692 Chapter 19
Discharging: Unlike batteries Ultra Capacitors store
their energy over their entire voltage range which makes
the design of the boost complex due to the wide input
voltage range and overall efficiency.
19.10 Batteries
Batteries offer a means of producing a smooth, ripple-
free, hum-free, portable power supply. A battery’s
capacity is rated in ampere-hours (Ah). Three facts
about batteries are:
- An ampere-hour can be a 1 A drain for 1 h, 0.5 A
drain for 2 h, or 2 A drain for 0.5 h. - A 12 V liquid battery is generally considered
completely discharged when its voltage reaches
10.5 V. - Batteries for cycling service—i.e. powering ampli-
fiers etc.— are normally rated with:- A 20 h discharge rate, which means a 20 Ah
battery will deliver 1 A for 20 h and a 100 Ah
battery will deliver 5 A for 20 h. - A reserve capacity stated in minutes for a 25 A
discharge rate. - Discharging batteries below 50% shortens their
life.
- A 20 h discharge rate, which means a 20 Ah
A cell or battery is an electrochemical system that
converts chemical energy into electrical energy. When
the chemical action is reversible, the battery is a
secondary or rechargeable system.
To be rechargeable, the positive and negative elec-
trodes of a battery must be capable of being converted
back to their original state following a discharge. Thus,
the battery must be electrically recharged by reversing
the process that occurred during its discharge cycle.
19.10.1 Temperature Effects
The standard rating for batteries is at 25°C (77°F). Bat-
tery capacity is reduced at lower temperature. At freez-
ing, Ah capacity is reduced to 80%. At –27°C (–22°F),
Ah capacity drops to 50%. At 122°F, capacity is
increased by 12%.
Battery charging voltage is also affected by tempera-
ture. It will vary from about 2.74 V/cell (16.4 V) at
–40°C (–40°F) to 2.3 volts per cell (13.8 V) at 50°C
(122°F).
Temperature also affects battery life. While battery
capacity is reduced by 50% at –22°F, battery life
increases about 60%. Battery life is reduced at higher
temperatures. In fact, for every 8.3°C (15°F) over 25°C
(77°F), battery life is cut in half. This holds true for all
types of lead-acid batteries, sealed, gelled, and AGM.19.10.2 Cycles versus Battery LifeA battery cycle is one complete discharge and recharge
cycle and is often considered a discharge from 100% to
20%, and then recharged back to 100%. Other ratings
for depth of discharge (DOD) cycles are 10%, 20%, and
50%.
Battery life is directly related to how deep the battery
is cycled each time. If a battery DOD is 50% every
cycle, it will last twice as long as if the DOD is 80%. If
the DOD cycle is only 10%, it will last about five times
as long as one cycled to 50%. A 50% DOD is usually
recommended. A battery that has a DOD cycle of 5% or
less usually does not last as long as one cycled down
10% because at very shallow cycles, the lead dioxide
tends to build up in clumps on the positive plates rather
in an even film.19.10.3 Battery VoltageAll lead-acid batteries supply about 2.14 volts per cell
(V/cell), or 12.6–12.8 V for a 12 volt battery when fully
charged. Batteries that are stored for long periods will
eventually self-discharge. This varies with battery type,
age, and temperature. Self-discharge can range from
1–15% per month. Batteries should never be stored in a
partly discharged state for a long period of time. A float
charge should be maintained if they are not used.19.10.4 State of ChargeState of charge, or conversely, the depth of discharge
(DOD), can be determined by measuring the voltage
and/or the specific gravity of the acid with a hydrome-
ter. Voltage on a fully charged battery is
2.12–2.15 V/cell, or 12.7 V for a 12 volt battery. At
50% DOD the voltage is 2.03 V/cell, and at 0% DOD it
is 1.75 V/cell or less. Specific gravity is 1.265 for a
fully charged cell and 1.13 or less for a totally dis-
charged cell. Many batteries are sealed, therefore,
hydrometer reading cannot be taken.19.10.5 False CapacityA battery can meet all the tests for being at full charge,
yet be lower than its original capacity because the plates
are damaged, sulfated, or partially gone from long use.