Battery sizing FAQ

[veggie]

This may be of some use to people sizing their battery banks.

Cheers,
Veggie

[mike90045]

Thanks !

Now I have to dig up a Charging FAQ, so the depleted batteries do not sulphate 

[rcavictim]

Thanx for posting that Veggie.  That is a very nicely written primer on the subject of planning a battery bank for the loads at hand.

[WGB]

So the short hand version is,
man I need a lot of batteries!

[mobile_bob]

i personally ascribe to the determination of how many amp/hrs do i need over a 24 hour period, then

i take that number and divide by 3, then multiply the result by 10, this gives me the total battery bank size in amp/hrs
needed to allow me to cycle between 50 and 80 SOC daily, and then go from 80-100% once every week to ten days.

so if i figure the need is 300amp/hrs per day, divided by 3 = 100, therefore 100 x10 = 1000amp/hr bank

i can then figure on running the battery bank down to 500 amp/hr, then recharge to 800 amp/hrs daily
this of course means i need to put back 300 amps/hr daily from my charging system, plus about 10-15%  
or about 345amp/hrs daily. (the extra 10-15% covers the battery charging inefficiency of the cells themselves)

from that i can then figure that a 1000amp/hr flooded lead acid battery bank can take approx 25% max charge rate
in amps or about 250amps maximum, which means technically if i can provide those 250amps it would take approx 1.4 hrs
of run time per day.

however the reality is likely closer to 2 hours run time per day because of charging system limitations and lower charge rates
are easier on the batteries anyway.

this means  i would need approx 175 amps of charging capability, to get the charging done from 50 to 80% daily in about 2hours
of run time.

one can also reduce the system requirements in amp/hrs if he can schedule some of those heavy loads to be serviced during that
two hour charging window each day, removing those sheduled loads from being serviced by the battery/inverter system not only reduces
their size but improves on system overall efficiency because those scheduled loads do not need to be provided for by multiple step conversions.

bob g

[Lloyd]

Again Professor what you say.

But what I also find important is Peukerts law http://www.mpoweruk.com/performance.htm.

I never see anyone think about that in their anecdotes of bank size. It was ok when we mostly used banks for simple discharges...today we are expecting our banks to run power to meet what it is we expect from the grid. A bank sized nominally for a 24 hr load will be absent the Peukert equation for such equipment as AC, well pumps, and any of the other grid run draws that we have become so accustomed to that draw large amp loads.

Most banks problems are a result of poor planning, poor installation, and outdated thinking. We strive to reach all of the efficiency in charge equipment, while ignoring the basics of the system...first use good  type three tinned cable...spend a couple of extra bucks in the quality/size of cable, use lead plated copper terminal lugs, and you will see the bank efficiency increase the over all system efficiency.

Lloyd

[mobile_bob]

Lloyd:

thanks for the link, i think a lot of folks will find it very helpful

when i posted my method of determining battery bank sizing, i incorporated most of what is needed,
and tried to present it in a way that most folks would take the time to understand,

it seems too often folks will just take a stab in the dark, or take what some salesman tells them they need,
or some other well intentioned but ill informed person tells them, and they end up with either a massively oversized
battery for what they have to charge with, or one that is on the other extreme and terribly undersized.

as they say batteries don't die of old age, they are murdered.
this seems true of the first set for a great many people, they need to kill a set first before they will take the time
to actually study up and learn exactly what they need for batteries and as importantly what and how large a charging system
they need to maintain the bank properly.

even well designed and engineered systems have issues with undercharging if the system is not checked out properly and monitored
on an ongoing basis,, resulting in loss of capacity a bit by bit, until what started out to be a nice size battery bank is just not able to carry
the load for the needed length of time.

a set of batteries that might have lasted 15 years can be cut short and start to have serious problems at even a year from chronic undercharging.

and of course that same set can be murdered in less than half a year by chronic overcharging, overheating and lack of water.

there is so much good info out there, if a person takes the time to read it.

my problem is i have spent the last 10 years researching flooded lead acid batteries, and now i am strongly considering AGM batteries and have to
start over again to learn about their use, charging and maintenance.

we need a white paper that condenses all the info into something that is readable and understandable to the average diy'er in my opinion.

lets see now, who can we get to take on that project, hmmmmmm

how bout it Lloyd?



bob g

[Lloyd]

Hi Professor,

While you were responding here, I was busy putting up a story here http://www.microcogen.info/index.php?topic=682.0 of a certain true life story.

I will take some time and put together a wp on bat banks.

Right now my money, for most situations is still on conventional lead acid bat banks, especially if I can interest the owner in some proper maintenance. The internet is full of peeps that will say that the water saver caps are a waste of money, usually at $4-5.USD per cell (one 12v=6 caps), time the # of bats....but de-watered bats is the number one killer, outside of leaving a bat in an un/partially charged state. Typically a good 12v 240 amphr bat(2x6for12) is $250-300USD. 6 caps(12v) x5=$30, 2 6v to make 12v 2x6x5=$60USD, worst case cenero=about 2% of investment, which I call cheap insurance.

A well maintained conventional, will out cycle an agm, at half the initial cost, I think we are looking at about about .06 cent KWh for amortization of a well maintained conventional wet acid, and some where north of .12 cent KWh for a similarly maintained AGM, you can multiply by 3 the KWh for gels.

For the new tech in bats right now it's just below the gels.

Remember we can have the most efficient charge sys, and with a poor un-thought out bat bank in tow the sys efficiency can drop by 50%.

That's my story.

Lloyd