Battery management basics

02 Jan 2013861

After voltage and current, the most useful measurement available from a battery condition monitor is the SOC (state of charge). However, estimation of the SOC 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 current drawn form (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” started 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 batter at a current 1/20 if 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 100Ahr, then the 20-hour rate for that battery is 5Amps (because 100/20=5). Likewise, a 40Ahr battery would have a 20-hour rate of 2Amps (40/20=2).

If higher current 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 Amp in 100Ahr battery), the available capacity falls to about half of the stated capacity. The battery will be flat after about 1 hours 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 20hours’ resting or at a discharge rate closed to the 20-hour rate.)

When the battery is being charged, the voltage is no longer a reliable estimate of the state of charge, and so the MTX 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 manufacture 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.

The effects of cell deterioration on the available capacity are significant. If the battery is charged for long periods, gassing takes place. The gases are Hydrogen and Oxygen, derived from the water in the battery acid. Loss of this water needs to be made up by topping up the cells if possible, or by avoiding lengthy overcharges in sealed cells.

Other irretrievable effects include sulphation(encouraged by leaving the battery flat for long periods), and deterioration of the cells’ plates. If the battery voltage falls below 10.7V (for a nominally 12V battery), and charging is not started, sulphation of the plates can begin. The MTX has alarm setting features, that user can free to set the alarm value in over-voltage or low-voltage condition in 12V, 24V, 48V, 96V battery system. If the alarm is triggered, it is important to reduce the current drawn or stop charging process immediately, to avoid permanent damage to the cells. If the alarm is ignored, the total number of charge/discharge cycles which the battery will survive before it loses a substantial fraction of it nominal capacity may be substantially.

All of these and other effects reduce the available charge after full charging the battery. If the effects are ignored, the MTX will incorrectly estimate that more capacity is available at any state of discharge than is actually the case. If so, it is wise to alter the nominal capacity stored in the unit to match the reality of the battery’s condition.