Battery Bank too big for 900W?

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?

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

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 ...

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?

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

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

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.