BATTERY MANAGEMENT BASICS
After Voltage and Current, the most useful measurement available from a
battery condition monitor is the sate of charge of the battery. However.
estimation of the state of charge of lead-acid batteries is never exact. The
problem of making accurate estimates results from the characteristics of
the cells, the electrolyte, and the history of currents drawn from (discharge)
and supplied to (charge) the battery.
The basis for the best capacity estimates is that the starting condition is
known. The only well-established "known" state of a battery is when it is
fully charged after a long period of trickle or float charging, usually on a
shore or regulated alternator-driven charging system. Discharging a fully-
charged new battery at a current 1/20 of the manufacturer's stated
capacity will discharge it fully in 20 hours. This current is known as the
"20-hour rate".
So, for example, if a battery has a stated capacity of 100 Ahr, then the 20-
hour rate for that battery is 5 Amps (because 100/20 = 5). Likewise, a 40
Ahr battery would have a 20-hour rate of 2 Amps (because 40/20 = 2).
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If higher currents than the 20-hour rate are drawn from the battery, the
available capacity is reduced. For example, if it is steadily discharged at 10
times the 20-hour rate (50 Amps from a 100Ahr battery), the available
capacity falls to about half of the stated capacity. The battery will be flat
after about 1 hour instead of the expected 2 hours. (However, if the battery
is left to recover with the heavy load removed, most of its remaining
capacity will return after perhaps 20 hours' resting or at a discharge rate
close to the 20-hour rate.) The BM1/BM2 makes due allowance for these
effects when estimating the battery's state of charge and the expected time
to discharge the battery fully.
When the battery is being charged, the voltage is no longer a reliable
estimate of the state of charge, and so the BM1/BM2 integrates the Ampere
hours added to the last known capacity to estimate the battery's state of
charge on a continuous basis. Allowance for charge efficiency (not all
charging current results in useful charge in the battery) is also computed.
Available battery capacity is significantly reduced at temperatures
significantly below 20C. The value quoted by the manufacturers is valid at
20C. However, at 0C the capacity may be only 90%, and at -20C may be
only 70% of the 20C value. A small increase in capacity is achieved at
battery temperatures above 20C, rising to about 105% of the nominal value
at 40C.
