Lead Acid
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.
Downsides:
- Very expensive
- Very expensive to dispose of - Cadmium is considered VERY hazardous.
- Low efficiency (65-80%)
- Non-standard voltage and charging curves may make it difficult to use some equipment,
such as standard inverters and chargers.
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..."
Christopher Freitas Xantrex
Downsides:
- Low efficiency - may be as low as 50%, typically 60-65%.
- very high rate of self-discharge
- high gassing/water consumption
- high internal resistance means you can get large voltage drops across series cells.
- high specific weight/volume
- can reduce the overall efficiency of the solar system as much as 25%
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.
Other Technologies:
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.
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