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.