Author Topic: Battery rating mumbo jumbo (Read 5947 times)

Crispy · « **on:** October 13, 2012, 09:55:30 AM »

Hello,

I am trying to sort out all the numbers used by battery manufacturers for their amp hour capacities. It gets quite confusing at times.

First a little background info.

I am on the grid and have no fantasies about being able to go totally off grid. The project I am proposing is in the hobby / backup category with the goal of having a useable daily capacity of 90 amp hours on a 24v bank to run some lights, tv, dvr, modem and 2 laptops but also build in enough capacity to go 2 days without input if cloudy and no wind, plus extra spike capacity to power a 240v well pump for minimal needs and also the fridge and small 4 cu ft chest freezer, so maybe a total of around 400 amp hours useable which I understand useable to mean no more than 50% SOD of the batteries.

My calculations show that factoring in inverter losses that to power everything except the well and fridge and freezer would draw close to 20 amps with spikes of 40 amps ( that's a guess ) with everything running at once.

I am thinking that my "normal use" of 90 amp hrs a day with 400 amp capacity ( to 50% DOD ) would be considered light duty on the bank and I could have good performance for my needs using GC8 batteries, perhaps 2 strings of 3 each in series paralleled together or 3 strings if necessary. So, finally, here's the question:

Based on these specs, how many of these would I need to get to 400 amp hour capacity at 50% SOD? I am never sure if the numbers they give are based on 50% SOD or totally discharged (80% ? ) which would not be good, right?

20 amp hr rate: 165
5 amp hr rate: 130
Minutes at 25 amps: 318 So roughly 5 hours at 25 amp draw, but to what state of discharge is that??
Minutes at 56 amps: 121
Minutes at 75 amps: 85
Volts: 8

I have no doubt that the knowledge and expertise I have seen from the members here will ensure I get this right the first time.

Thank you all for your input

Crispy

dnix71 · « **Reply #1 on:** October 13, 2012, 12:43:49 PM »

You want the bank small enough to be fully recharged during the time of year with the least amount of solar, but still large enough to meet your needs.

That may not be the dead of winter. In south Florida we get lots of sun and cooler weather (better panel output) in the "winter," but in summer it's hot and overcast even though the sun is higher and the days longer.

How fast you draw down affects the usable capacity, hence the different ratings based on amp draw. You will shorten battery life by going 80% DOD, plus the voltage at that level may not be enough to run your inverter. Because you are not using DC appliances you cannot allow your bank voltage to sag below the inverter cutoff, even momentarily.

50% DOD is okay for what you are doing. There are 12v traction batteries with 90 amp-hour ratings, so two of those in series would be 24v at 90 [round up to 100] amp-hours. If you want 400 amp hours for extended bad weather, then that would be a bank of 8 (4 x 2) and that will not work well. You will have trouble keeping that many batteries in a parallel/series combo properly equalized.

If you used a 12v bank, you would need 8 in series for 400 amp-hours. I have five 75 amp-hour batteries and those are charged by 700 watts [nameplate rating] of solar. 8 in series didn't work. They didn't stay charged. I went to 7 and am now on 5.

400 amp-hours at 24v is the same rating as one of the battery powered pallet jacks we use at work. You can get 400 amp-hours out of that for years if you have the high amps to properly recharge it and cook the electrolyte at least once a week. To recharge that lift would take solar with lots of clear skies, or a grid charger, or genset.

gww · « **Reply #2 on:** October 13, 2012, 03:55:57 PM »

dnix71
Are you describing a 400 amp hour 24 volt battery bank?

Crispy
Are you asking about a 800 amp hour battery bank that you can discharge giving you 400 amp hours use at 50% dod?

15 of the listed batteries? 5 parellel banks of three in series?

Am I missing how this works? Probly.

Thanks
gww

PS I always heard you never want to parellel more then 4 banks.

dnix71 · « **Reply #3 on:** October 13, 2012, 04:42:38 PM »

I am hoping he wants 400 amp-hr total capacity, not 800 and only use half. An 800 amp hr battery is a cherry-picker lift battery.

http://www.wholesalesolar.com/products.folder/battery-folder/Surretterolls.html

Product 9990011 matches that and is $3k. That's whole-house grid-tie backup size.

http://www.ebay.com/itm/New-24-Volt-12-125-15-Forklift-Battery-Free-Shipping-Made-in-USA-/320975042390?pt=LH_DefaultDomain_0&hash=item4abb9a7756

875 ah at 6 hours. $3.5K

gww · « **Reply #4 on:** October 13, 2012, 07:11:10 PM »

dnix71
I'm not say I wasn't reading it wrong I was just trying to see if we were all talking the same language or if I was missing something. I miss or missunderstand things and also how things work all the time. Trying to learn though.
Thanks
gww

Crispy · « **Reply #5 on:** October 13, 2012, 08:40:49 PM »

Hey guys,

I probably buried my question in too much detail. I tend to over think things and end up including details that really aren't pertinent to the question.

I'm trying to get a handle on the specs that battery manufacturers use. From your answers, it sounds like the amp hour ratings they give need to be cut in half to determine what you can safely use without harming the battery. Correct?

Then there are the other specs such as 20 hr amp rate 165. What does that mean exactly and is it useful information? In the example I gave, there was no AH rating given for the battery, just this. It seems like what it is saying is you can pull a load of say 10 amps for more than twice the time period as 20 amps so apparently grade school math don't work on batteries

My understanding of things is if I want a 400 ah bank using 8v 200 ah batteries, I could achieve that by series connecting 3 strings of 4 batteries per string with the strings being parallel, so each string would be 8v and 800 ah and connecting the 3 strings together in series would give me 24v and 800 ah. How am I doing?

Just want to be sure I understand the specs so I size my bank correctly.

Thanks, Crispy

dnix71 · « **Reply #6 on:** October 13, 2012, 10:44:42 PM »

Short answer about amp-hr rating. Yes, use less than 1/2 and the battery will last much longer. Also use the 6 hour rating for that purpose because you will be discharging overnight, not over 20 hours.

gww · « **Reply #7 on:** October 14, 2012, 06:00:06 AM »

Crispy
It would be 4 strings of 3 batteries in each. 8x3=24 In your exanple each string would be 200 amh for a total of 800.
Hope this helps
gww

mab · « **Reply #8 on:** October 14, 2012, 03:35:53 PM »

Hi Crispy,

Quote

20 amp hr rate: 165
5 amp hr rate: 130
Minutes at 25 amps: 318 So roughly 5 hours at 25 amp draw, but to what state of discharge is that??
Minutes at 56 amps: 121
Minutes at 75 amps: 85
Volts: 8

I've never got my head around the minutes at x amps rating, but the '20 Amp hr rate: 165' is the 'C20' rate; means that if you discharge over 20 hrs to 100% discharged, then the AH capacity is 165 (i,e, you can draw 8.25 amps (165/20) for 20 hrs to reach 100% discharged).

if you draw more current then the battery efficiency is lower (Peukert effect?) so the useable capacity is less:
C5 rate: you can draw 130/5 = 26A for 5 hrs to 100% discharged.

correspondingly, if you draw current at just C100 then your Ah rating will probably be >200.

so it helps to have an idea what you major loads are drawing as these will generally determine which rate you need to use to calculate Ah needed to stay <50% DOD.

ghurd · « **Reply #9 on:** October 14, 2012, 06:44:54 PM »

Quote from: mab on October 14, 2012, 03:35:53 PM

Hi Crispy,
...
so it helps to have an idea what you major loads are drawing as these will generally determine which rate you need to use to calculate Ah needed to stay <50% DOD.

Cudos to mab. Good short answer for a complex question.

G-

Crispy · « **Reply #10 on:** October 14, 2012, 08:25:42 PM »

Quote from: mab on October 14, 2012, 03:35:53 PM

so it helps to have an idea what you major loads are drawing as these will generally determine which rate you need to use to calculate Ah needed to stay <50% DOD.

I think I'm getting the idea. The lower the amp draw, the higher the amp rating on the battery to a degree. Since loads vary over time it seems those specs are of little use unless you have a steady draw over a set period of time.

My load calculations are as follows: @24vdc

Living room tv, dvr, modem, router, 2 laptops, 2 cfl lights add up to 11.5 amps for 6 hours per day - plus 15% add for losses=80 ah/day

Side by side refrigerator/freezer and small chest freezer are 15 amps at (guessing) 12 hrs/day runtime plus 15%=207 ah/day

Well pump 230vac 8 amp comes to 46 ah for 1 hour per day runtime. I only plan on using battery power for the well in grid down situations.

So my loads not counting the well come to 287 ah/day. I figure with an 800 ah bank (400 useable) I am at 287/800=36% discharge.

Adding the well would put me at 333/800 or 42% discharge.

Without the well it would be a steady 12 amps with spikes to 27 amps if both the fridge and chest freezer ran at the same time.

Based on this, do you think I'm on track? Is the 15% I allowed for inverter and other losses seem reasonable?

I am on grid, but this would be an off grid setup. My plan as it stands now ( open to suggestion and modification) is for a 1200w solar array which on a good day I may see maybe 4kw out of it and a 10' axial turbine for icing on the cake, providing anywhere from a big fat zero to as much as 6kw per day with any excess going to heat water.

Thanks for all the input guys. It is much appreciated.

Crispy

Crispy · « **Reply #11 on:** October 14, 2012, 08:34:23 PM »

Wondering if my conversion from ac to dc on the well was correct? 8 amps @ 230 ac probably should be 80 amps dc @ 24v without the extra 15% ?

ghurd · « **Reply #12 on:** October 14, 2012, 08:39:16 PM »

Quote from: Crispy on October 14, 2012, 08:25:42 PM

I think I'm getting the idea. The lower the amp draw, the higher the amp rating on the battery to a degree. Since loads vary over time it seems those specs are of little use unless you have a steady draw over a set period of time.

Correct.

Generically, I throw in 20% for inefficiency, however my typical systems are never more than C/20 load.
Much of a real working system is designed by seat-of-the-pants, with a good strong dose of experience.
G-

gww · « **Reply #13 on:** October 14, 2012, 08:40:00 PM »

Crispy
My math probly sucks and I may not know what I am talking about. If I take 287 amps and times it by 24 I come up with 6888 watts used each and every day. If you get 4000 watts off you solar after discounting about 30 percent, you have a shortfall of 2888 watts. You would have to live in a great wind area to get this with a ten footer and have a tall tower. Hows your wind?
cheers
gww

Crispy · « **Reply #14 on:** October 14, 2012, 09:14:28 PM »

Quote from: gww on October 14, 2012, 08:40:00 PM

Crispy
My math probly sucks and I may not know what I am talking about. If I take 287 amps and times it by 24 I come up with 6888 watts used each and every day. If you get 4000 watts off you solar after discounting about 30 percent, you have a shortfall of 2888 watts. You would have to live in a great wind area to get this with a ten footer and have a tall tower. Hows your wind?
cheers
gww

Your math looks good to me

This is a hobby with an added benefit of keeping some essentials going in a grid down situation. We can be flexible with what we power off the system as conditions permit. Worst case scenario, we leave the tv and laptops off, light a candle and get 2 days worth of refrigeration off the stored 400 ah, and find something else to do besides tv and surfing the net

Wind here is hit or miss but usually if the grid goes down here it's during winter storms when it's windy, gotta love those noreasters. The 10 footers are supposed to put out 800 watts so even at 500 watts with a steady wind, I have my extra 3 kw in 6 hours.

I'm a self confessed "weather weenie", so I will usually know when the potential is there for a power outage and can plan to have the bank topped off.

I know, I could accomplish all that and more probably with a $800 genny, but that's no fun

gww · « **Reply #15 on:** October 15, 2012, 05:29:40 AM »

Crispy
You sound like you are trying to end up with about what I am trying to end up with.
I am going with a little more solar and a little less turbine (2- 8' ers) and about half of the battery. I am a year off from install so I hope you keep posting as you go. I will enjoy your successes and try to avoid your pitfalls. I added it up yesterday and with no batteries and nothing wired and installed I am at $7300. I am committed now.
good luck
gww

Crispy · « **Reply #16 on:** October 15, 2012, 06:03:23 AM »

Quote from: gww on October 15, 2012, 05:29:40 AM

Crispy
You sound like you are trying to end up with about what I am trying to end up with.
I am going with a little more solar and a little less turbine (2- 8' ers) and about half of the battery. I am a year off from install so I hope you keep posting as you go. I will enjoy your successes and try to avoid your pitfalls. I added it up yesterday and with no batteries and nothing wired and installed I am at $7300. I am committed now.
good luck
gww

gww,

So far, I've spent $28. Hugh's book should be arriving any day now.

I think it shall be I enjoying your successes as I'll be spending the next 6 months here asking questions. I don't want to be on here asking why my pole collapsed while raising it. For me, the homework comes first. Sounds like you have the same strategy. Maybe we can travel this road together. PM me if you like and let me know more specifics of your project like what your goals are and what you've purchased so far.

I do hope to get my mill built over the winter and work on the tower in the spring after the garden is in. Speaking of homework, I need to find out if a DIY install still qualifies for the federal 30% tax credit. Something tells me it doesn't

I'll be selling a 2007 Kawasaki VN900 with 10K miles to help fund my project. Any takers out there?

Crispy

DamonHD · « **Reply #17 on:** October 15, 2012, 07:23:14 AM »

"If I take 287 amps and times it by 24 I come up with 6888 watts"

YES, a Watt is power (Amps * Volts, ie assuming you meant this 24 was your system voltage: putting units on would hep)

"... used each and every day."

NO, you would then have to multiply by 24 to get Watt-hours, which is energy, ie 165,312Wh or 165kWh, which is about what my whole house and family of 4 uses in a *month*.

Don't mix power and energy: it'll make a mess of your sums.

Please use units to make your working clear: is that 24 a voltage or a number of hours?

This is why you get marks for showing working at school. (Trust me, I'm our local school's "Numeracy Link Governor"!)

Rgds

Damon

gww · « **Reply #18 on:** October 15, 2012, 03:26:39 PM »

Damon
Some times my fingers move slower then my brain, no let me refraze that some times my brain just works slow. Have you ever gave a depasition? You say what you think you mean while somebody writes it down. When you reread it you have to call them and say you need to change this word or that word. The person writing it down says, "Thats what you said" then you say "That may be what come out of my mouth but thats not what I was trying to say." I will try to do better.
24 volts
cheers
gww

DamonHD · « **Reply #19 on:** October 15, 2012, 04:47:11 PM »

gww, oh yesssss, been there, messed that up, I have, guilty as charged! B^>

Rgds

Damon

gww · « **Reply #20 on:** October 15, 2012, 07:45:04 PM »

Crispy
I am only planning now because I have no choice. I live in a different state then I will a year from now when I retire. I actually started trying to built wind turbines out of junk. Then my plant closed and I had to move. I was now an hour closer to work and in town. I didn't have anything to do so last winter I started building solar panels in a spare bedroom. The spring came and I built two of Hugh's 8' turbines. Then I saw an outback system on craigs list. Then I wanted to fill the charge controller to capasity. Solar panels came down to $0.92 per watt delivered so I bought 1400 watts. Then I joined this forum and started asking questions. I wanted to grid tie but electric companys don't like home made. I would say I did it back wards. Then I got to thinking about the cheep shipping where I live now so I bought 1400 watts more in solar.

I knew nothing and nobody who has wind or solar till I joined this site. The brains here are amazing.

I would say I did it backwards but I did do it. I believe I will end up with a mirror image type system that you described above except for battery capasity.

I am trying to learn as much as I can before I move further. I am not going to buy more solar (I think) untill I can get some of this stuff installed. I welcome any imput you have to give and am selfish enought to watch you like a hawk in the hopes of learning something. I will help you if I can but you need to understand that smart people here are always correcting my misconceptions.
In frendship
gww

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Author Topic: Battery rating mumbo jumbo (Read 5947 times)