Page 30 - Energize July 2021

NEWS

How to choose a battery for a solar application

Information from Silicon Engineering

hen specifying a storage battery for a solar application, a number of factors
must be taken into account. If we consider a 200 Ah battery requirement
Wfor a solar installation, the first measurement would be the discharge rate.
Stationary batteries re usually specified at C10 (20 A for 10 hours) and solar batteries at
C100 (2 A for 100 hours). These discharge rates give different capacities.
For example, the solar GEL battery Type ES-12200 is rated at 220 Ah @ C100,
200 Ah @ C20 and 187 Ah @ C10.
Knowing the actual discharge time may allow us to optimise the battery further.

TYPES OF BATTERIES
SLI
The lowest cost battery is the SLI (starter, lights, ignition) battery which is normally flooded
so must be kept upright at all times. It is designed for vehicle applications where the
design criteria are high start amps for a few seconds, low cost, low mass, and a life of
three years. These SLI batteries are brilliant in achieving these objectives but when used in
solar applications are very disappointing. put another way, have a lower mass.
They are not designed for deep discharge and service life of less than 50% of the expected If it moves, then one of the Lithium
three years is often the result of deep discharge. We do not recommend this technology and do batteries is probably the best option.
not offer this battery. SLI batteries could cost 50% to 75% of an AGM technology battery. Theoretically they are three to eight
times lighter, but in practice one can
AGM expect a mass saving of 50% when
AGM (absorbent glass mat) technology batteries use “sealed” cells with vent caps to allow compared with lead acid. This loss of
hydrogen to escape and are referred to as VRLA (valve-regulated lead acid) batteries. They size advantage is due to the technology
usually use a flat plate design. These have excellent short-term current ability and are used and extra electronic and ancillary
extensively in UPS applications where deep discharges are infrequent. equipment which protects the Lithium
The failure mechanism of a lead acid battery is positive plate corrosion. The published battery from under- or over-charging,
anticipated life is often based on the period the positive plate takes to corrode away without over-current and high temperature, and
other failure mechanisms. is usually called a battery management
AGM cells are often quoted as having a 12-year lifespan but often give an operational system (BMS).
life of five to seven years due to other failure mechanisms. These batteries are susceptible to Lithium batteries have high cycle life of
high ambient temperatures and high ripple currents which must be avoided. Drying out of the 3000 cycles. They also cost more.
battery is one of the failure mechanisms. This is rated as 1200 cycles at 50% DoD.
BATTERY COSTS
GEL Typical cost per cycle at 12 V:
The lead acid GEL battery has a jelly electrolyte which has been created by adding silicon LiFePO4 = R5,00/cycle
dioxide (SiO2) to the acid electrolyte. This causes cracks in the GEL when it sets and allows AGM battery = R4,46/cycle.
the recombination of O2 from the positive plate through the GEL to the negative plate. GEL Flat plate GEL = R3,32/cycle
technology is used in flat plate and tubular designs. GEL batteries have more electrolyte and Tubular GEL = R2,18/cycle.
drying out is less of a problem than with the AGM technology.
The GEL battery also has a better cycle life than an AGM battery, but the cost is about 10% All “sealed” batteries have the problem of
to 15% higher. These batteries are rated at 1800 cycles at 50% DoD. starting their life when they are made and
The best technology for solar applications is the tubular cell and is used for professional and not when one takes delivery.
commercial applications. The capital cost is higher than for flat plate cells but the amortised life Silicon Engineering ensures its
cost per cycle is lower. For comparison, tubular GEL batteries have a German-tested cycle life, “sealed” batteries are generally not older
under IEC solar cycles, of 6400 cycles. than six months when sold. The German
BAE OpzV Tubular GEL battery is only
Lithium manufactured to order, so delivery is 14 to
The Lithium battery family includes a number of types. LFP (Lithium iron phosphate LiFePO4), 16 weeks.
NCM (Lithium nickel manganese cobalt oxide), LCO (Lithium cobalt oxide, LiCoO2), LMO
(Lithium manganese oxide LiMn2O4), and LTO (Lithium titanate). These generally trade off Contact Irene Blythe, Silicon
energy density for safety. Engineering, Tel 011 474-3423,
Lithium batteries have the advantage over lead acid of higher energy density or to Irene.b@silicon-online.co.za

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