INCORPORATED
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General Information
ITC-Series
Inverter
Page 26
10/07/05
This rule of thumb can be used to estimate the minimum alternator size required for your application. It may
also be used in calculating the minimum size battery required when operating from an alternator and battery
combination; a photovoltaic panel and battery combination; or when operating from battery alone. The following
examples should help to clarify the use of this rule of thumb.
Example
What is the DC current draw of a 12-volt DC input inverter when it is operating a vacuum cleaner with a
nameplate rating of 6 amps at 120 volts AC?
The appliance rating is given in amperes, so we must first calculate the power it consumes. Then the
rule of thumb can be used to find the DC input current of the inverter.
Output power = 120 volts x 6 amps = 720 watts, and
DC input current = 720
10 = 72 amps DC.
This information to estimate the DC input current requirement for an inverter will allow you to size an
alternator or charging system to supply an inverter for continuous operation. This rule of thumb will be used
later in the discussions on battery sizing.
Battery Terminology and Ratings
Batteries used for automotive applications generally are lead-acid storage batteries. They can be separated
into two categories according to their use: engine cranking batteries and deep cycle batteries. The engine
cranking battery is specifically designed to supply hundreds of amps for a short period of time to start an
engine. Cranking an engine usually uses a small portion of the battery's total capacity and once the engine is
running, the engine‟s alternator quickly recharges the battery. The deep cycle battery is specifically designed to
deliver current for extended periods of time and can be almost totally discharged before recharging.
The "deep cycle" lead-acid battery is designed to withstand the deep discharge/recharge cycling that is typical
of most inverter installations. These batteries are available in the "maintenance free" style where the electrolyte
does not need to be checked or replenished and they also are available in the gelled electrolyte style or "Gel
Cells". Deep cycle batteries are generally advertised for use in recreational vehicles or boats and are
sometimes referred to as RV or marine batteries.
Battery Council International (BCI) is a voluntary industry organization that has helped to standardize battery
ratings. Ratings in use at this date are:
CCA (Cold Cranking Amperes): Rating in amps a battery cold soaked at 0°F or
–20°F will carry for 30
seconds and maintain a minimum terminal voltage of 7.2.
MCA (Marine Cranking Amperes): Rating in amps a battery cold soaked at 30°F will carry for 30 seconds
and maintain a minimum terminal voltage of 7.2.
RC (Reserve Capacity): Rating in minutes a battery will carry a 25-amp load at 80°F and maintain a
minimum terminal voltage of 10.5.
AH (Amp Hour): At the “20 Hour Rate”, also called the C Rate, a battery having a 100 AH rating must carry
a 5 amp load for 20 hours (100AH ÷ 20 hours = 5 amps) and maintain a terminal voltage of 10.5 at
80°F. (Two 100AH 12volt batteries connected in series provides 100AH at 24 volts. Two 100 AH
12volt batteries connected in parallel provides 200 AH at 12 volts.).
CCA and MCA are used for sizing an engine cranking battery and have no bearing on a battery's cycling ability.
RC is a rating given to cranking batteries to provide an idea of how long a vehicle could be driven if the vehicle
charging system were to fail.
The most important and probably least understood battery capacity rating is the ampere-hour. One simple
reason the AH rating is misunderstood is that a battery rated at 100 AH cannot always deliver 100 AH. The
underlying reason is the efficiency with which the battery converts its chemical energy into electric energy. The
AH capacity of a battery is affected in the following ways:
Discharge rate: A battery becomes less efficient as the discharge current increases. For example, a typical
