Question on battery sizing - part 2

System · September 2007

This is a new thread without the talk about peak oil, the end of the world, etc. Yea, I know I started it

I'm still undecided on straight GT or GT with a battery bank. However, this is very interesting and I'm beginning to understand a few things so for the sake of this thread:

1) I want a Grid Tied system with a battery Backup and I'm leaning in the direction of the Xantrex XW system.

2) I'm installing a 10kw PV system. The panels are not purchased so I'm not locked into the SPR-210s.

3) With power conservation we could get by on 10kwh/day but our current usage is about 25kwh/day. The system would need to sustain us at these levels indefinitely without backup from the grid (if necessary). Of course, we would adjust our usage based on conditions; weeks of summer sunshine gives us more power, winter storm season we go into a more conservative mode.

According to Solar Guppy, we would need at least 800ah worth of batteries if we went with a pair of Xantrex XW6048 inverters. This would give us, ~38kwh of backup. If I calculate a 10% loss and 50% DOD, I would have about ~17kwh of usable backup, which would last about 1 1/2 days if we were conservative.

Does this sound about right?

I would need 4 charge controllers. What is the max number of charge controllers the system could employ? I may want to add batteries later.

Also, the PV panels would need to be wired in some way (that I do not understand) to keep the input voltage to the charge controllers below 150V. Is this correct?

I'm also undecided on AGM vs. FLA batteries. One is less expensive but requires maintenance, an enclosure and venting to the outside. The other is more expensive but easier to install and maintain but might not last as long. My batteries would be installed in the basement.

BB. · September 2007

Re: Question on battery sizing - part 2

You are talking about a very large system (battery wise) if you really want to go off-grid for long(er) periods of time. Does you basement have access for crane delivery of larger type batteries? Or, should you be looking at building a battery shed with better access and to just keep them outside of your home (ventilation, fire, chemicals)?

I am not the battery expert here--but I would tend towards the larger cells/batteries to reduce wiring costs and maintenance issues... And generally, those larger batteries will be flooded cell types...

-Bill

rplarry · September 2007

Re: Question on battery sizing - part 2

I have had both types of batteries, agm and fla. My fla batts were Trojan T-105 and needed watering, cleaning, and equalizing. I only had 14 of them, but that means 42 cells to tend to, plus corroded interconnect cables to clean and sometimes replace because the corrosion had eaten them up. Then trying to equalize them with enough charger power and maintain enough power to the batteries to keep them at equlize voltage while also supplying power to other loads. Enough, I switched to agm batteries, now I only have 6-2v batts, they each weigh 267#, but in 3 years I have not touched them, sometimes I go out and watch them charge (when I am really bored), but mostly I just leave them alone. The agm are also quite a bit more efficient than the fla batts, mine seem to be about 96%. I have never regretted switching to agms'. Your mileage may vary a bit, who knows?
Larry

System2 · September 2007

Re: Question on battery sizing - part 2

rplarry wrote: »

sometimes I go out and watch them charge (when I am really bored), but mostly I just leave them alone.

That's funny

I'm going to watch the socks in my sock-drawer after work.

System2 · September 2007

Re: Question on battery sizing - part 2

BB. wrote: »

You are talking about a very large system (battery wise) if you really want to go off-grid for long(er) periods of time. Does you basement have access for crane delivery of larger type batteries? Or, should you be looking at building a battery shed with better access and to just keep them outside of your home (ventilation, fire, chemicals)?

I am not the battery expert here--but I would tend towards the larger cells/batteries to reduce wiring costs and maintenance issues... And generally, those larger batteries will be flooded cell types...

-Bill

Nope, cant access the basement with a crane. Not withough major damage to the rest of the house

I don't really know what makes a good battery. However, I did find the some AGM 12v 250ah batteries for about $412 each. It seem like I would need 8 of these wired in 4series-2parrallel to get 500ah @ 48v. Double this (total of 16) for the second inverter and I'm looking at about $6500.

Any suggestions as far as AGMs

Here is the link for the batteries noted above:
http://www.batterystuff.com/batteries/pv-solar/agm/UB8Dagm-45964.html

System2 · September 2007

Re: Question on battery sizing - part 2

Does anyone know when the Xantex XW stuff will be available for purchase?

mike95490 · September 2007

Re: Question on battery sizing - part 2

Maybe if you look at the cycles you expect to put the batteries through, that may help your decision:
AGM : _____ cycles
FLA : ______ cycles

Somewhere I thought I heard that AGM have shorter lifetimes when cycled. If you plan on discharging every night, that's a cycle to the battery. If you plan on grid tie, and only cycling the batteries when the grid is sick. AGM would be the winner hands down, 3 cycles a year, they should last well, and with zero maintenance.
Maybe you want ot only have lifeline loads on the batteries - HVAC, water pump, sump pump, a few light circuits, outlet for TV, and fridge, and use the PV power for everything else, in the afternoon after the batteries have bulked up.
Of course, the end of the world scenario will be even more different.

System2 · September 2007

Re: Question on battery sizing - part 2

mike90045 wrote: »

Maybe if you look at the cycles you expect to put the batteries through, that may help your decision:
AGM : _____ cycles
FLA : ______ cycles

Somewhere I thought I heard that AGM have shorter lifetimes when cycled. If you plan on discharging every night, that's a cycle to the battery. If you plan on grid tie, and only cycling the batteries when the grid is sick. AGM would be the winner hands down, 3 cycles a year, they should last well, and with zero maintenance.
Maybe you want ot only have lifeline loads on the batteries - HVAC, water pump, sump pump, a few light circuits, outlet for TV, and fridge, and use the PV power for everything else, in the afternoon after the batteries have bulked up.
Of course, the end of the world scenario will be even more different.

Well, I'm told that with a 10kw GT w/BB system and XW inverters, the smallest battery bank I should have is 800ah at 48v. If I'm going to go that big, I might as well have the whole house tied in. Then I could determine during the day what is criticle and needs power.

niel · September 2007

Re: Question on battery sizing - part 2

chuck,
whatever your reasons for doing solar it doesn't matter. maybe some are going overboard with an end of the world scenario, but in any case there have been grid problems in the past that were longterm. i site a few years ago when the niagra power system failed and they blamed it on an ohio power company saying something to the effect they had failed to cut tree overgrouth as the reason for it. (i didn't buy that explanation) hey, it could've just as well been a terror attack and we are not immune from one at any time no matter who is in charge of the country. the bottom line is you want a backup like so many others that are here. do keep in mind that of that 800ah of battery that you should not go below 50% so only 400ah would be available for use. to use more would lower life expectancies of the batteries, but in an emergency you could go a bit further with them in my opinion to a max of 75% depth of discharge or so(no more than once or twice in the batteries' lifetime should this occur to be clear) unless lives are on the line and there's no choice but to exhaust the batteries to death.

System2 · September 2007

Re: Question on battery sizing - part 2

Interesting article at the Sun Xtender website:
http://www.sunxtender.com/otherpdf/sunextenderbatterysizingtips.pdf

Many battery manufacturers will advise sizing the battery for cyclic applications to a maximum
depth of discharge of 50%. That would mean doubling the size of the battery. Some batteries
have trouble recovering from deep discharges. That would mean for the 60 AH/Day load with
5 days of autonomy or 300 AH that they would advise using a 600 AH battery.
The Concorde Sun~Xtender® does not have this limitation. We consider that the battery is
replaceable when it does not provide 80% of its original capacity. Therefore, we recommend
that the same 300 AH requirement be divided by 0.80 to provide a reliable battery system. We
would recommend using a 375 AH battery. That represents a significant savings.

niel · September 2007

Re: Question on battery sizing - part 2

as a general rule, what i said holds true. you found an exception as the sunxtenders can take a deeper discharge and i saw that the max cycle life was obtained between 60 and 70% dod, but closer to 60% dod and can take discharges at higher rates. these batteries are rated to be charged on their full ah rating per 20hr rate as told to me by the company too so if you have say the pvx 1040t it is rated at 104ah and can be fed a charge at that rate, though i would not do it personnally. yes, i have one of them and they really are nice well made batteries, but you pay a premium for it too.

System2 · September 2007

Re: Question on battery sizing - part 2

According to Solar Guppy, we would need at least 800ah worth of batteries if we went with a pair of Xantrex XW6048 inverters. This would give us, ~38kwh of backup. If I calculate a 10% loss and 50% DOD, I would have about ~17kwh of usable backup, which would last about 1 1/2 days if we were conservative.

Does this sound about right?

Your loss estimate is probably pretty close but may be a couple of points low, consider battery discharge efficiency, voltage drop and inverter efficiency to get actual loss. If 11kW/h is your daily requirement in "conservation" mode, then 1.5 days autonomy is not realistic for CT if you expect to go without the grid during prolonged cloudy conditions

I would need 4 charge controllers. What is the max number of charge controllers the system could employ? I may want to add batteries later.

The number of controllers required is not related to battery capacity, it's related to array capacity. Adding batteries is not recommended after about 75 cycles, that is why I recommended the small starter battery to give you some experience with batteries and set a reasonable level of expectation from them without spending a small fortune on the wrong capacity battery. The NAWS "Deep Cycle Battery FAQ for Solar Electric Systems" here http://www.solar-electric.com/deep_cycle_batteries/deep_cycle_battery_faq.htm has a wealth of information to help you decide which type of battery you select.

Also, the PV panels would need to be wired in some way (that I do not understand) to keep the input voltage to the charge controllers below 150V. Is this correct?

Yes, that is correct. To keep your 10kW/h array under 150 Voc, your array would need to be wired in a series-parallel configuration. When people speak of x number of strings of x number of panels, they are refering to series-parallel wiring. For instance, lets say your panels are 40 Voc. Without getting into temperature coefficients and NEC requirements yet, each of your strings could have up to three panels in series (3 series wired panels X 40 Voc = 120 Voc) to stay below the 150 Voc as specified. Those strings would then be connected in parallel. I don't know if you understand the difference so I'll just describe it as best as I can, Barney Google can help too. The series wiring is (+) to (-) to (+) to (-) to (+) to (-) for the three panels in the first string. Wire each subsequent set of three panels in this manner. Then connect the free (+) wires at one end of each string together and the free (-) wires at the other end of the strings together. These are then run to their respective connections in the charge controller. If, for example, each panel puts out 5 amps, one string would put out 120 volts @ 5 amps (amps remains the same in series connections). This string would be rated at 600 watts. Each additional string of panels you connect in parallel, as described above, would add 5 amps @ 120 volts. Amperage is additive in parallel wiring, voltage is additive in series wiring. Battery "banks" can be wired this way as well. Draw a few strings out on paper and you will get a good understanding of it. Mull that over for a bit and let us know what you come up with.

Cheers,

Bad Apple

Question on battery sizing - part 2

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