www.nutsvolts.com/index.php?/magazine/article/november2011_Decker
LEAD-ACID BATTERY
CONSTRUCTION
Lead-acid batteries can be broadly divided into two
categories: the starting/lighting/ignition (SLI) type found in
cars, trucks, and motorcycles; and the deep cycle type
found in boats, golf carts, and forklifts. SLI batteries are
designed to deliver maximum peak current, but they are
not tolerant of being deeply discharged. In contrast, deep
cycle batteries can tolerate deep discharge without
damage, but they generally have a lower maximum “peak”
current output than SLI batteries of similar size.
Regardless of type, every 12V lead-acid battery is
constructed from six individual cells connected in series.
Each cell produces about 2.108 volts at room
temperature. This means that a 12V lead-acid battery
actually produces about 6 x 2.108V = 12.65V at room
temperature when it is fully charged. However, the cell
voltage drops rapidly with temperature. At 0°F, the fully
charged voltage is only 2.086V per cell and the output is
around 6 x 2.086V = 12.52V. As we’ll see later, a built-in
temperature sensor allows the Battery Marvel to
incorporate temperature into the appropriate internal
calculations.
STATE OF CHARGE (SOC)
A very common measure of battery condition is “state
of charge” or SOC. SOC is expressed as a percentage
from 0% (fully discharged) to 100% (fully charged). The
Battery Marvel continuously estimates SOC by measuring
the no-load output voltage of the battery, the ambient
temperature, and using a set of lookup tables in Flash
memory. The Battery Marvel issues an alert if the
estimated SOC drops below a minimum threshold.
Notice that a low SOC doesn’t necessarily mean that
a battery is bad. It might simply be low on charge because
the headlights were left on or because the vehicle’s
charging system is not working, for example.
CAPACITY
An ideal battery has no internal resistance and is able
to supply infinite current. That’s a nice goal, of course, but
it isn’t achievable in the real world. All real batteries have
some internal resistance and therefore an upper limit on
the peak current they can supply.
To illustrate, let’s assume that we have a 12V lead-acid
battery with an internal resistance of 0.02 ohms. The peak
current output of this battery is 12.65V/0.02 ohms = 633
amps. This is the current that would flow if a zero ohm
load (a short circuit) was placed across the terminals. If
the internal resistance of this battery increased by merely
0.01 ohms, the peak current output would drop to
12.65V/0.03 ohms = 422 amps. Thus, even a small change
in a battery’s internal resistance makes a big difference in
the peak current output.
■ FIGURE 1.
CRANKING MATH
To continue with our discussion, let’s assume that a
particular automotive starter motor has a DC resistance of
0.08 ohms. We’d expect this starter motor to draw 12.65V
/ 0.08 ohms = 158 amps of peak current when it was
connected to an “ideal” battery. Incidentally, it is
interesting to point out here that the instantaneous current
actually fluctuates rapidly, depending on the mechanical
load on the starter motor at any instant — whether a
winding is energized (and which one), whether a magnetic
field is just starting to form, already established, or
collapsing, and many other factors. The peak current and
the average current through the motor are quite different,
as well.
If we connected our starter motor described above to
a battery with 0.02 ohms internal resistance, we’d have a
0.02 ohms + 0.08 ohms = 0.1 ohms total load which
would draw 12.65V/0.1 ohms = 127 amps (peak). The
minimum voltage measured across the starter motor (and
also the battery posts, assuming the connecting cables
have no resistance) would be 127 amps x 0.08 ohms =
10.16V. We can see that even with a fully charged,
healthy battery capable of delivering 633 amps, the
voltage across the battery terminals would drop to around
10.16V when the starter motor was engaged. This is
illustrated in Figure 1.
As lead-acid batteries age, their internal resistance
gradually increases. This is mainly due to a chemical buildup of hard lead sulfate crystals on the internal metal plates
called “sulfation.” The rate of sulfation increases as the
battery’s SOC drops, and it also increases with
temperature. Poorly charged, hot batteries experience a
November 2011 35
