Batteries
The battery stores energy ready to use in the way of electrical charge.
Capacity
The maximum amount of electrical charge that the battery
can store is called "capacity" and is measured
in AH (ampere x hour). The capacity depends on
the level of the discharge current. At high currents
the battery behaves like it has less capacity.
That is in part because of its internal resistance that wastes energy.
The manufacturer specifies the capacity for different discharge currents. The "nominal capacity" corresponds to new batteries and low level discharge current. Aging batteries lose capacity.
The batteries can be charged and discharged (cycle) several times in their life, but it is advised not to reach a deep discharge in order to not shorten its life. The measure of discharge is a percentage of the capacity and it is called "depth of discharge" (D.O.D.). The manufacturer specifies the number of cycles available as a function of D.O.D.
A common question is what capacity is needed for a specific power load connected during a specific time.
The following expression illustrates the answer:
T [hour] x P [watt]
120
C [AH] = _________________ x ___________
V [volt]
D.O.D. [%]
C [AH]: capacity in ampere x hour
T [hour]: time that the load is connected to the battery in hours
P [watt]: power of the load in watt
V [volt]: voltage of the battery or bank of batteries in volt
D.O.D. [%]: depth of discharge in percentage
The expression increases 20% the nominal capacity in order to take into account the dependency of the capacity with the discharge current level.
Example: If we want to determine the capacity of a 12 volt battery or bank of batteries with a permitted D.O.D. of 50% for a 1000 watt load during a half an hour the calculation is:
0.50 x 1000 120
C [AH] = ___________ x _____ = 100 AH
12
50
Types
The batteries are classified in different ways according
to their chemistry and use.
By chemistry: nickel-cadmium, lead-acid with several
variances such as wet, gelled, dry, etc.
By use: automotive (starting battery), deep-cycle, etc.
The ones recommended for use with inverters are lead-acid/deep-cycle.
Lead-acid batteries are cheaper than others with different chemistry.
Deep-cycle type can be discharged more deeply than the starting ones, which affects its life less. They can be repeatedly deep discharged and recharged.
The starting batteries are for very high current during a short period of time. It is recommended not to surpass a D.O.D. of 20%. A starting battery in a good condition car experiences a D.O.D. of around 2% after starting and then it is recharged.
Deep-cycle batteries have thicker plates in its cells than the starting ones, permitting the passage of high current for a long period of time. The recommended maximum D.O.D. for these batteries is 50%.
State of charge
An easy way to determine
the battery state of charge is by measuring at open circuit
(no load connected) the voltage between terminals. Those values
depend on the battery type and should be checked in the
manufacturer's specifications.
In general it is:
During a battery charge following the manufacturer's specifications, it is normal to get a voltage of around 15 volt and a float voltage for standby conditions of 13.2 volts. Leaving the just charged battery at rest for a while lets the voltage stabilize at around 12.8 volts.
It is strongly recommended to read the instructions and specifications from the battery manufacturer.
Bank of batteries
The group of two or
more batteries is called "bank of batteries". Batteries
are grouped in order to increase the voltage or
increase the capacity and/or increase the current.
Note
Sometimes the manufacturers make reference to
Reserve Capacity (RC) and Cold
Cranking Amp (CCA).
Reserve Capacity (RC): measurement in minutes of the time it takes to a totally charged battery to be totally discharged (10.5 V) delivering a constant current of 25 A at 80°F.
Cold Cranking Amp (CCA): maximum current in amperes that a battery can deliver during 30 seconds maintaining its voltage at not less than 14.4 V at 0°F (applicable to starting ones).