A brief overview of the different types of batteries that may be used in solar electric and backup power systems.
The common automobile batteries in which the electrodes are grids of metallic lead containing lead oxides that change in composition during charging and discharging. The electrolyte is dilute sulfuric acid.
Even after over 100 years, the Lead-Acid battery is still the battery of choice for 99% of solar and backup power systems. With the better availability during the last few years of the new AGM batteries and the true deep-cycle batteries, we feel that there is little reason to use any other type. Industrial type batteries can last as long as 20 years with moderate care, and even standard deep cycle batteries, such as the golf car type, should last 3-5 years. Intermediate batteries, such as the S460 and other batteries made by Surrette should last 7 to 12 years.
NiCad (Nickel Cadmium)
Alkaline storage batteries in which the positive active material is nickel oxide and the negative contains cadmium.
My impression of traditional pocket plate NiCads--this is a turn of the century technology--is that they have many good points--low self-discharge, non-freezing, and so on--but their CYCLE LIFE IS NO BETTER THAN, IF AS GOOD AS properly chosen lead-acids. To put it another way, they have long life in *chronological terms,* but not in *cycle* terms. This makes them good for emergency/standby systems, but not for systems with a daily cycle. Not recommended for most solar or backup power systems.
NiFe (Nickel Iron)
Energy storage density = 55 watts per kilogram
Alkaline-type electric cells using potassium hydroxide as the electrolyte and anodes of steel wool substrate with active iron material and cathodes of nickel plated steel wool substrate with active nickel material. This is the original "Edison Cell". Very long life.
..."Our experience with customers using alkaline batteries in stand alone AE systems suggests that they may have as many drawbacks as advantages when compared to lead-acid type batteries. We suggest that potential alkaline users evaluate the economics and performance claims carefully to determine the suitability of any battery being considered..."
This also means that the output voltage varies with load and charge much more than other batteries. If you are using an inverter, the inverter needs to be designed with these voltage swings in mind. You may not be able to use NiFe's if your system depends on a stable voltage, for example if you are running certain common DC appliances such as a refrigerator directly off the batteries. Also when using NiFe's to power DC lighting, you will notice the light intensity fluctuates. One could always use a voltage regulator to feed those appliances that need it, but that would decrease the efficiency even more.
Currently, it appears that the only source for new NiFe batteries is from Hungary, and we have heard mixed reports on them. In short, we do not recommend them unless they are nearly free. The high losses in charging and discharging will add an extra 25-40% to the size of the solar panels you will need for the same energy usage.
In short, despite some hype about long life and thousands of cycles, we feel that overall these batteries are a very poor choice for all solar applications.
There are other type of batteries out there, but most are far too expensive for any but the most specialized application - such as space shuttles.