Installation Manual

32
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 C20) 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
(C20). Alternatively, the same battery can support a controlled 34 Amp draw over a
10 hour period (C10), meaning it supplies 340 AH capacity for that period of time.
Batteries which are discharged should be recharged as promptly as possible.
A Renewable Energy 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 (C20) rate of the battery bank, or
the C4, C5, or C6 rate of the battery. Using the Absorption charge time calculation
( charge current of 10% of the C20 rate of the battery bank will take approximately
4.2 hours, plus the untimed bulk phase (usually about 1-2 hours) to bring the
bank from 50%-100% state of charge. This is an ideal scenario as a lower charge
current will often result in deficit charging as this increases the required Absorption
charge time 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 a full state of charge on a regular basis.
This often causes sulfation problems and capacity loss which then needs to be
addressed through corrective equalizations.
Finally the last issue with some systems, after installing/commissioning end users
will inadvertently add more loads after the installation causing problems with how
often the battery bank needs to be recharged, thus increasing cycle life usage.
This needs to be avoided, and can be by properly educating the customer at the
time of sale.