A look at emerging rapid-test technologies for
deep-cycle, lead-acid batteries
The secret of battery runtime lies
in the capacity. Capacity defines
the energy a battery can hold.
The definition for capacity is usually
given in ampere-hours (Ah); it specifies
the elapsed time when discharging a
battery at a calibrated current to
the end-of-discharge voltage. Portable
batteries commonly use a one hour
discharge; larger batteries are rated at
either a five or 20 hour discharge period.
Lead-acid batteries come in two
basic architectures: deep cycle and
starter types. The deep cycle battery is
designed for maximum capacity and
high cycle count. This is achieved by
installing thick lead plates. Typical
applications are golf carts, wheelchairs,
people movers, scissor lifts, and
inverter power for RVs. In comparison,
starter batteries are made for
maximum CCA (cold cranking amps).
The battery maker obtains this by
adding extra plates to get a large
surface area for maximum conductivity.
Capacity and deep cycling are less
important for automotive because the
battery is being recharged while driving.
If continuously cycled, the thin lead
CAPACIT Y :
The Key to Battery Runtime
by Isidor Buchmann
plates of the starter battery would wear
down rather quickly. As a rule of thumb,
the heavier the battery, the more lead it
contains and the longer it will last.
What is the
Capacity and CCA?
The characteristics of the lead-acid
battery can best be explained by
making capacity responsible for energy
and CCA for delivery. Capacity and
CCA do not age at the same pace. The
CCA tends to stay high through most
of the battery’s life, and then drops
quickly towards the end. This often
leaves us stranded when all of a sudden the car won’t start in the morning.
In comparison, capacity decreases
gradually. A new battery is designed to
deliver 100% of its rated capacity. As
the battery ages, the capacity steadily
drops and it should be replaced when
the reading falls below 70%. Capacity
measurement is considered a more reliable state-of-health indicator than CCA.
Let’s look at the aging mechanism
of capacity and CCA. Figure 1 shows
two lead-acid batteries: one with high
capacity and one that has aged. The
build-up of so-called “rock content” as
part of aging robs the battery of usable
energy although it may still provide
good cranking power. Figure 2 illustrates
a battery with high and low CCA by simulating free-flowing and restricted taps.
The third criterion of battery
runtime is state-of-charge (SoC). The
battery capacity is always measured on
a fully charged battery and the most
simplistic method of estimating SoC is
reading the open terminal voltage
(OTV). This approach is accurate if the
battery has rested for at least four hours
after charge or after applying a load.
The rather long rest period is the
required recovery time to pacify a
battery when disturbed. You should
also be aware that different plate
compositions alter the OTV reading.
Calcium raises the voltage by 5-8%,
affecting SoC estimation. Calcium is
an additive that helps in making the
FIGURE 1. Battery capacity illustrated in available liquid. Both batteries are fully charged,
but “rock-content” limits the capacity on the second battery. A) New battery has high
capacity; B) Aged battery has low capacity.
54 SERVO 02.2008
Battery capacity is commonly
measured by applying a full discharge.
While this method provides accurate
readings, it is cumbersome, time
consuming, and wears the battery
down unnecessarily. During the last 15
years, several rapid-test methods have
emerged that eliminate the need for
discharge (so the manufacturers claim).
Introduced in 1992, AC conductance
became a popular in testing scheme,
from which CCA is estimated. This non-invasive method was hailed as a major
breakthrough because the test only
takes a few seconds and the instrument