Battery capacity question

[Oly]

Howdy all,

I have a question about battery sizing.  I just bought an off-the-grid cabin with a small 12V solar setup.  It's got 6 smallish 6V deep cycle batteries that I'm told are about 10 yrs old, and 2 PV panels about 2x3 feet each. The previous owner left the lights on for 10 days straight and drained the batteries down to 8V; now the generator won't charge them past 12 Volts.  They quickly discharge. So, I'm probably going to have to invest in some batteries.  I may have the opportunity to buy some new 1850 Amp hour 2volt batteries cheap.  I could get 6 of them for $600, which is about what I might spend on a good set of Surette or Trojans with about 500 Amp hours.

So, my question is, is it stupid to put these huge capacity batteries on a system supplying relatively little PV input?  Will I kill the batteries by running them down if I don't keep them fully charged back up?  Would I be better off just buying lower capacity batteries that I could reasonably keep charged?  As a side note, I will be looking to add more panels and possibly wind power to the system in the long term, but that would be a few years out.

Any input is appreciated, I'm new to this.  Thanks, Eric

[cdg378]

Those panels are pretty much geared towards "maintenance charging" of your battery bank. I would start with about 6 - 6v golf car batteries and add them as you add charging capacity to your system. This would give you over 600 amp hrs of storage for about 300.00.

Get an inverter if you don't have one, 700 watt at Walmart for under 70.00, they are on sale right now (web price only) for 59.00.

Invest in a small generator or build yourself a 12v alternator and lawn mower engine setup on the cheap if you are handy to recharge the bank quickly when needed.

I have owned off grid cabins in the past and found this avenue to be the best road to take, it worked well for me without giving up any comforts of home (I never had solar either).

my 0.02 from experience...

[cdg378]

Also...you could spend more on agm batteries and reduce maintenance, gassing... I don't know your budget, but my experience with remote cabins is you don't want to start putting expensive components in because they may (will) come up missing someday.

Make it all plug and play as much as you can, take things like inverter and generator back home with you when you leave.

[Gary D]

Another option is to check water levels. If low, add to fill line (distilled water) and use a desulfator/charger on them. Some on this board have had good luck bringing dead batteries back to service with them. I just picked up a vector charger at Wallmart 2-10-20-35 amp charger with a reconditioner/desulfator setting for under 70 bucks. If you're taking up an ac generator to the cabin anyway, could try it and see if it works for you.... probably not worth 2 cents  Gary D.

[picmacmillan]

hi oly........i have put a really good battery file in my files..i put it in there because i felt it was too large to put with your post...if you go to my files look there it is called "batery pdf" and i got it from tazman.com....i just went to tazman .com but it isn't the same information as i have in my files...go look in my files and i suggest eceryone  else to go look there too...there is a lot of useful information on batteries and charging etc....it would actually be good for everyone as it is very informative..definately worth keeping around...hope this helps you ...pickster...

[ghurd]

Batteries self discharge about 1% per day.

Those big ones will need 19 amp-hours per day from the solar just to stay even, so they are too much for the panels you have there.

G-

[juiced]

I just learnt a disturbing lesson.

   Converting from DC to AC requires almost 2x the battery banks (or power source)!

 (Ill quote rather then link)

"One of the biggest mistakes made by those just starting out is not understanding the relationship between amps and amp-hour requirements of 120 volt AC items versus the effects on their DC low voltage batteries. For example, say you have a 24 volt nominal system powering a load of 3 amps, 120VAC, which has a duty cycle of 4 hours per day. You would have a 12 amp hour load (3A X 4 hrs=12 ah). However, in order to determine the true drain on your batteries you have to divide your nominal battery voltage (24v) into the voltage of the load (120v), which is 5, and then multiply this times your amp hours (12 ah). So in this case the calculation would be 60 amp hours drained from your batteries - not the 12 ah. "

[ghurd]

Hey Juiced

You knew this! They just confused you with too much fluff. I had to get a calculator.

Watts is watts is watts.

a   120v x 3a = 360w

a   24v x 15a = 360w

a   120v / 24v = 5

a   3a x 5 = 15a

a   24v x 5 = 120v

(The a's are only there to keep the server from changing the numbers around)

G-

[juiced]

<As Ghurds wind machine quickly blows the smoke away....>

   I see Sensei! <doh>

 I dont why i didnt even bother checking that with ohms...  I did find it at around 3:30 am though.. :p

[troy]

The only disturbing part is the conversion losses.  Flooded lead acid batteries are about 80% efficient.  So if you take 80 watts out of your battery, you have to put ~100 watts in to make the battery fully charged again.  To make matters worse, most inverters are only around 80% efficient overall.

Taken together, thats:

.80 x .80 = .64 or 64% efficient at best.

On top of that, battery chargers aren't 100% efficient either, it takes more than 100 watts of energy into the charger to put exactly 100 watts into the battery.  So now we have maybe 50% overall efficiency.

It's those pesky laws of thermodynamics again.

Good luck and have fun!

troy

[Tyler883]

"Will I kill the batteries by running them down if I don't keep them fully charged back up?  Would I be better off just buying lower capacity batteries that I could reasonably keep charged?"

Hi Eric,

If you don't mind, I'll take a stabb at this, and try to create a few general rules that you can base your decisions on...

1. You need a system where your power consumption is a little less than you capability to charge the batteries
   
2. A battery that is kept fully charged( and is never discharged) has the greatest chance of lasting many years without prematurely dying on you.
   

[juiced]

Im trying to stick with what i learnt at the beginning. And based on what i was reading above it seems more and more realistic.

   you need min 75% more panels then storage. You need to portion your storage so you only use 45% of the actual power capabilities. This is what looks (by all the numbers out there) to me like a long-life system

[nothing to lose]

I kinda just figure the batteries self discharge a certain amount, find that amount and have more charging capacity that that and they stay fully charged. Now figure what your gonna use and that's how much more charging capacity you need.

So if 1890amps hrs discharges 1% per day I would need 19 amps per day charging to keep it full charged. Now they should last forever, but their not doing me any good if I don't use them. So if I need to get 200 amps a day out I need 200 amps charging per day +19amps for self discharge.

 If I only plan to use them once per week (like saturday) and only use 200amps that day, I need 200amps/7day = 26.8 amps charging per day to keep them full + the 19amps they self discharge.

And at 200amps out of a 1890amp bank I am drawing only around 10% of the bank so they should stay healthy.

Probably not the correct way to figure things, but it's my general easy way to geusstimate something.

I think I would go with cheaper smaller batts myself though as others mentioned. Things go wrong, I'd rather pull out 1 or 2 12V or 6V batts if I had to and still have power than pull out 1 2V and lose everything. Bad things do happen randomly.

For instance today I was stuck at the rock house I rented for about 3 hours with a dead battery on my truck. I could have easily pulled 1 12V battery to run my truck and kept the system up at the same time. Or I could have pulled 2 6V trojans and jump started the truck while still keeping the system up on the rest. (if I had everything set up, I could have connected the battery charger, but that was at home)

  Personally I didn't care to go to that trouble and took a nap instead and jump started it off the wifes car when she got there.

Sometimes it's good to think about such things when dealing with remote locations

I didn't have a battery charger with me, had I needed to go somewhere right then I could have pulled one battery to get the truck started. For my only bank in a remote location I dought I personnally would go less than 6V batteries.

[Oly]

Thanks to you all, you've been quite helpful. I think I'll keep the system simple and stick to some new 6V batts (if I can't resuscitate the existing batts.)

[juiced]

If i have my PV system hooked up to a battery bank (or single battery) can i simultanously charge and use the bank?

  So lets say my bank is being charged @ max power (12 noon, 'perfect day') can i be running my invertor + load at the same time?

[juiced]

BTW, based on the above numbers; I think my 65w array could charge a 18AH battery from dead to full in one day. So since we dont want dead batts, i would want 4x4.5AH batteries hooked parallel, correct?

[ghurd]

A 65w PV puts out about 4a. 4.5 hours of sun is 18ah.

BUT...

The battery voltage will rise to where the controller will cut the charging back.

Thats where it gets sticky. The higher the charge current to capacity, the faster regulation cones in to play.  

If you put a 50a charger on the 18ah battery, the voltage will rise to regulation in a minute or 2, not the 21 minutes and 36 seconds that formulas say. It will not be charged.

Better figure 2 days.

G-

[nothing to lose]

Yes.

Basically if you are producing more power than you are using then you are charging the batteries. If your producing = power then your neighter gaining or lossing. If your producing less power than your using your discharging the batteries what ever amount you need above what you are producing.

Can't perfectly balance a system, but if you could that would be the best way to do it and aviod the losses with a battery. Like say you use 50amps a day and your pannels produce 50amps a day. If you could do it, having full charged batts in the morning you would not need to charge them, use the 50 amps as you make it you have no battery losses, end of day you have full batts incase you needed them at night.  Can't do exactly that, but the closer you get the better.

So say you have clothes needing washed, while the pannels are producing wash the clothes, that power is not having loses going into and back out of a battery. But if you put all that power into the batteries durring the day then wash clothes at night using the batteries you have the losses. So actually when ever posible it is probably best to use the power while it's being made. Any extra power not being used will go to charge the batteries, but I think the power that is being used will go directly to the device using it thus bypassing the batteries.

I was at the rock house yesterday and the batts there were getting low on me. I needed to run a car a bit anyway so I pulled it to the house and connected jumpers to the battery bank. Inverter jumped back up to 13V led again soon as I connected the cables to the batteries which were down to around 11V or 11.5 volts, and I still had the same loads running. I know I didn't raise a 345amphr battery bank no 1.5-2V in a couple seconds. I was using the power first and the unused power was going into the batteries.

Don't know how much charging I did, little chrysler idling for about 20 minutes and cheapy jumper cables, but my bats are better charged now anyway. Not a good way to charge, but all I had there at the time and it works.

[juiced]

Could this be why my battery booster packs show full, but are not?

  Tom was right, i need some field experiance to advance at this point.

[ghurd]

Yes. It is reading surface charge. The solar/other controller will read the same thing, and cut back or stop charging. It is not full, but adding more charge at this time will start doing damage.

Ain't this fun?

G-

[juiced]

hehe...

   a 75ah is 85$ and about the size of a car batt. They are AGM i belive.