SPECIFYING BATTERIES, CHARGERS
& ALTERNATORS
The inverter will require Deep
Cycle Lead Acid Batteries
of appropriate capacity.
Lead-acid batteries can be categorized by the type of application: automotive service -
S
tarting/
L
ighting/
I
gnition (
SLI
, a.k.a. cranking) and
deep cycle
service
SLI Batteries
Everybody is familiar with the SLI batteries that are used for automotive starting and
powering vehicular accessories. SLI batteries are designed to produce high power
in short bursts but must be constantly recharged (normally with an alternator while
driving). Vehicle starting typically discharges 1%-3% of a healthy SLI battery’s capacity.
The automotive SLI battery is not designed for repeated deep discharge where up to
80 % of the battery capacity is discharged and then recharged. If an SLI battery is used
for this type of application, its useful service life will be drastically reduced
Deep Cycle Batteries
Deep cycle batteries are designed with thick-plate electrodes to serve as primary
power sources, to have a constant discharge rate, to have the capability to be deeply
discharged up to 80 % capacity and to repeatedly accept recharging. They are
marketed for use in recreation vehicles (RV), boats and electric golf carts – so they may
be referred to as RV batteries, marine batteries or golf cart batteries. There are two
categories of deep cycle lead acid batteries – wet and sealed. A wet cell battery has a
high tolerance to overcharging. However, it will release hydrogen gas when charging
that must be properly vented and the water level must be checked frequently. Sealed
batteries can either be Gel Cell or AGM (
A
bsorbed
G
lass
M
at). Both the Gel Cell and
AGM are maintenance free, have no liquid to spill and gassing is minimal. The Gel Cell
is the least affected by temperature extremes, storage at low state of charge and has
a low rate of self discharge. An AGM battery will handle overcharging slightly better
than the Gel Cell
Units of Battery capacity
The battery capacity is the measure of the energy the battery can store and deliver
to a load. It is determined by how much current any given battery can deliver over
a stipulated period of time. The energy rating is expressed in
A
mpere
H
ours (AH).
As a bench mark, the battery industry rates batteries at 20 hour rate i.e. how many
Amperes of current the battery can deliver for 20 hours at 80 º F till the voltage
drops to 10.5 Volts for 12 V battery and 21 V for 24 V battery. For example, a 100
AH battery will deliver 5 Amperes for 20 hours. Battery capacity is also expressed as
Reserve Capacity (RC)
in minutes. Reserve capacity is the time in minutes for which the
battery can deliver 25 Amperes at 80 º F till the voltage drops to 10.5 Volts for 12 V
battery and 21 V for 24 V battery.
Approximate relationship between the two units is as follows:
Capacity in AH = Reserve Capacity in RC minutes x 0.6
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