IEEE Electrification Magazine - September 2017 - 20

Since the batteries'
life span is
dependent on the
temperature, the
battery space should
be temperature
controlled.

The BMS is also responsible for
ensuring that there is an adequate
voltage balance between cells in
the system. This primarily requires
compensating for individual selfdischarge rates between the cells
by draining the cells with the lowest self-discharge rate through a
resistor, but could also be done by
active electronics. Such cell balancing is often performed when
the batteries are not in use, e.g.,
during nighttime for a ferry that
only operates in daytime.
The most critical failures are those that can lead to an
internal short circuit in the cells that consequently can
lead to a thermal runaway. A properly designed BMS will
prevent this scenario, except if there is some mechanical
abuse or defect in a battery cell originating from the
manufacturing process.
Manufacturing defects can produce a short circuit that
is not detectable by the BMS. It is then important that the
battery system be designed such that the consequence of
an internal short in a cell is acceptable from a safety point
of view. Because of this, the DNV GL rules require that a
possible cell thermal runaway must be confined at the
lowest level possible. This implies that the module design
should inhibit thermal propagation from cell to cell or, at a
minimum, that the thermal runaway should be confined
within a battery module.

DNV GL Class Requirements for a
Safe Battery Installation Onboard a Ship
A ship's battery system location is referred to as the
battery space. This can be a dedicated room with only
the battery system inside or a larger room where additional equipment is located. To have a safe battery
installation, the DNV GL rules set some requirements
for the battery space.
To minimize the risk of mechanical damage to the
battery system, the battery space must be located aft of
the collision bulkheads, and the boundaries of the battery space must be a part of the ship's structure (or
something similar).
To reduce the probability of a thermal event, heat
sources or high fire risk objects are not allowed to be
in the battery space. For example, it is not acceptable
for a hybrid ship to have the battery system located
in the same room as the combustion engines. If the
neighboring room to the battery space is a room
with, e.g., a diesel engine, then the walls must have a
fire rating of A60. [The A rating means testing based
on a normal international organization for standardization fire curve (cellulosic fire). The item should
maintain its specified insulation performance, integrity, and load-bearing capacity for 60 min.] In case

I E E E E l e c t r i f i c ati o n M agaz ine / SEPTEMBER 2017

there is a thermal incident with
the batteries, the room and the
equipment inside the space must
be regarded as lost. Hence, the
battery space should not contain
redundant systems supporting
propulsion and steering. Since
the batteries' life span is dependent on the temperature, the battery space should be temperature
controlled. If the battery system
is liquid cooled, then the ambient
temperature of the battery space
is not of similar importance.
Since a battery cell has a risk of going into a thermal
runaway and releasing flammable, toxic, and corrosive
gases, it is required that the battery space have ventilation. At a minimum, there must be an emergency ventilation system that starts up in case of a gas release. Since
the gas will be hazardous, the ventilation ducting system
must be separated from any other ducting systems
onboard. The outlet of the ventilation ducting system
must be to the free air outside the ship.
Some battery systems have enclosed battery modules
directly connected by pipes to the exhaust ducting system from the battery space. Other battery systems are of
a more open type and might ventilate the flammable
gases directly into the battery space. If such a battery
system is installed on the ship, then the room should
have gas detectors, and all equipment in the space must
be disconnected (or designed to be explosion proof) in
the case of a gas release to prevent ignition of the flammable gases.
A fire extinguishing system must be installed in the
battery space. There is no established standard fire
extinguishing material for Li battery fires, and many
different materials are available (e.g., water mist, waterbased foam, Novec, and FM200). The challenge is to find
an extinguishing system that both puts out the cell/
module fire and, at the same time, cools down the surrounding modules so the fire does not spread. DNV GL
rules recommend the use of a water-based, fixed fire
extinguishing system. Such a system will cool down the
battery modules to an extent that the fire does not
propagate. Portable fire extinguishers are not recommended for larger battery systems because of their limited capacity, difficult accessibility for covering the
battery on fire, and possible hazard for crew entering a
battery space on fire.

DNV GL Class Requirements for BatteryPowered Electrical Systems for Propulsion
Charging and discharging of the battery system are controlled by a battery charger, normally a bidirectional
ac/dc or dc/dc converter (Figure 15). The BMS provides
voltage and current limits for the battery charger. DNV

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