■ FIGURE 2.
much higher rate of sulfation than do fully charged cold
batteries. Unfortunately, the effects of sulfation are
cumulative and largely irreversible. The ever-increasing
internal resistance gradually reduces the peak output of
the battery, until it can no longer start the vehicle. This is
how many lead-acid batteries die.
STATE OF HEALTH (SOH)
Let’s assume that a certain battery has suffered some
sulfation and its internal resistance has increased from
0.02 to 0.04 ohms. When the starter motor is engaged,
the battery sees an effective load of 0.04 ohms + 0.08
ohms = 0.12 ohms. The current flow will be 12.65V / 0.12
ohms = 105 amps (peak). The voltage across the starter
(and battery posts, assuming the cables have no
resistance) drops as low as 105 amps x 0.08 ohms = 8.4V.
If a battery has good SOC and SOH, the voltage
should remain above 9V when the vehicle is started at
70°F. If it doesn’t, it could indicate that the battery may
require some maintenance (such as adding water to one
or more cells which may be low on electrolytes), or that
the battery may need to be replaced.
Each time your vehicle is started, the Battery Marvel
computes an SOH score (0 to 100%) based on the
temperature, the SOC, and the voltage drop measured
while your starter motor is engaged. A score of 100%
means the battery is very healthy, i.e., there’s very little
drop in voltage and minimal internal resistance. A lower
score is evidence of increased internal resistance and a
reduced ability to deliver current. The Battery Marvel
watches for deteriorating SOH and issues an alert if it
becomes critical.
Complete kits are available from the Nuts& Volts
Webstore at http://store.nutsvolts.com.
Assembled and calibrated units are available at
www.batterymarvel.com.
36
November 2011
CHARGING SYSTEM
Once the engine starts, the alternator works to
replenish the energy used by cranking. Modern alternators
are designed to produce a varying AC voltage (linked to
engine RPM) which is rectified to create an unregulated
DC voltage. The unregulated DC voltage then goes to a
linear voltage regulator which provides approximately
14.4V DC for powering the electrical and charging
systems.
The regulated charging voltage can vary from about
13. 5 to 14.8V DC, depending on the make and model of
the vehicle, whether headlights or other accessories are
on, SOC, and temperature. Some vehicles have a fixed-output voltage regulator while others are manually
adjustable. Many newer vehicles feature a variable voltage
regulator with built-in temperature compensation. This
allows a higher charging voltage when the battery is cold,
and a lower charging voltage when it is hot. The Battery
Marvel continuously monitors your vehicle’s charging
system and issues an alert if any problems are detected.
SURFACE CHARGE
When the vehicle is switched off, you might expect
the voltage across the battery terminals would
immediately drop back to 12.65V once the engine
stopped, but this is not the case. Instead, a residual
“surface charge” slowly dissipates over a period of several
minutes to several hours. Surface charge is due to a nonuniform concentration of ions in the electrolyte near the
surface of the plates.
During charging, ions build up near the plates where
the electrochemical reactions are taking place. When
charging stops, the ions begin to slowly migrate away from
the plates, seeking to establish a uniform concentration
within the electrolyte. As the ions slowly diffuse
