Help with number of batteries

CVN-71 · September 2009

Hi folks, I'm in process of designing a system and would like some advice on what type of AGM battery bank I should use, matched to the final number of panels I will have. I'm new to solar, so please bear with me if my terminology is off and for this long winded thread but i know the experts will have a lot of questions if I didn't.

First let me give a little specifics. Recently refinanced my house and took out a little equity to purchase a small off-grid system. I may want to expand later to bigger and better things but now budget is driving factor. Because of the rebates/incentives I wanted to go solar based, rather than inverter/battery only. My main plan was to use system for power outages, something we get a lot around here, summer and winter. I have been using a generator, but what a PITA, noise, filling it with gas every few hrs, etc.

My emergency load will be a few CFL lights, a few hrs a night, a small TV a few hrs, coffee pot in the morning, maybe running the fridge a couple of hrs at a time per day just to keep food from spoiling, or oil furnace fan to keep the house from getting too cold till grid comes back. I'm currently using a Kill-a-watt to determine actual load, but it's difficult because a summer outage is way different than a winter outage in terms of what is needed. I'm sure the winter load is what I'll use. I may also use 20% of battery capacity daily to reduce current electric bill as well. I'm trying to get the system in and working then add more capacity as needed and budget allows. The big factor is batteries. From what I gathered, it's not wise to add new batteries to an old bank, so I want to get the bank sized right to start. I want AGM because bank will be in the garage and possible sparks from car ignitions, gas vapors tools etc may be encountered. And it could get below freezing at times.

This is what I have planned so far for system:

- 4, 205W Evergreens wired in 2 series at 24v (future expansion to total of 8 205w panels at 1640w)
- Outback Flexmax 80 (25 ft from panels)
- Magnum 120/240 pure sign (4 ft from batteries)
- South facing roof with unobstructed sun from 10am to 3pm (after two more trees are cut down) :-)
- Still have generator for cloudy conditions, and Magnum charger connection to shore power.

So, what's missing is designing battery bank for current system that will also work for expanded system. I was thinking 8-Concorde 6V 220ah batteries. How far off am I and if so can some of you experts recommend a tweaking.

Thanks!
Bob

Cariboocoot · September 2009

Re: Help with number of batteries

The first question you're going to get asked is: "Why are you doing this?"
If it's to save money on electric, forget it; solar power is in the $1-2/kW hour range and your electric rates are no doubt much lower than that. In that respect, look into conservation first. Grid-tie solar would be second choice, depending on the incentives in your area.

If you're planning this for emergency back-up power, you should think about how much you'll really need to use it. Often times a small, fuel efficient generator makes more economic sense for the occasional power outage.

But if the outages are frequent, and/or of long duration it may be practical. Likewise if there is no grid power available.

That said, the next issue is planning a battery bank for expansion. You don't. Adding new batteries to old is a no-no, as the new ones will be 'dragged down' by the old and you won't get as much life out of them. As for system design:

Determine potential loads. Your maximum cumulative load (everything that will be on at once) will give you the info for sizing the inverter. Your Watt/hour usage between charge times (usually daily for best results) will tell you how big a battery bank you need. That in turn determines how much solar power you need to properly recharge them. Designing a system from any other POV tends to leave it lacking in some respect.

That said, some nominal calculations based on your info ...

(4) 205 Watt panels = 820 Watts PEAK output. 820W divided by 24V = 34A peak charge current. Charge current should be 5%-15% of the bank's Amp/hr rating (20 hr). At around 10% the panels should be able to keep up 340 Amp/hrs, so your (8) 220 A/hr batteries would be large @ 440 A/hr total, but 5% of that is only 22 Amps charge current so it might just make it. However, none of that includes system losses due to inefficiencies in components. To be on the safe side you could divide the batteries into two separate banks and use a switch http://store.solar-electric.com/basw1300amp.html to choose between 'A' and 'B' when necessary.

Your 440 A/hrs @24V and maximum 50% Depth Of Discharge would yield approximately 5,280 Watt/hrs per day, which is quite a bit. But like I say, the panels are slightly undersized for that much battery.

Check out my sig for my off-grid set-up. I should add what it runs: 'frige, lights, radio, water pump, sump pump, computer/satellite set-up ... it's a bit marginal for all that.

niel · September 2009

Re: Help with number of batteries

my take on his purpose is to replace the generator as the backups. yes, they can be a pain and even unreliable under some conditions, but there is a limit to what you can store in batteries too so a genny may be good to keep around as an option to a solar setup. now it wouldn't make sense to get this just to wait for an outage so this would be a candidate for a battery backed gt system so that when the batteries are full that you can offset your bill with it as you expressed a desire for. this would be nearly all of the solar power it harvested after the batteries have charged up and would allow for more than just 20% of it to be used.:D
now i don't know what your load requirements will be to know if you are sizing the bank well enough. agms are good batteries to have and are more efficient too, but you don't want them to dip below 50% during any outages as this will take away from their lifespan. as your presenty number of proposed batteries will be roughly at a minimum charge level with the pvs you propose, do know that you can expand upon the pvs quite far with those batteries as they can take an amp for every amp hour capacity if need be. i don't think you will go that far with it, but the point is you can continue to expand the pvs to your desire to allow lowering your electric bill even farther. it is true that this battery backed gt system is not as efficient as a pure gt system, but you indicate you need the backup power so it may be viable.

DeltaFox · September 2009

Re: Help with number of batteries

http://www.freesunpower.com/battery_designer.php

This might help alittle ,I hope it opens up for you.

CVN-71 · September 2009

Re: Help with number of batteries

Thanks for the tips guys. I do loose power a LOT. Our grid sucks. Last winter out 7 straight days during ice storm. My Onan generator would not start. :-( I had a small Walmart generator and ran that for the entire time. All gas stations within 15-20 miles were shutdown as well. Most of the roads were impassable so even if I could reach a gas station, it would take hrs. I need more reliability, luckily, I could siphon gas out of a car to keep generator running. We lost power 2 weeks ago in a T-storm and again last week when it rained. I live in an area where the entire power lines are surrounded by trees. The slightest wind or storm, or teenager slamming into any pole along the main road and my power is dead. Summer or winter it doesn't matter, we loose power a lot. When the storm hit last winter, I couldn't get my Onan in for repair, the repair centers (ones that were open, were clobbered. My small generator couldn't even run a coffee pot, but it did keep my marine fish tank alive (which is a big concern and my primary load concern), also ran a small TV and the satellite. I waited in line for 8hrs at Sears for the privilege to purchase another Generator, and got it on the 7th day of the outage, just as the shore power came on. Needless to say, generators are OK, but don't always cut the mustard.

So, I do plan to enlarge the panel capacity, 4 panels now and prob 2 more in DEC before winter and maybe 2 more in March (if this recession recovers). I think this will be enough to charge the planned battery bank. As I said, I know it's not good to add batteries later so am going to get 8 of the 6v's up front. If the 4 panels I get initially are not enough to keep them full, I can use shore power to supplement until I do. At least if the power goes out I'll be able to run critical loads for a couple of days, have enough time to find a gas station, and recharge with the generator if needed.

icarus · September 2009

Re: Help with number of batteries

You do realize that a battery based PV system nearly doubles the per KWH cost of the energy. If your goal is to achieve energy independence, a grid tie system gives you twice the bang for the buck. If your goal is to provide emergency power, a standby generator is WAY cheaper per KWH.

A point that is often lost on newbies, is that the grid is your friend in some ways, and is in fact the cheapest "battery bank" out there. Those of us who live off grid do so (usually) because grid power is too expensive to bring "the last mile".

Just my opinion however,

Tony

icarus · September 2009

Re: Help with number of batteries

A PS to my previous, since your reply came in at the same time,

I would consider either converting the Onan to LP and stock pile a supply for the LONGEST expected outage, or trade away all the gennies you have and get either a honda eu or an LP Onan. THEN I would consider a grid tie system with perhaps a smallish battery back up. IMHO, in NH if you expect that the major outages are likely to be in the winter ( and the consequences more severe) then your potential solar gain might also be limited.

I think you should do some more research to avoid the "ready, fire, aim".

Once again, only my opinion.

T

niel · September 2009

Re: Help with number of batteries

sorry tony as i don't see him doing a ready fire aim and he never said he was looking for energy independence, but only trying to keep power going during chronic outages. his generator failed and a straight gt system does nothing for him in an outage with a genny that can and has failed as his backup. an offgrid solar setup would waste too much of the excess power by not selling to the grid. like it or not there is a place for battery backed gt systems and nobody told him to get rid of the genny as he may still need it for long term outages, but he will be able to stretch power usage too if need be from the battery backed gt system. this still will sell power back to the grid for any excesses so i don't see your aversion to my advice to him on this. you tell him basically to get a generator so why not tell him to get 3 of them rather than a battery backed gt system if you are that against it? that's dumb.

icarus · September 2009

Re: Help with number of batteries

My point is Niel, IMHO, the better bang for the buck would be a reliable genny with a reliable fuel source. The reality is, in many cases, if you have a 7 day weather related outage, the chances of having good insolation is fairly slim in winter NH.
by doing that can be put to other uses, such as grid tie, or better gennies etc.

In my personal experience, the mistake folks make, is buying cheap gennies, then they let them sit without exercising them regularly, then they don't provide fresh fuel, and when it comes time that they need them, they don't work properly. As with most things, doing it right in the first place is usually the least expensive alternative.

I am not suggesting that the OP is guilty of any or all of the above, merely illustrating what happens. It seems the best strategy is to define the loads, define the worse case outage time expected, and then calculate the life cycle cost of any given system INCLUDING battery replacement, exercise fuel cost, running fuel cost etc.

I realize that cost is not everything to everyone, but I think it makes sense to look carefully at the alternatives. The reality is, in spite of the OP's experience with his Onan, Onan gennies can be had in the 5-10 KW range for ~$2-800 that run on LP. They are fairly fuel efficient, and are very reliable. We have one that runs 18 hours a day for 125 days a year. We are in year six, with 5000 hours, and it has only required oil changes and head decarbonizing every couple of years.

Tony

mike95490 · September 2009

Re: Help with number of batteries

In the snowy winters, what temps to you start to have problems with propane not vaporizing to a gas ? Is this a problem with generator carbs iceing up like airplanes can?

Cariboocoot · September 2009

Re: Help with number of batteries

Back up generators: start them once a month whether you need to or not. Nothing worse for a motor than letting it sit for months on end and then expecting it to fire off when you really need it.

There is really no problem starting gas generators at low temperatures if the maintenance is kept up. You will need to let it warm up before putting a load on. If it's really, really cold (-40!) you can fire off a propane space heater off an hour and warm up the area around the gen first.

And a little fuel line anti-freeze is always welcome.

BB. · September 2009

Re: Help with number of batteries

Actually, the problem with carburetor ice in airplanes is not a cold weather issue... It is worse around the mid 70's (if I recall correctly) and high humidity.

If the temperature is warmer, the carb will not ice... If it is colder, the air holds less moisture and it is not a problem.

According to this link, the theoretical maximum temperature for icing is around 72 degrees with aviation fuel because of the 40F drop of fuel vaporization in the carb. (which is not gasoline). Lycoming says:

Lycoming publishes a temperature range of twenty to 90 degrees F. for carburetor ice.

One thing I did not know from the above link--Methyl Alcohol makes icing at higher temperatures much worse:

Adding alcohol to gasoline dramatically increases the ambient temperature where ice forms. Evaporating methyl alcohol creates a temperature drop of 300 degrees F. As a result, carburetor ice occurs over a wide range of temperatures

In the US, various forms of alcohol (usually Ethanol?) can be added to fuel (why--is a political issue). But, depending on your local fuel supply, it can affect the operation of a gasoline engine with respect to icing.

-Bill

niel · September 2009

Re: Help with number of batteries

look guys, he tried the genny route and had 2 of them with one failing and difficulties with fuel. there's nothing wrong with getting a battery backed gt system as that is what he inquired about based on his difficulties. how cost effective is it with 2 gennies on hand and 1 working with limited fuel for whatever reasons. what tony proposed was for him to get a 3rd generator. come on now, how cost effective is that?

and there's fuel costs and availability issues and i'm getting tired of tony advising against my advice just because he has an aversion to a battery backed gt system. there is a place for it that would fit his needs quite well and he has spent lots of money already with unreliable results. you just cannot justify telling him to go buy a 3rd generator:roll: and that advice from you with your constant aversion to battery backed gt systems will cause some to shy from even asking about it, which i'm beginning to think is one of your goals. sorry, but a generator with or without a gt system isn't a cure-all or the only viable answer and the man ruled out the generator as his main fallback. as far as costs go, all answers are expensive in one way or another, but don't ask him to repeat a mistake.
telling him to get another generator under the guise of giving him another option is bull as he stated the generators are unreliable for him with 2 of them present just because he inquired about a battery backed gt system and i concur with it being a good option. the man i'm sure considered costs already as he found he needs a more reliable solution that a 3rd generator will not fill.

Slappy · September 2009

Re: Help with number of batteries

yea neil I could have not said it better! the man has 2 gens already, and some times he is having trouble with the gens/fuel, he would also would like to supplement his grid use also with this system while the the next grid power failure is around the corner. At least with the PV set up, he can use the lights....etc...with out stumbling in the dark to get the genny going if need to be.

But I wish I was smart enough to answer his question, I am still learning my_self.

icarus · September 2009

Re: Help with number of batteries

With out getting into the personal issues, and fuel availability issues not with standing: Please at least consider the possibility that either the selection of the genset(s), or it's it maintenance may be part of the problem. As I have often stated (too often perhaps!) gensets that are mismatched for their loads and or are not properly taken care of are nothing but trouble. That said, a well taken care of, well matched genset (with a store of proper, fresh fuel) will in all probability serve as the cheapest most reliable back up power system. I have no aversion to any particular system or solution INCLUDING battery backed up grid tie, but have merely suggested that there are other alternatves.

If the OP wishes to build a system strictly for back up, that is his choice and the advice he gets here is second to none IMHO, BUT I think we do a disservice to someone not to point out other options and let him/her come to his own conclusions.

Tony

Cariboocoot · September 2009

Re: Help with number of batteries

Personally I think a battery backed-up grid-tie set up is the electrical system equivalent of heaven. But it doesn't come without a price, and when you're looking at things pragmatically you have to consider that. I don't know what's gone wrong with the generators the OP has, but I'll bet they can still be put back into working order for less than what the G-T system costs.

Mind you, if money is not an issue ...

What's the electric rate in NH? I seem to recall that it has co-op power everywhere? Got any financial incentives for buying solar stuff? That always helps!:D

niel · September 2009

Re: Help with number of batteries

tony,
do not make it as if it's my personal issue as it is you that keeps contradicting advice on battery backed gt systems no who mentions it and it is your personal aversion, not mine. keep make believing you're shaking a palm tree up there and let the guy choose something besides a 3rd generator if he wants to.
on that note drop it and let the op chime in for the real info he was seeking.

icarus · September 2009

Re: Help with number of batteries

I have no idea what that means,,,, but I will consider my knuckles wrapped,,, and shut up.

T

niel · September 2009

Re: Help with number of batteries

now that came out slightly wrong as the meaning was, it is you that seems to have it personal about advising everybody against battery backed gt systems and you are trying to make it seem like the problem is me. fists are not clenched here, but let people have the opportunity to have a battery backed gt system if they so wish to. if you would like, you can state in your own thread your views on it pro and con without denying or scaring off those that want to know or want to have such systems. if you like i'll even sticky it if it's good enough.
now again, after my clarifying things somewhat, i expect anymore posts here to help him in his quest.

Kamala · September 2009

Re: Help with number of batteries

Not really my area but, after reading the OP, I thought... fix or replace the main genny.

If running the genny is so troublesome, maybe add a battery based UPS kind of set up. If the outage is short, no need to run the genny. If it's longer, then genny can be used to recharge batts.

I also think that the amount of insolation during a NH winter must be considered. Along with nearby (or not so nearby) trees and/or mountains. The low sun casts much longer shadows.

niel · September 2009

Re: Help with number of batteries

you are quite right kamala, but why just have an ups just sitting there as the excess power, after the batteries are charged and awaiting an outage, can be sold too, right?

Kamala · September 2009

Re: Help with number of batteries

CVN-71 wrote: »

...bear with me if my terminology is off...

Because of the rebates/incentives I wanted to go solar based...

Thanks!
Bob

Neil, you're right! I think that the OP may be considering GT or possibly including it in future expansion.

Something I missed since he didn't directly refer to it.

Craig

Help with number of batteries

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