Battery Bank Sizeing

[r77]

Hello everyone, I need some advise on calculating what size battery bank I require,

Am planning an off Grid system, to be independent, and avoid blackouts, and secondly to weigh up the cost difference between the state supplied and home generated units, I could do most of this on paper, but I still would not be covered for blackouts, this and the rising cost of electricity is a big concern, either way, here is what I plan.

First for me, is costing the system to provide the cheapest possible KW per Dollar, so here goes.

At present I consume 93Kwh per calendar month, and there is on average 30.4166 days per month, so if I divide my 93,000 watts by the 30.4166 days, I get 3,057.5409 watts per day, or basicly 3.0575409 Kwh per day, if this is correct so far, then if I multiply my per day usage by seven this will provide me with my weekly Kwh usage, which in the above case this time using my calculator, would be 3,057.5409×7
=21,402.7863 watts, or to round it up would be 21.403 Kwh per week,? ( something tells me that x amount of watts used per day dont coorelate to the same watts being reffered to as Kwh's, watts being a measure of energy the Kwh's a measure of energy over time ?)

have I got anything right so far, and if so, how do I calculate the Ah required for a 12 volt batery bank that will go a week between charges, the power I consume will be the same each day, drawn at the same daily rate, there will of course be times every day when current willbe drawn at higher rates, I will overcome these spikes in the daily usage cycle, by oversing the bank to compenste,


any help will be much apreciated

Thank you kindly, John

PS, I hope ive put this in the appropriate spot, if not, please move as desired.

[Tom Reed]

Wow, you are doing very well with your electrical usage @ 3kw per day. We're off-grid here and use 10 kw per day for 2 people. The typical sizing for a battery bank is 3 days and you don't want to draw the bank below 80% state of charge. Then there is inverter efficiencies to convert the DC into AC which you can roughly figure 80%. So a 14kw bank would cover you. The final number for a 12vdc bank would be 1167 AH.

I would suggest you look into a 24 volt system if you haven't purchased anything yet.

[mike90045]

Assuming 3KWh / day is correct, you want to size battery bank for 3 days before you hit 50% and HAVE to start generator.  Daily discharge should only be 20% down from full.    You generally want to recharge when you hit 75% of full, because Lead Acid batteries begin to sulphate at about 80%. 
Once a bit of sulphation happens, the acid is permanently weakened, and the battery looses capacity.   If this was to happen weekly, and you lost 5% capacity each time, your batteries would be ruined in a year. 

Waiting a week between recharges, is not a good policy.   

In the winter, I go out each morning that I can, to crank up the genset for an hour. This lets the generator run at full load recharging batteries, and then the solar (if any that day) can top them off.  If it's too stormy any one day, I skip walking to the generator, and hit day #2 on the batteries.  I've seldom hit  day #3 when I would have to brave whatever storm is there, to do a mandatory start on the generator.


If  you have a lot of 12V gear, you "could" build up a 12V system, but be advised, losses from the high currents involved in 12V systems, will cause you issues.

A "standard" 12V, 100ah deep cycle battery holds 1,200 Wh (1.2KWh) of power, 50% of which you are allowed to use.

You could use 9 KWh in 3 days, calling for a 18KWh bank.  At 12V, that would be 1500 amp hours.   But with 48V,  only 375AH of batteries. 

Parallel batteries are a poor choice, because of unequal current sharing between batteries. So using low voltage, single cells, in series, is a better option, in a series circuit, all current has to be the same.

stuff I did:   solar: http://tinyurl.com/LMR-Solar , battery lugs http://tinyurl.com/LMR-BigLug
Setting up batteries http://tinyurl.com/LMR-NiFe 

here's a couple of links to another forum, that has some more specifics on what you are asking:

http://www.solarpaneltalk.com/showthread.php?5334-Solar-Off-Grid-Battery-Design
http://www.solarpaneltalk.com/showthread.php?7592-So-You-Want-To-Go-Off-Grid-Battery
http://www.solarpaneltalk.com/showthread.php?2144-How-Many-Batteries-amp-Why-Tutorial

[r77]

Hi again, thank you all for replying, I certainly have a lot more thinking to do, I could be wrong about my 3.057 Kwh per day, I deduced this figure from my electric bill, ie, 93 units burned, each unit amounting to 1 Kwh, and if my previous calculations were ok, that leaves me with 21 or so Kwh a week, which I thought to be a lot, the house is only occupied 8hrs a day,  I only run energy saver lights lights, the fridge freezer although A rated is likley the biggest user, and will be replaced with a gas one, therws a 90 watt circulating pump for oil heating, and a 100 watt blower on the oil heater, which wont be in action much as I have a solid fuel stove to replace it , theres no electric water heaters of any kind, the shower pump is electric but the hot water is from the heating system, and I cook in gas, I hope I have interped my figures correctly, from my bill, please comment in this,

As for the batterys, I have been told that large batterys in series are a bigger problem than in paralell, it has something to do with the further away batterys in the line have to discharge through the more exhausted ones closer to the load,  and also have to deal with the added resistance of them, please tell if anyone has experianced this,

I did plan to use 12 volt 240 Ah batterys  in paralell as to increas the amps available, I could find no reason for using 24 volt batterys, as the cable to the inverter is very short and would be sized to minimise losses, however as I said, am very new to all this, so please advise me of what benefits can be gained or lost  between differing voltages,

and thank you for pointing out the inverter loses, I plan on using a phoenix multi plus, which has both the provision to charge the batterys too, AND, has the abillity to paralell with my generator, which means when I anticipate a big load I can start the genny, and set how much power it can give to the load, the exess genny power can charge, or be dumped for heating, has anyone used one of theese inverters,

I dont mean to ask so much, but I have little to no experience,

look forward to hearing from you all, thanks, John

[r77]

Thank you Tom, and Mike90045,

after reviewing what you wrote, I looked at some Inverter specifications, and can see why you both mention about rising the voltage, I see in most cases the efficency of these units goes up as voltage rises.

Most interesting is your advise in relation to chargeing, this is realy helpfull to know, I did know a little about sulphication, did not know how very damaging it could be, my initial reason to charge once a week was to keep cycling down so as to extend battery life, and to keep fuel cost down, but I can see now how badly that would have ended,

so from what you advise in relation to baterys I will only discharge to 50% and then recharge,

and on the subject of chargeing, would it be enough to recharge at an amp rate of 20% of used bank capacity,

for example if I had 24 volt 1000 Ah bank, discharged to a depth of 50%, to replace this discharged 50% would take 5 hours @100 amp, should I charge faster, or may be slower, advise please,

also thanks for the links, will have a look through them,

John

[mike90045]

....   As for the batterys, I have been told that large batterys in series are a bigger problem than in paralell, it has something to do with the further away batterys in the line have to discharge through the more exhausted ones closer to the load,  and also have to deal with the added resistance of them, please tell if anyone has experianced this .... 

I can assure you as an electronics engineer, that SERIES batteries have far less issues than ones in parallel.

Read this http://www.smartgauge.co.uk/batt_con.html about battery connections, and why parallel is not good.

[mike90045]

....so from what you advise in relation to baterys I will only discharge to 50% and then recharge,

and on the subject of chargeing, would it be enough to recharge at an amp rate of 20% of used bank capacity,

for example if I had 24 volt 1000 Ah bank, discharged to a depth of 50%, to replace this discharged 50% would take 5 hours @100 amp, should I charge faster, or may be slower, advise please,

also thanks for the links, will have a look through them,

John

Batteries are rated in charge rates of something like C/10   If you have a 100 ah battery, and it's spec is to be charged at a C/10 rate, then 10 amps (one tenth) of the Capacity is the desired charge rate.    Recharge rate has little to do with how deeply the battery is discharged (the mfgs recomended model is based on battery capacity, not amount removed) 

Additionally, just to confuse things more, the battery will accept a charge pretty readily, till it is about 80% full.  Then it starts slowing down, and the charge rate decreases, so you may go from 50% to 80% in 3 hours, and then the last 20% may take another 4 hours.   And if you remove 20% of charge, internal losses mean you will have to replace all that you used, plus another 20%, for a total of 20% out, and about 25% goes back in.   That last 20%, your genset will be loafing along at a greatly reduced load, burning fuel.   Solar makes a good buddy for this, jam charge in with the genset in the early AM, and let the solar top off the cells over the rest of the day - saves a lot of fuel.

[r77]

Read the article on paralell connections, had the same idea to connect up my batterys too, came to this conclusion because even to me it made sense to eleviate resistance differences in the cable lengths, it makes even more sense now why a higher system voltage is also a good idea,and  thank you for the link,

I have one more issue in relation to the generator I currently have, it does supply the correct voltage for the inverter i plan to use, but it is not AVR controlled, only capacitor, my question is this, is stable voltage critical, or is the waveform more important, I have no way of testing waveform, but if its a big issue I will have to get someone to test the genny, also, is it common place for inverter manufacturers to note such tolerances on their products, and what exactly do they refer to it as, i see a lot of references in their litrature but its confusing when i dont know what to look for,

any advise welcomed, thanks, John

[thomasonw]

John,

Hello.  Will pass on our experience of living in effect 'Off the Grid' for 9-10 months out of the year.  We are full time cruisers currently in the Puget Sound area.  As background we have:

• 12v 1500Ah 'industrial' battery, aka Fork Lift battery.
• 480w solar panels with MPPT charger (installed last Spring)
• 120A DC Generator (Kubota diesel based)
• 270A alternator on  main engine that delivers around 200-240A into the battery while underway.

During the core of the Winter months (early November till mid Feb, or perhaps the 1st part of March) we tend to Come In and plug into shore power.  The rest of the time we are 98% independent of shore power sources.   We tend to stay on one place 4-5 days, and then move on.  During this moving time (about 2-4 hours typical) the mains alternator will recharge the batteries.   I would estimate our daily energy usage at 2.5-3.8KWh / day powering a small AC refrigerator, Engal Freezer, water pumps, a couple of hours of TV and lights.  A BIG variable (surprised us) is how much we use the computers - two lap-tops.  That alone can add as much as 1Kwh / day of usage.


<PRE-SOLAR PANELS>
I had originally sized the battery system to provide power for 4-5 days while 'at anchor' with the idea of recharging while underway.  The batteries we have can do that, unless we use the computers every day.  However doing so we are not able to totally replenish the batteries while underway.  We figured we needed 1-2 hours of underway for each day at anchor.  And on short movements (many are 2 hours or so) we came up short and needed to run the generator to make up the gap.

Due to the battery size we would need to run the generator many hours (4-8) to totally top off the batteries.  And this we found unacceptable.  In practice, we ended up running the generator each day 1-2 hours around 1pm.  We  picking this time mostly because we both tended to be up and active with other projects at that time and were not as bothered by the noise as say at 9am, or 9pm.  Plus, typically the wind would pick up in the afternoon to mask the sound.

So in practice we ended up using our system like this - running the generators every day for an hour or 2 as opposed to waiting 4 days and needing to run it 6-8hrs.  So, in effect the large (4-5 day) battery bank was not needed.

I will also add a wild-card was the cloths washer.  We have a small all-in-one, and need to run a load every 2-3 days.  We try to do this while underway, or while running the generator.

Now, when we added the solar panels we found we no longer needed to run the generator daily.  IN fact, after installing the panels we did not run the Generator until mid September, and that was only while doing Landry.  But we do need to point out that we did run an energy depletion each day, but it was much smaller and we were able to replace the power when moving - even on the short 2 hour trips.. 

Should also point out - we have a diesel boiler that provides daily hot water needs (however, during June and July we could at times turn on the Electric water heat and use excess solar!).  If we depended on electricity for out hot water needs the picture would be much different.

So, I think bottom line:

• Going into this we - like you - though to get a very large bank and not need to 'hear" the generator but every 4-5 days.
• In practice, such a usage required SUCH long run times on that 4/5th day it drove us nuts.
• Instead, we settled into a pattern of running the generator daily for 1-2 hours.
• Solar Panels greatly reduced the time needed to run

Doing it over again, I would size the battery bank for 2 days MAX and plan on replenishing the batteries every day through a combination of Solar and Generator.  Though in our case with the boat movement every 3-4 days I might size around that patterns + Solar panels.  This might drive me to a slightly larger bank then a straight 2 days max.  But doing it over again, I would NOT have gotten the 1800lbs of batteries we currently carry.

And of course, if you installed co-gen system to capture heat - that could be used daily for hot water in the afternoon and into the evening.  Further justifying a point of daily generator running.

And finally, just a point of clarification:  Issues occur with a large bank of parallel batteries, not series.  Unless you are talking about small Li-Ion cells in mass.   Those need special charge controllers to deal with serial cell issues.  For  land based system I would look at 24v or 48v FLA battery.  Given your low usage, you might be able get away with one bank of 2v L-16 sized cells ala these:  http://dynobattery.com/Spec2vL16.pdf

And side note:  I selected 12v to keep all the boats systems the same voltage as I have two  (redundant) DC systems:  House and Starter that can be used interchangeably.  This drove the need for both to be the same, and the engine was already a 12v system. . . .   

[Mad_Labs]

Batteries discharged to 50% and then left to sit for days until charged won't last long. In fact, batteries discharged to 50% won't last long even if you do charge 'em right back up.

I have a 1200aH bank of golf cart batteries and use about the same amount of juice that you do. I never go more than a day with no solar before I charge 'em. These batts are coming on 4 years old and are showing some signs of age. I have been very good about not discharging below 70% but I don't do the absorption stage very well or often, mostly due to not having enough solar.

Thing about battery banks is that they are like children, the need constant attention and feeding. And if you don't have the charging source, a bigger bank can be a liabililty. When I first moved off grid and bought the batteries, I only had a few panels and a small charger and it was hard to keep the batteries topped up.

Anyway, I think you'd be fine with a 1200 aH bank like mine but you are going to need to charge it way more than once a week.

Jonatahn

[r77]

Good day everyone, once again I thank you for your help, mike90045, realy apreciate you explaining about charging time and and amp considerations, this will certainly help me balance out an apropriate system.

thomasonw, good to hear from you, thank you for taking so much time in explaining your setup, from what you wrote about chargeing times, I can see the sense in using a combined aproach, initially i planned on useing a generator only, but thanks to mike90045 I now understand the finial stages take a lot longer than I was aware, will need to look into either wind or solar assist.

I see you mentioned diesel boiler, further down the line I plan to build a steam boiler, being a welder this wont be much of a problem, but geting it to heat my home AND run a generator, to guess what, " charger batterys" poses some issues that I need to work on, " high pressure for the steam generator ", while integrating a low pressure side, or even cycle to heat radiators, anyway, I will get the battery bank up first,

heres are links might interest you as you have a boat, first is finished product, second details construction,

http://www.steamboating.net/page6.html
http://www.offroadfabnet.com/forums/showthread.php?t=6906

thanks for your help, John

Ps,regarding your chargeing cycle, how long do you estimate your battery bank to last,

[r77]

Hi jonathan, thanks for commenting, very true about my initial chargeing plan, following a link mike90045 posted, am now acutely aware of this issue, my hope is to discharge to a maximum of 30%. and charge every 2-3 days, deppnding on final system size, am also thinking of puting the batterys on a bed, that I can viberate to slow or break down any formation of crystals, thinking about a 30 second session, just before chargeing, and depending on feedback, am wondering if there needs to be a time gap between this ajitating and commencing chargeing, does anyone have an opinion on this viberating idea, will it bennefit the batterys any?,

thanks everyone, John

[Ronmar]

DO you know the difference between a 24 month car battery and a 48 month car battery?  the answer is the depth of the sump to catch the debris that sluff off of the plates and ultimately shorts out the cells.  I would NOT reccomend shaking the cells, I doubt you will shake off any sulfur anyway, at least not without shaking off lead also.

the best way to avoid sulfation is to maintain your cells above 80%

[thomasonw]

Good day everyone, once again I thank you for your help, mike90045, realy apreciate you explaining about charging time and and amp considerations, this will certainly help me balance out an apropriate system.

thomasonw, good to hear from you, thank you for taking so much time in explaining your setup, from what you wrote about chargeing times, I can see the sense in using a combined aproach, initially i planned on useing a generator only, but thanks to mike90045 I now understand the finial stages take a lot longer than I was aware, will need to look into either wind or solar assist.

I see you mentioned diesel boiler, further down the line I plan to build a steam boiler, being a welder this wont be much of a problem, but geting it to heat my home AND run a generator, to guess what, " charger batterys" poses some issues that I need to work on, " high pressure for the steam generator ", while integrating a low pressure side, or even cycle to heat radiators, anyway, I will get the battery bank up first,

heres are links might interest you as you have a boat, first is finished product, second details construction,

http://www.steamboating.net/page6.html
http://www.offroadfabnet.com/forums/showthread.php?t=6906

thanks for your help, John

Ps,regarding your chargeing cycle, how long do you estimate your battery bank to last,

John,

Hello.  Guess I could have been a bit more explicit on the Diesel Boiler - it is not a steam boiler, but a hot water boiler.  (http://itrheat.com/products/hurricane-heating-systems/).  The hot water is used for domestic heating, hot water, and can be (in the future, once I finish the project) be used for pre-heating of the engines.

Funny though  - when in Portland a slip-mate of ours had an actually steam powered boat.  In fact, the ONLY (according to him) twin-engine steam vessel in private ownership (small boat) in the USA.  Has an interesting history that started with a wonderful ships wheel he built and had in the living room  . . . ."Hey Dave, you really need to build a boat to go with that wheel" and he did!


On battery life, we are using these: http://dynobattery.com/prod04.htm - the D85-29's, and my notes have in it 'designed for 4,000 cycles at 80% DOD'.   This seems excessive (the DOD), but that is what I collected when I was researching batteries.  And cycles is an interesting topic that many folks do not seem to think about.  From my memory the common 'golf cart' batteries last longer then typical RV/Marine deapcycle batteries (~500cycles vs. 100 cycles, IIRC).  While the L16 Floor Sweeper can have 50% to 100% more cycles (750-1,000?) , and these fork lift ones even longer life (2-4,000?).


More reading, looking at these three batteries from the Trojen Website:
http://www.trojanbatteryre.com/pdf/agm_trojan_productlinesheet.pdf    Group 27 battery = 550x at 80% dod
http://www.trojanbatteryre.com/PDF/Premium_Trojan_ProductLineSheet.pdf   Golf Cart/L16 = 1,000x at 80% dod
http://www.trojanbatteryre.com/PDF/Industrial_Trojan_ProductLineSheet.pdf  Industrial = 1,600x at 80% dod

Like I said, 4,000x at 80% dod seems a bit high for my Dyno industrial batteries, but the point is:  All batteries are not created the same and looking at expected lifecycles is important vs. just looking at AH capacity when comparing costs.  And of course, not discharging too deep helps as well.

[Tom Reed]

Another thing to think about when selecting batteries is how the plates are alloyed. In forklift/industrial batteries one of the criteria for a long lasting battery is how well it holds up to vibration and shaking from vehicle movement. Antimony is alloyed with the lead components to make them harder to withstand fatigue better. The down side to this is that they have a higher self discharge rate.

Solar batteries are alloyed with calcium since they are stationary. The calcium alloy is softer but has a much lower self discharge rate.

And then there is the AGM/gell batteries which have some advantages when generator charging because they can accept higher charge rates and don't need maintenance, but are can be quickly damaged by an over charge.

Our off grid system has a 1055 ah 48vdc bank of Hawker PV1 batteries. These are 865 pound monsters and the 4 of the weigh about 3500 lbs. Life expectancy is 10-20 years.