panel/battery sizing

jtdiesel65

Hi,

I think it's time for new batteries. I have 1000 Ah of Surrette T12-250 @24v. I'm also moving my equipment out of the house into an out building. I seem to get anywhere from 2.5 to 4 days out of a full charge. The batteries were put in service in 1999 and specific gravities seem to be down in the 1.25 range. Some cells have fairly cloudy electrolyte. There are also a couple cells that seem to be bad with specific gravities way down in the 1.1 range. So, I'm not sure really what the capacity of these are anymore. At any rate, I was thinking of replacing these with a bigger bank. I want to use 2.7 KW of PV. What is the biggest battery bank that could be paired with this array?


My PV is actually 3.3 KW with 2 1360w arrays each on an MX60 and a 600w array on a solarboost. The 600w array probably hasn't been much of a contributor as it's shaded heavily from 10am to 1 pm. I haven't moved it due to logistics and I'll probably not use it after I move the other equipment, because the rack, etc may not be worth the expense.

thx

Cariboocoot

Re: panel/battery sizing

So the question is "what is the largest battery bank that can be recharged by 2.7 kW array?"
That depends on a couple of factors. One being how much sun you actually get on your array. You can use the PV Watts program http://rredc.nrel.gov/solar/calculators/PVWATTS/version2/ to get a reasonable idea of that. The other is what rate of charge is best for your batteries.

In very generalized terms:

2700 Watts @ 77% efficiency (typical) = 2079 Watts / 28.4 charging Volts = approx. 73 Amps.
At 5% that would be peak current for 1460 Amp hours of battery. At 10% (better target, in my opinion) it would handle 730 Amp hours. Remember that the "tall" batteries really need the higher charge rate to stir the electrolyte properly.

Under the same circumstances the additional 600 Watt array would add 160 to 320 Amp hours capacity.

Sounds like your existing bank was sized fairly well for the whole array: maximum of about 89 Amps which would be around 8.9% of 1000 Amp hours.

jtdiesel65

Re: panel/battery sizing

Thanks.

Unfortunately the 600 w array was sized to the bank in 1999. I added the other 2.7 KW in a couple phases starting in Dec 2006. I suppose that's why the batteries are in such poor shape today.

I'm in vt. I'd like to get more days out of the bank. I don't see much that can be done in the bad winter months, but I think the larger bank would help ride through longer no-sun stretches in late fall and spring.

The first mx60 array went online Dec 2006 and the second mx array went online June 09. I have no data for the 600w array since the solarboost has no data logging. I don't think it's contributing much due to shading and mismatched voltage set points between the mx-60's and the solarboost.

kwh on mx 60 array(s)
2007 2008 2009 2010
Jan 57.4 67.7 88.4 76.5
Feb 68.1 63.3 87.4 94.4
Mar 75.7 72.4 94.5 120.9
Apr 56 71.6 89.6 131.5
May 84.5 167.8 114.4 151.2
Jun 125.3 145.5 172.3* 205.2
Jul 138 164.9 227.9 321.8
Aug 140.7 132.1 260.2 240.8
Sep 86.2 87 126.8 146.7
Oct 73.1 79.7 103.8 113.7
Nov 59.4 54.9 114.4 107.1
Dec 25.8 47.4 77.2 66.1

float minutes on mx60 array(s)
2007 2008 2009 2010
Jan 1236 493 267 755
Feb 1999 845 3755 ?
Mar 2586 3143 2961 8879
Apr 2346 5551 2613 9018
May 5922 878 2502 12742
Jun 3208 494 5072* 4925
Jul 1299 ? 4139 2493
Aug 1680 2000 2961 4172
Sep 3052 3059 5675 3952
Oct 1867 1175 2106 3397
Nov 1234 119 2171 1352
Dec 98 0 556 393

*=Jun 2009 and later includes sum of both arrays.

Our steady state usage seems to be around 4.5-5.5 kwh/day. I tend to use a dehumidifier and air conditioners in summer when the bank is full. That's why production is much higher. In May, there is no reason to use an air conditioner which is why the large float minutes. As you can see Winter is not so great. In the winter of 2009-2010, I ran the generator about 35 hrs. This year has been completely different.

Cariboocoot

Re: panel/battery sizing

At that usage rate your looking at approximately 250 Amp hours on a 24 Volts system:
250 * 24 = 6 kW hours
With a 1000 Amp hour bank you could get up to two days use with zero charging and 50% maximum DOD. That's not bad.
Using the rule-of-thumb array analysis for production you'd get something like this:
2700 Watts * 4 hours = 10800 Watt hours per day / 2 (usual 50% AC out of array size) = 5400 Watt hours. So far it all looks workable.
Recharging in Winter is bad. The sun just doesn't appear for very long. December looks like your worst month with a two year average off the two arrays of 4.7 kW hours per day. By the time that gets to the AC side of things you're short half the power you need.

Other than running the generator as needed or installing a ridiculously over-sized array I don't see any simple answer.

jtdiesel65

Re: panel/battery sizing

Well, thanks again. One thing I'm a bit baffled at is how the minimum amperage fits in with equalizing. It seems like the two things accomplish the same thing. It would seem to me that as long as you have amps going into the battery and not perpetual loss, equalization would take care of moving the electrolyte around. I must be missing something.

Unfortunately, offsetting winter with wind isn't on option here. I have a small stream near the house, probably too small for hydro. I'm not sure how workable hydro is in freezing weather anyway.

Cariboocoot

Re: panel/battery sizing

jtdiesel65 wrote: »

Well, thanks again. One thing I'm a bit baffled at is how the minimum amperage fits in with equalizing. It seems like the two things accomplish the same thing. It would seem to me that as long as you have amps going into the battery and not perpetual loss, equalization would take care of moving the electrolyte around. I must be missing something.

Absorb and Equalization have similar properties: a fixed charge Voltage with diminishing current. But there is a subtle difference that is important. In Absorb you are "putting back" the Amp hours used and remixing the electrolyte to prevent sulphation and stratification. Equalization is a corrective measure where the Voltage is pushed even higher than normal on a battery that is already fully recharged. The idea here is to reverse any sulphation that has already occurred. Equalization is hard on a battery and should only be done on an "as needed" basis (my opinion - some manufacturers do not agree). Good Absorb charging should reduce the need for frequent or prolonged Equalization.

Unfortunately, offsetting winter with wind isn't on option here. I have a small stream near the house, probably too small for hydro. I'm not sure how workable hydro is in freezing weather anyway.

Wind is not very dependable in most places. Tends to be an expensive investment too. Micro hydro is going to freeze for sure - unless you have some secret underground stream to tap! If you can't "conserve down" to available Winter power levels or add more panels the generator becomes the only practical option.