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NiFe Batteries

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BigBreaker:
It would be five times not 20% more.
A 200 amp-hour (labeled) lead acid run down from 100% to 80% and then recharged has one fifth the effective capacity of 200 amp-hour ni-fe battery that can be run from 100% down to 0%.  The fair comparison would be a 200 amp-hour lead acid and a 40 amp-hour ni-fe.  The lower you go on lead-acids before recharging, the shorter their life.  It may still make sense for you to buy lead-acids but don't expect them to last long if you are running them all the way down.

scottsAI:
No argument, the NiFe battery handles abuse Way better than Lead acid, if abusing it is an every day thing NiFe is the best choice. But just what does abusing it mean? Lets read on about golf carts.
Looking for applications abusing lead acid battery's.

Do we really need to work at the top 20% to get decent life from a battery?

Fork lift or golf carts are good examples.
Pick Golfing since often recommend golf cart batteries for RE applications.
Ask around at local golf courses with electric carts:-). Some golf courses replace the batteries every couple year. Others let them go until near dead. What I hear a set of batteries may last 3 to 8 years.

http://golfcartsalesllc.com/tech.shtml

"Batteries last from 3 to 8 years under normal circumstances". (bottom of page)
Most RE systems are designed for a number of days power.

Worst case for MI is around 11 days, to size a battery for this is way over Kill, worst case does not happen Every year! Just may seem like it.
Battery charging efficiency, to operate above 80% SOC is deliberately working at the lowest charging efficiency of the battery. Makes NiFe look good.
If a golf course can get 3 (and more) years with deep cycling their lead acid batteries, why can't RE? With better care extending it out to the 7-8 is very reasonable expectations. Mine have 3 years and still going.
I ran into a guy off grid using solar panels, he had a HUGE battery 11 days only discarged to 50%, designed for worst case. His battery was about 5 years old and not holding a charge past 5-6 days forced to run the generator. Equalizing charge was done Quarterly using a generator. Expensive battery. Solar panels were sized about 20% larger than his normal (average) loads. Realized due to the battery charging efficiency above 80%, the batteries were  fully charged only once each quarter. Batteries were sulfating due to all the time below fully charged. Considering the charger efficiency and battery charging efficiency the battery on the average was never above 80% SOC! To be honest I did not figure this out until years later. I had him deep cycle the battery which brought back some life. Told him to deep cycle the battery to 50% before each equalizing charge. I took care of a 1Mwhr battery in the Navy. We discharged the battery 25% or more before each equalizing charge. Battery was replaced every 7 years, still had most of its capacity. Very expensive battery. The battery was pampered, frequent SP readings, watered weekly, cell voltage measured every hour etc. any changes the battery was immediately gone over to see what was going on.
What are the dangers of following this advice?

Why saving 75% on the cost of a battery.

Deep cycle lead acid batteries are designed to be deep cycled. To baby them is a huge waste of money. Yes the life may be longer than the 25% sized battery, yet the life will NOT be 4x. Charging system must be much larger to keep battery nearly fully charged more frequently than once each quarter. Doing an equalizing charge more than quarterly actually abuses the battery. :-) don't you love it?

Understand industrial batteries life are around 20 years, if you get 5 or so years from your battery, by 20 years you will have spend less than an industrial battery.
In summery

I would size NiFe to handle what I needed and make the Lead acid 20% bigger to match the capacity.
Be careful of optimizing one efficiency, each system has many possible efficiency's, to over optimize on one and ignore the rest will not get the results your looking for.
Have fun,

Scott.

BigBreaker:
True deep cycle lead-acids are a different beast.  Fork lift batteries don't last forever, but close!  They can really take a beating too.  Also the capacity below 70-80% SoC for lead-acids is like a built-in reserve which is handy.  In most circumstances lead acid is cheaper.  In five years we may have super batteries that with all the best features, so a cheaper near term solution could be smart.
I don't want to pamper a battery.  I don't want to think about equalization charges or whether I'm burning through battery life by going too low in SoC.  I'd rather have a smaller bullet-proof battery for 48 hours and a generator.  But that's me.

shay:
The off grid battery system in practice is well described by Scottsai. My system (currently lead acid golf cart batteries) does not use a charge controller. Either additional loads are added or panels subtracted to control things. The angles of the sun and hours of sunlight during the year call for some monitoring but usually it purrs along for a month at a time and I supplement electrolyte at the same time of making other changes. I'll be changing to NiFe batteries soon (see other comments by me-Shay).
Usually, operating the lead acids in the top 10-20% of charge is not possible without having 'sleepy' batteries. This is aggravated by age when individual cells differ and begin to slack off or take more of the load. I've never gotten a weeks worth of energy out of lead acids (40-220Ah golf carts) here on the homestead. Dissimilar cells can drag a system down. Equalization helps, so do it once a month if possible. In practice, the details of tending lead acid deep cycle batteries (replacement, especially) add up to using Edison/NiFe cells if you can get them for less than 7-10 times the cost of cheap golf cart deep cycle lead acids. A bad cell can be replaced. No controller needed. Complete discharge doesn't damage the battery, etc. I expect that the bank size can be reduced by half as well.
Do your homework when sizing and purchasing NiFe cells. I'm currently haggling over misrepresentation, "bait and switch type" tactics with a NiFe seller. Changshou Hongba makes good NiFe's. More to follow........  

shay:
$400 for 500Ah NiFe from China in plastic case.

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