Xantrex Technology Xantrex Battery Monitor - Manual (Page 10)

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1 Battery Monitor Basics
Why should I monitor my battery?
Batteries are used in a wide variety of applications; mostly to store
energy for later use. But how do you know how much energy is
stored in your battery? No one can tell you that by just looking at it.
Some basic battery knowledge and good monitoring is essential if
you want to maximize the lifetime of your expensive batteries. The
lifetime of batteries is dependent on many factors. Battery lifetime is
reduced by under-charging, over-charging, too deep discharging, too
fast discharging and too high ambient temperature. An advanced
battery monitor like the Xantrex Battery Monitor can give you
important feedback about the performance of your batteries and
help you extend their lifetime.
How does the Xantrex Battery Monitor work?
The capacity of a battery is rated in amp hours (Ah). For example, a
battery that can deliver a current of 5 amps for a period of 20 hours
is rated at 100 Ah (5 × 20 = 100). The Xantrex Battery Monitor
continuously measures the current flow in or out of the battery so it
can calculate the amount of energy removed from or added to the
battery. But since battery age, discharge current, and temperature all
influence the battery’s capacity, you cannot rely on an amp-hours
reading. When the same 100 Ah battery is discharged completely in
two hours, it will give you only 56 Ah. As you can see, the battery’s
capacity is almost halved. When the temperature of the battery is
also low, its capacity is decreased even more. This is why simple
amp-hour counters or voltmeters cannot give you an accurate state-
of-charge indication.
The Xantrex 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 your battery. This parameter is given
as a percentage, where 100.0% represents a fully charged battery
and 0.0% a completely flat battery. You can compare this to a fuel
gauge in a car.
The Xantrex Battery Monitor also estimates how long the battery can
support the present load (time-to-go readout). This is actually 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 too