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Author Topic: Battery Bank  (Read 8099 times)
JMPC
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« on: February 26, 2011, 09:46:17 AM »

Hi all,

Please could i ask for comments on what is wrong, or right, or with my maths/assumptions below.

My home uses 10kw a day, i am 'offgrid' and therefore need a battery bank to supply power. I have the means to generate 2kw an hour for 5 hours each day.

I should not be discharging the battery bank to less that 50% of its capacity so therefore I require a battery bank capacity of at least twice what I expect to consume, so a capacity of 20KW, or 20,000 watts.

I should not  recharge batteries at a faster rate than 10% per hour of their total capacity, so therefore if I am charging them over a 5 hour period i can 'put in' 2kw, or 2,000 watts an hour.

Is this correct?

Also, do I have to be recharging 12volt batteries with a 12volt power source, or can i use a 24 volt power source?


Many thanks, JMPC
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billi
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« Reply #1 on: February 26, 2011, 09:49:29 AM »


Hi

Quote
Also, do I have to be recharging 12volt batteries with a 12volt power source, or can i use a 24 volt power source?


You can , but i think you need a special controller for that ,
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« Reply #2 on: February 26, 2011, 10:15:49 AM »

and you should not be discharging your batt bank more than 12 percent,,, that is if you want them to last any length of time. if you take them down  50 percent they will die very quickly.
       you can charge at any votage you like,you just need the proper charger.however 12 and 24 volt are more common and the inverters more easily obtained, but,, but,, if you configure the voltage to say 48volt then the wiring becomes less hassle and less expensive,plus the batteries like it because they seem to get the charge more evenly and give it out more evenly which makes them last longer. if you work on a voltage of 120volt (10 x 12volt batts)you would need a 120volt alt to deliver,,,be it wind turbine or 10 x 80watt x 12 volt solar panels,this system can grab any sunlight going and process it much quicker than the ordinary 12 volt system.( but you need to be carefull,, very carefull,)
                                         biff
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Countrypaul
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« Reply #3 on: February 26, 2011, 10:31:24 AM »

JMPC you don't say how you would use the 10kwh during the day, but if you could arrange for most of this usage to be during the generation period that would avoid having to discharge your batteries as much (at all?) during this period and therefore prolong their life and improve overall efficiency. The less "use" that you can make of the batteries the longer they should last. Even if your useage is flat over the day, you should be able to keep the discharge to 40% rather than 50% since for 5 hours/day the batteries should not be discharged (ie whilst charging).

Paul
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martin
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« Reply #4 on: February 26, 2011, 10:46:20 AM »

Don't know where the "50%" per day discharge came from - your batteries won't last very long at all treated like that - I'd sooner aim for 10-20% (maximum) per day to give them a chance of any sort of longevity.......
Once you've done the sums and fainted at the costs involved of a decent sized battery bank of quality, it'll rather bring home the need to "start at the other end" and minimise electrical consumption first - 10 kilowatt hours per day is around the national profligate average!
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Justme
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« Reply #5 on: February 26, 2011, 10:53:09 AM »

50% comes from it being the point at which battery life, bat replacement & charging time & costs seem to converge for the least cost of ownership. Oh & its bad to just discharge by a small amount & then recharge again for FLA cells.
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Navitron solar thermal system
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1200watts solar 120vdc
FX80 Solar controller
Victron 12v 3000w 120a
6kva genny
6 x 2v cells 1550amp/h 5C
24 x 2v cells 700amp/h 5C
Total bank 4350 amp/h 5C
JMPC
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« Reply #6 on: February 26, 2011, 11:08:29 AM »

thanks everyone for your advice. Has helped. Taking on board what everyone is saying, I shall opt for a 48 volt bank, (thanks biff) and be conscious to ensure max power consumption takes place at the same time as charging, (thanks Paul) . In this way I can reduce overall size and costs.

thanks, JMPC
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martin
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« Reply #7 on: February 26, 2011, 11:14:47 AM »

I think battery "type" needs to be firmly set out before loudly trumpeting "50%"-  there are many batteries on the market that'll last a year or 18 months given that sort of treatment, which someone new to the field may be misled into believing are suitable for the job (I have in mind a certain UK concern making ludicrous claims for it's "leisure" batteries)
« Last Edit: February 26, 2011, 01:35:48 PM by martin » Logged

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dhaslam
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« Reply #8 on: February 26, 2011, 11:26:27 AM »

There is also an efficiency loss  from generation to storage to  end use of   30-40%.       

http://wiki.xtronics.com/index.php/Sealed_Lead_Acid_Battery_Applications
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tz0c0s
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« Reply #9 on: February 26, 2011, 03:43:18 PM »

Martin, I would not ask you to name & shame. But what leasure batteries would you suggest forum members avoid ??

Also, are we yet at the point where LifePo4 batteries make sense ?

Andy
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martin
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« Reply #10 on: February 26, 2011, 04:08:07 PM »

To be frank, I'd suggest avoiding ANY battery labelled "leisure" which is usually a weasellism for "not very good at all"..... whistlie

Here's a thread on the subject (one of many!) - http://www.navitron.org.uk/forum/index.php/topic,8958.0.html
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« Reply #11 on: February 27, 2011, 12:19:33 AM »

For low power applications you might get away with some new 12V LiFeYPO4 monoblocks that have appeared from China.  Claim that they have cells that are matched well enough to not require individual cell balancing (provided you stay within a safe operating area Voltage range of 11.0V to 16.0V).  They are available up to 90Ah size.

http://www.ev-power.eu/?p=p_48&sName=lifeypo4-batteries-%2812v%29

I mailed the distributors and asked about using them with the programmable controls on the Morningstar lead acid solar charge controller and they reckoned it was ok.  With an inter-block balancer (like wot I bought for my slightly wayward AGM bank), it should be ok to run two of these lithium monoblocs in series for 22-32V (24V nominal).  32V is a bit steep for the inverter I have (it will shut down with an over-Volt warning) but you could just charge them up to 30V and they'd never be damaged (just never fully charged).  I'd need to have a more aggressive LVD safety on the inverter though.  The inverter will shut down at 21V but this is already below the safe operating range of the battery.  Over discharge causes immediate and permanent damage to these cells but they don't care about being left at partial charges for months (they recommend a slow full charge once a year to the upper Voltage limit).

I might get a little 20Ah guinea pig to slaughter on my spare Morningstar controller and inverter in some sort of battery alien autopsy experiment... Then I can move on to lightly killing a more expensive 90Ah pair on the main system before finally biting the bullet with a full set of 200Ah cells that will only be made to feel brutalised Roll Eyes

Apparently, the distributor can tell if you've over charged the big wide cells because they bend into the shape of a banana if you do. 
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3.58kWp & 800Ah LiFeYPO4 off-grid(ish). See 'Cobbled together PV in W.Sussex' (in "Show Us Yours")
Justme
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« Reply #12 on: February 27, 2011, 11:30:57 AM »

If you care to read all of this thread you will find out about a bat that is on the market now that beat all the others in its price range hands down.















Or I could just tell you that they are Hawker 12FV120 AGMs.
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Navitron solar thermal system
30 x 58mm panel 259L TS
1200watts solar 120vdc
FX80 Solar controller
Victron 12v 3000w 120a
6kva genny
6 x 2v cells 1550amp/h 5C
24 x 2v cells 700amp/h 5C
Total bank 4350 amp/h 5C
EccentricAnomaly
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« Reply #13 on: February 27, 2011, 02:54:58 PM »

Or I could just tell you that they are Hawker 12FV120 AGMs.

But don't you have to sign up for a tour of Afghanistan to get one?

Excellent thread on the canal boat forum, thanks for that.
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Outtasight
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« Reply #14 on: March 02, 2011, 10:14:29 PM »

There is also an efficiency loss  from generation to storage to  end use of   30-40%.       

That thought hasn't eluded me.  I'm thinking of downsizing my battery.  If I limit the use of battery at night to just lighting (and not trying to run the fridge over night or anything outlandish like that), I can use a much smaller battery that is cheaper to replace when it wears out and just use intelligent load management in the day to maximise use of the available power.  A small battery (maybe just 100Ah @ 24V) will charge up quickly in the morning and leave the rest of the day to power things around the home directly through the inverter - at 92% efficiency (ok maybe less when taking wiring into account, but avoiding the battery losses).
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http://solarbodge.blogspot.com/
3.58kWp & 800Ah LiFeYPO4 off-grid(ish). See 'Cobbled together PV in W.Sussex' (in "Show Us Yours")
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