Installation Guide

4-44 975-0691-01-01 Revision B
Theory of Operation
How State-of-charge and Amp-hours are Calculated
The capacity of a battery is rated in amp-hours (Ah). A battery that can deliver a
current of 5 A for a period of 20 hours is rated at 100 Ah (5 × 20 = 100) at a C20
discharge rate. The Battery Monitor continuously measures the current flow in or
out of the battery and calculates the amount of energy removed from or added to
the battery. However, battery age, discharge current, and temperature all
influence the battery’s capacity. When the same 100 Ah battery is discharged
completely in only two hours (a C2 rate) it will provide only 56 Ah. In this case, the
battery’s effective capacity is almost halved. Additionally, when the temperature
of a battery is low, its battery capacity is decreased even more. This is why
simple amp-hour counters or voltmeters cannot provide an accurate state-of-
charge indication.
The Battery Monitor can display both amp-hours removed (not compensated)
and actual state-of-charge. Reading state-of-charge is the best way to read the
battery. This parameter is given as a percentage, where 100.0% represents a
fully charged battery and 0.0% a completely flat battery. It can be thought of in
comparison to a fuel gauge in a car. Discharging lead-acid batteries lower than
50% of SOC is typically not recommended as it may affect battery life.
How the Charge Efficiency Factor is used
Charge Efficiency can be defined as the ratio between the energy removed from
a battery during discharge and the energy replaced into the battery during
charging. Not all energy transferred into the battery during battery charging is
available during subsequent use of the battery. The charge efficiency of a brand
new battery is approximately 90%, which means that 10 Ah must be transferred
into the battery to get 9 Ah actually stored in the battery. This efficiency figure is
called Charge Efficiency Factor (CEF) and decreases as batteries age. The
Battery Monitor automatically calculates the CEF of the battery as the battery is
used. The level at which the CEF is recalculated is variable and is linked to the
discharge floor set in F1.3.
The CEF algorithm starts operating when 10% of the difference between 100%
and the discharge floor is discharged from a fully charged battery. For example,
assume the discharge floor is set to 50%. Then the CEF algorithm starts
operating when 100% - 50% / 10 = 5% is discharged from a fully charged
battery. When the discharge floor is set to 0%, this level is 100% - 0% / 10 = 10%.
How Time Remaining is Computed
The Battery Monitor calculates how long the battery can support the currently
active load. This is referred to as Time Remaining. Time Remaining indicates the
time left until the battery needs to be charged again. If the battery load is
fluctuating heavily, it is best not to rely on this reading since it is a momentarily
calculated readout. This means that it will always reflect the time remaining as