IEEE Electrification Magazine - September 2017 - 19

The DNV GL class
rules require that
voltage, temperature,
and current be
measured by
the BMS.

a complete battery system. The
DNV GL rules require that the battery system be certified before it is
installed onboard a ship. The certification consists of a design verification per the applicable class
rules, in addition to safety, functional, and electrical tests.
A Li battery cell needs to operate within specific voltage and
temperature levels to ensure safe
operation. This is achieved through
a battery management system (BMS), which performs
control, monitoring, and protective functions for the
battery system (Figure 14). If the batteries operate
outside the safety limits, the BMS will activate an
electrical disconnection of the battery system. The
DNV GL class rules require that a battery system have
an integrated BMS, without which the system cannot
be certified.
The operating temperature of the battery cell must be
kept in a specific range. When the temperature is too
high, the electrolyte will start to vaporize into flammable gases; and, if heated further, the battery cell can initiate a thermal runaway (exothermic reaction) that
might lead to self-ignition of the flammable gases. If the
battery cell is charged at too low a temperature, Li plating can occur. This will shorten the life of the battery
and increase the probability of an internal short in the
cell. The safe temperature range depends on the type of
Li battery chemistry and is typically between 0 °C and
60 °C. Additionally, the life span of the battery cell
depends heavily on the cell temperature during charging
and discharging. The optimal operating temperature is
normally around 20-25 °C.
Equally important is that the voltage of an Li battery cell is kept within a specific range. If the charging
voltage is too high, excessive chemical reactions or
side reactions occur, and the cell becomes thermally
unstable. If the discharge voltage is too low, the negative electrode can be damaged, and Li plating can grow
from one electrode to the other and cause an internal
short circuit in the battery cell. The voltage range
depends on the type of Li battery chemistry and is typically between 2.5 V and 4 V for a battery cell.
The charging and discharging current must also be
controlled. As the current goes in or out of the battery
system, the battery cell is heated and its temperature
increases. The amount of current that a cell can handle
(typically called the C-rate) will depend on the battery
system construction and the charge/discharge time.
The DNV GL class rules require that voltage, temperature, and current be measured by the BMS. If any
values are outside the specified limits, the BMS generates alarms, and, if no action is performed, the BMS
disconnects the battery system. To handle a situation

in which this automatic disconnection fails, an independent
manual emergency disconnection
option that the crew or captain
can activate to disconnect the
battery system is also required.
DNV GL rules require that the
BMS monitor and calculate the
battery's state of charge (SOC)
and SOH. The term SOC describes
the energy in the battery system
available for use depending on
specific conditions (i.e., within a certain power range).
A fully charged battery system has an SOC of 100%.
Li-ion batteries will unavoidably degrade with age
and use. As the cell is charged and discharged repeatedly, the ability to accumulate ions at the negative
electrode will gradually decrease. The SOH reflects
the general condition of the battery and the ability
to deliver the specified performance compared to a
new battery.
To calculate the available energy in a battery system, both the SOC and SOH must be considered. The
available energy is needed to give a clear indication of
how far a ship may go on its remaining battery energy
and for ensuring that a hybrid system gets its maximum potential fuel consumption benefits. Methods
for estimating SOC and SOH should be based on best
industry practice for the relevant battery technology.
Such methods may use a combination of measurements, elec t ro ch e m i c a l m o d e l s , a n d prediction
algorithms and take into account factors such as battery characteristics, operating temperature, charge
rates, cell aging, and self-discharge. The accuracy of
the calculated available energy of the batteries in the
different operation modes is one of the challenges
that future re search and industrial development
should improve.

BMS

Battery Cells

Figure 14. The BMS monitors currents (A), temperatures (T), and
voltage levels (V) within the battery system.

IEEE Elec trific ation Magazine / S EP T EM BE R 2 0 1 7

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2017