A battery is dependent on all of its cells.
The only way of controlling each cell in a battery is to have a battery
monitoring system that continuously measures total current and individual
cell voltage. In any in case of a deviation greater than a certain value
the battery monitoring system should send an alarm signal. With such a
system any failing cell can be identified at an early stage and by replacing
that individual cell further damage is avoided. This method will guarantee
a battery’s health through its whole life cycle.
A UPS battery is usually in one of three states:
|1. Float charge
This is the most common state. In some cases, it
can even turn out to be the only state. The battery is charged with
a low current equivalent to its self-discharge with a certain margin
When the battery
is supporting a load due to a power outage. In this state the battery
is discharged with a high current for a short period of time. The
current could be -(4 × C10) Amp during 10 min.
When the battery
is recharged after a period of discharging. In most cases the battery
is recharged with an increased voltage during a certain amount of
time, for example 24h. The current is usually limited to +(0.2 ×
Note: The term C10 refers to a battery's nominal capacity in Ah, when discharged
during 10h to a final voltage of 1.8V per cell.
Discharging battery cells connected in series.
When discharged a battery must continuously generate a certain amount
of current and voltage in order for supported equipment such as an UPS
to work properly. A battery's terminal voltage is the sum of the cell's
unloaded voltage, EMF, minus its inner- and connecting lines' resistance
multiplied by the current. Therefore, terminal voltage drops as a result
of an increased current. A defect cell usually has higher internal resistance
resulting in lower terminal voltage than other cells in the same battery.
At high load current it is fairly easy to detect a defect cell by measuring
voltage on each cell or block of cells. However, this requires that each
cell's voltage is measured simultaneously. With a battery monitoring system
that stores measurement values and compares new values with historical data
very small variations in a cell's internal resistance can be identified well
before the cell is ruined.
Charging battery cells connected in series.
A battery containing a number of cells connected in series is recharged
using a voltage recommended by the manufacturer multiplied with the number
of connected cells.
Fig.1 below shows a battery with 4 cells charged with
2.25 per cell, a total of 9 volt. Total voltage and average cell voltage
is generated by the charger. However no cell has recommended recharge
voltage of 2.25 volt. Therefore, the battery manufacturer has stated the
maximum amount a cell's voltage is allowed to deviate from the average.
This value is usually +-0.10V which in this case means that each cell's
voltage must be between 2.15 and 2.35 volt.
Fig1. Battery with normal cell voltage.
Fig.2 below shows the same battery with a defect in cell
#3 resulting in an increased internal resistance. An increased internal
resistance results in an increased cell voltage, in this case 0.3 volt.
This means that the other cells' voltage drops with 0.1 volt. The water
in Cell #3 is transformed into explosive hydrogen gas. In case this problem
is not discovered also cell #1 and cell #4 will fail as a result of low
charging voltage. Furthermore, low charging voltage will result in the
formation of highly insoluble lead. The water in Cell #3 is transformed
into explosive hydrogen gas. In case this problem is not discovered also
cell #1 and cell #4 will fail as a result of low charging voltage. Furthermore,
low charging voltage will result in the formation of highly insoluble
lead sulphate on the cell plates. cell #2 will stay unaffected until all
water in cell #3 is consumed.
Fig2. An increased cell voltage results in decreased voltage over remaining cells.
In this particular case a Battery Monitoring System would
trigger an alarm as soon as voltage over cell #3 exceeds 2.35 Volt.
charging it is difficult to discover individual failing cells when reading
data from more than one cell per channel. If cells in Fig. 2 are grouped
in pairs, average voltage in group 1 (cells #1 and #2) is 2.15 Volt and
in group 2 (cells #3 and #4) 2.35 Volt. Both these values are within limit.
a battery’s total voltage does not give any information about
the state of health of individual battery cells.
voltage over a group of cells gives only an average value for each
cell within the group.
each individual cell is the only way in guaranteeing each cell is
receiving the correct voltage.