User Manual

34
DISCHARGE RATES
Discharge and recharge rates should be considered when selecting and sizing
a battery bank. Battery manufacturers publish multiple discharge rates for each
battery model, which range from 100 hrs to 1 hr. These are often referenced for
various type of applications. The most common in Renewable Energy applications
is the 20 hr rate as this closely matches a 1 day period. The rating, (ex C/20)
refers to a controlled load (Amps) which can be placed on the battery for a period
of time before the battery voltage reaches 1.75 VPC (volts per cell). A high amp
draw may be run for a shorter period of time and vice versa.
As an example, a 400 AH battery can support a controlled 20 Amp draw for 20
hours (C/20). Alternatively, the same battery can support a controlled 34 Amp
draw over a 10-hour period (C/10), meaning it supplies 340 AH capacity for that
period of time. Batteries which are discharged should be recharged as promptly
as possible.
An off-grid PV system should be designed to provide a charge current that is
capable of recharging the batteries quickly, efficiently and within the window of
time when the system is generating peak power (peak sun). The charge current
should be within 10-20% of the 20 Hr AH rate (C/20) rate of the battery bank,
or the C/4, C/5, or C/6 rate of the battery. Using the Absorption charge time
calculation (charge current of 10% of the C/20 rate of the battery bank will take
approximately 4.2 hours, plus the Bulk phase (usually 1-2 hours) to bring the bank
from 50% to 100% SOC. This is an ideal scenario as a lower charge current will
often increase the required charge time beyond the available sun hours
causing deficit cyling and/or the use of supplemental charging such as a
generator. Often customers who need to supplement charge with a generator
do not run it long enough to allow the batteries to reach 100% SOC on a regular
basis, causing sulfation problems and capacity loss which then needs to be
addressed through corrective equalizations.
In some situations, after and energy audit has been completed and the system
is sized and installed, the customers will inadvertently add additional and/or
unexpected loads. As system size is calculated with a desired depth of discharge
and charge requirement based on expected usage, this increases both the storage
needs and subsequent charge source to bring the battery bank to a full charge.
It will often cause problems as a result of deficit cycling and over-discharge which
will shorten cycle life.
To avoid this, the customer should be made aware of the importance of properly
sizing the system to meet their specific needs. Discuss any expected short/long-
term changes in usage and set the expectation that storage and charging needs
may increase to meet their changing requirements.