Power backup: Predicting battery life expectancy
Uninterruptible power supply (UPS), power
inverter and hybrid solar photovoltaic (PV) systems have become central to the
design and implementation of standby power supply strategies. At the heart of
these systems are rechargeable batteries, elements as important as the design
of the systems themselves - and possible weak links unless they are maintained
and serviced regularly.
Correct battery care will help ensure
the reliability and predictability of power provisioning systems. For example,
batteries – particularly commonly-specified flooded lead-acid batteries - must
be selected correctly and sized appropriately to ensure they meet their maximum
expected life span.
Battery suppliers and power provisioning
specialists are able to provide information for sizing batteries which varies
according to a number of criteria.
In this light, an understanding of the
various operating parameters, maintenance requirements and cost is vital in
choosing the most advantageous battery for any particular application.
It’s important to be aware of the
conditions that negatively affect performance and battery durability. What
factors should be considered?
Ambient
operating temperature.
High ambient temperatures shorten
battery life by a factor of two for every 10-degrees Celsius increase above 25
degrees C. So when batteries are exposed to large swings in temperature, life
expectancy can vary significantly. This is why modern batteries, although
marketed as ‘maintenance free’ must be regularly checked and analysed to
prevent untimely failures. In typically hot South African climate conditions,
batteries with a stated 10-year lifespan will generally require replacement
after seven years unless carefully maintained.
Ripple
currents.
Regular checks will help to identify and
rectify issues such as damaging ripple currents that can also cause premature
aging. A ripple current is generated by an uninterruptible power supply (UPS)
system or inverter which can cause an internal temperature rise due to power
losses within the UPS/inverter capacitor which in turn raises the overall
battery temperature with consequent loss of performance and longevity. Maintenance
involves replacing AC and DC capacitors on a regular basis.
Depth
of discharge.
Battery longevity is directly related to
the level and duration of discharge and the stress inflicted upon it. To
protect a battery from over-discharging, the specified ‘end-of-discharge’
voltage - the point at which a battery is considered fully discharged – must be
respected. It is good practice to stop discharge at this point otherwise the
result will be a reduced battery lifetime.
It is good practice never to discharge a battery beyond a maximum of 50
or 60%.
Overcharging.
Overcharging a battery is equally
damaging as undercharging it. Batteries should always be allowed to cool after
charging because the heat generated during the recharge and discharge cycles will
accelerate grid corrosion, which is one of the major causes of battery failure.
Sulphating.
Batteries should not be stored in a
discharged state. The sulphate that forms during discharge should not be
ignored because severe sulphation could make the battery impossible to recharge
fully.
Inactivity.
Inactivity can be harmful to deep-cycle
batteries. If they sit for several months, a ‘boost’ charge should be given
once a month in warm climates and every two to three months in cold climates.
This is because batteries discharge faster at higher temperatures than at
colder temperatures.
Maintenance
– the art of battery balancing.
Substantial savings can be realised by
regular servicing of a battery pack and managing its lifecycle. In this light, the
key to battery
longevity – and the maximising of an investment in batteries - lies in a
thorough an understanding of the status of batteries in terms of their duty
cycles and load factors.
Batteries such as those used for modern
energy storage systems – either linked to solar photovoltaic systems or used in
conjunction with commercial grade, long run uninterruptible power supply (UPS)
systems - are usually made up from strings of cells in series in order to
achieve the higher operating voltages required.
They are particularly vulnerable to
premature failure. The problem can be compounded if parallel packs of cells are
required to achieve the desired capacity or power levels.
Providing a solution to this problem is
the Powermode-supplied Q-on LR Battery Pack. It represents a ‘first’ for the
energy storage industry in that it is backed by a comprehensive three-year,
on-site guarantee.
Underpinning Powermode’s guarantee is
the ‘smart’ battery balancing/equalising technology built into the pack. This
includes a computerised harness that automatically monitors and reports - via a
‘cloud-based’ portal to the Powermode Management Centre - on a range of
parameters associated with individual battery cells in the pack.
Data streams containing information
critical to the well-being of these cells, including temperature, state of
charge and depth of discharge, are monitored and a tally of the number of
discharge/charge cycles is recorded.
In addition to a return on investment
(ROI) boost, this knowledge gives users the peace-of-mind that comes with
having Powermode in charge of the management and regular servicing of their
Q-on LR Battery Packs.
Importantly, the incremental investment
in a Q-on LR Battery Pack is more than offset by the savings associated with
the escalating costs of early battery replacement.
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