its always sunny you can reduce your ratio of panel to battery. For most of us who have short and/or overcast days in the winter, we want our ratio of panel to battery to be larger. How much larger? Depends on how much you want to run your generator. If you increase your ratio of array to battery high enough you can charge your batteries on cloudy days and avoid generator use.

With panels being inexpensive and batteries being very expensive, it seems to me that you should increase your array, not your batteries. If you don't want to increase your array be prepared to run your generator to keep your batteries charged.

Maybe I'm missing something, but I don't see how increasing your investment in batteries is a good thing.

--vtMaps

Feenix

Re: Battery Bank too big for 900W?

Hi!

The problem was with the charging of the batteries themselves, they were completed by 11AM. The problem was getting through the night without going below 50% SOC. Which, to me, seems like a storage problem and not a charge problem.

I called Wind and Sun, which is where I will be purchasing my batteries, and the sales guy did the calculation which agrees with what everyone is saying, that 400AH should be enough. The batteries I have now are mathematically rated 440AH, but that doesn't mean thats what they are actually giving me. The SG readings I've been taking are less than inspiring, so that must account for the poor storage in the winter.

In Southern AZ I get a MINIMUM of 4.5 hours of solar radiation per day (December 22nd), which means with my PV I'm pulling approx. 4kWh with my panels and only using ~1-1.5kWh/day.

The salesman's suggestion was to wait until winter, buy four new batteries and see how they work. Add four more within a month if this doesn't seem to work for me.

BTW, the salesman's name was Drew and he was really great. And thanks to you all for your help, too! If you do have any more comments, I would be happy for the input!

NorthGuy

Re: Battery Bank too big for 900W?

Feenix wrote: »

The problem was getting through the night without going below 50% SOC.

With 440AH array, your daily 1.5kWh loads are only 15% of the bank, so they shouldn't get discharged below 10% DOD. If they go below 50% SOC, they're probably severely damages. Most likely sulphation or other cause decreased their capacity to very low level.

With your loads and your sun, you can get away with only 220AH of healthy batteries.

Photowhit

Re: Battery Bank too big for 900W?

Feenix,

I think you have several misconceptions about solar and battery storage. Panel rating is optimistic, the NOCT(normal operating cell temperature) values will likely be 75-80% of panel rating, the trough put stored and used as AC will be roughly 50% of your panel rating times the hours of solar isolation, so a 900 watt array should be sized about right for a 1.5Kwh demand, you want to have some waste and reach full charge several times a week. There are some options to use some of this extra to aid in water heating or other loads, it's always nice to do laundry while the sun is shining...(load shifting)

You say you equalized, did you do a controlled equalize and watch until the SG quit rising? or a timed auto equalizing from your charge controller?

How old are you batteries? the not holding a charge over night is likely just reduced capacity as the batteries age, but this same reduction of capacity can be the reason your batteries are fully charged by 11am.

1-1.5Kwh isn't much electric, just a fridge in a warm climate with out A/C could easily draw this much by it's self, how are you measuring your usage? Do you have a shunt based battery monitor?

icarus

Re: Battery Bank too big for 900W?

If you at drawing your batteries more than 50% in one night,my guess is that you are underestimating you total daily useage. How are you arriving at the 1-1.5 kwh/day?

900 ah of battery, at 12 volts will deliver a out 5 kwh before getting to 50%. Something doesn't add up.

Please clarify.

Tony

Vic

Re: Battery Bank too big for 900W?

Hi Feenix,

But, I do wonder about the existing batteries and how they are charged;

What Charge Controller is used, and what is the Absorption voltage and duration? What brand are the batts? When you EQ the batts, what voltage do you use? Does the CC use a Remote Temperature Sensor on the batteries?

What SG readings are you getting when the batteries seem to be fully charged? Roughly, on average, how much water do these batteries consume and how often are they watered?

It is nice to see a sales person asking a customer to delay a purchase.

Am just thinking that you might want to make certain that the existing charging system, and settings are ready for new batts when the time comes. Vic

BB.

Re: Battery Bank too big for 900W?

Have you monitored the electrolyte specific gravity for your cells/battery bank? That will give you a better idea of the state of charge of your battery bank.

-Bill

Rybren

Re: Battery Bank too big for 900W?

If I understand your setup correctly, you have a 440AH 24V battery bank. Most folks around here recommend a bare minimum of 5% charge rate, with 10% being more or less ideal.

So, to achieve this 10% charge rate, you would need:

440AH*29 volts charging*1/0.77 panel+controller derating*0.10 rate of charge = 1657W panel


You have


440AH*29 volts charging*1/0.77 panel+controller derating*0.055 rate of charge = 900W panel

or roughly the bare minimum required to charge your bank. Going to a larger battery bank will only make things worse.

What Charge Controller do you have and what are the specs of your panels? Also what are your charging setpoints and how long are you holding in Absorb?

p.s. I don't know much, but I did stay at a Holiday Inn last night

westbranch

Re: Battery Bank too big for 900W?

lucky you... H.I. and all

mike95490

Re: Battery Bank too big for 900W?

Seems like your current battery bank has sulfated to the point it won't last the night, and the low capacity charges right up the next day.

BEFORE you buy new batteries, make sure you understand your loads and charging, so that you don't destroy your new batteries in a year.

Use your generator to charge the batteries first thing in the AM, and then let the PV do the equalize. Otherwise, you seldom have enough solar hours to recharge & EQ in one day.

icarus

Re: Battery Bank too big for 900W?

Just as an FYI, if you haven't read these, I urge you to do so:

http://www.batteryfaq.org/


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

Tony