Automotive Engineering - December 2022 - BET16

Powering
Light
Electric
Vehicles
Considerations when replacing lead-acid
batteries with Lithium-ion batteries
in light electric vehicles.
Concept of delivery electric tricycle
scooter with charging station on
city street. (Image: Shutterstock/
Scharfsinn)
L
Charging
The maximum charge voltage of a battery
pack depends on the chemistry of
the cells used. Comparing batteries with
similar nominal voltages, the charge voltage
for a lead-acid battery is different
from that of a Li-ion battery. For example,
for a vehicle with a nominal voltage of
24V, the lead-acid batteries can be
16
ight Electric Vehicles (LEVs)
such as golf carts have been
traditionally
Lithium-ion (Li-ion)
powered by
lead-acid batteries. Original
Equipment Manufacturers
(OEMs) are transitioning to
batteries as they
offer several advantages over lead-acid
batteries, such as higher power density,
longer run time, and zero maintenance.
However, a successful transition requires
careful consideration of the differences
in the cell chemistry and the battery
pack behavior.
Li-ion batteries are designed with a
Battery Management System (BMS) to
provide safety and intelligence, which
increases their overall complexity. This
article discusses some of the key considerations
that OEMs need to account for
when replacing the lead-acid batteries
with Li-ion batteries. The considerations
are categorized into four sections:
charging, regeneration, user interface,
and power tripping.
charged to a maximum voltage of 29V34V,
whereas a 24V Li-ion battery pack
with Lithium-Nickel-Manganese-CobaltOxide
cathode cells has a maximum
charge voltage between 28 and 29V.
Using a lead-acid charger to charge Liion
batteries can trigger over voltage
protection and result in reduced battery
pack capacity.
Similar mismatch exists with the
maximum charge current. The charge
current for Li-ion batteries varies with
the temperature and State of Charge
(SOC). Overall, lead-acid batteries can
support higher charge current. Using a
lead-acid charger to charge a Li-ion
battery can trigger an over current protection,
or an over temperature fault
due to the heat generated in cells with
higher charge currents. This will also
eventually degrade the lifecycle of the
battery pack.
The presence of a BMS in Li-ion batteries
brings additional levels of protection.
During a severe cell under voltage condition,
the BMS activates protection and to
bring the battery pack out from this protection
state, charging needs to be performed.
A lead-acid charger may not be
able to charge the Li-ion battery as it
looks for a valid pack terminal voltage
prior to starting the charge process. Since
the pack is in a severe under voltage protection,
the pack cannot provide a voltage
at the terminals. This results in a condition
where the pack does not get charged,
and therefore, the user may not be able
to drive the vehicle.
The solution is to replace the lead-acid
charger with a Li-ion battery charger with
a charge algorithm matching the chemistry
of the cells used. Using a smart charger
lets the BMS regulate the charge
voltage and current over digital communication
interface such as Controller Area
Network (CAN). The charger also needs to
account for the conditions during which
the battery cannot provide a valid voltage
on the terminals but requires a
charge voltage and current to come out
of that condition.
Regeneration
LEVs can generate regenerative current
while braking and transferring that
energy through the motor to be used as
the main braking mechanism for the vehicle.
The battery may absorb the energy
that is transferred. Lead-acid batteries
can do this under all operating conditions,
but Li-ion batteries are limited in
their capability to accept regen current at
certain operating conditions.
These limitations include when the
cells are cold, when they are fully
charged, and when a charge protection is
active, such as a cell over voltage or cell
over temperature fault. In addition, the
amplitude of regen current that the battery
pack can absorb also varies with
Battery & Electrification Technology, December 2022
Automotive Engineering - December 2022

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