This document describes the requirements and means of making a battery pack for powering an e-Bike class electric vehicle.

REQUIREMENTS

The battery pack in question must provide power to a DC / DC converter with the following features:

1. Nominal supply voltage: 36Vdc
2. Total Power: 350W

The design requirements require a battery pack in Lithium technology with the following features:

1. Rated Voltage: 36V
2. Output Current: 12A
3. Total Energy: 420W / h
4. Estimated Autonomy (Medium Operating Conditions): Approx. 2h

The battery pack must be made using 18650 lithium elements with 1.5A / h capacity.

REALIZATION

To create a battery pack with the features described above, it is necessary to make a series of strings to be connected in parallel.

Each string consists of a series of 10 batteries;

This imposes the voltage on the battery pack and the nominal voltage of a single 3.6V lithium cell, by connecting 10 in series, the final voltage will be 36V (nominal).

Parallel connection of ‘n’ strings determines the capacity of the entire battery pack as the capacity of the single string (equivalent to the capacity of the single battery) is summed to the elements that are placed in parallel.

Considering that each battery has a capacity of 1.5A / h, having to output a 12A current it is necessary to connect 8 strings (1.5Ax8 = 12A) in parallel.

It is therefore sufficient to follow the model “10s8p” which consists of 8 strings in parallel to make a battery pack with the necessary requirements; Each string consists of 10 series batteries (figure 1).

The most delicate aspect of this configuration, which certainly deserves more attention, is the management of the battery connection in series.

In fact, the good functioning of the whole package is subordinate to the operation of each single cell in series; it is sufficient that even one of the series batteries is malfunctioning to compromise the overall operation of the whole package.

More specifically, the state of charge of the battery having the lowest level in a string compromises the SOC of the entire packet; in fact this determines the total battery life of the battery pack to the detriment of its operation in the most serious cases.

For this reason it is necessary to balance the SOC of each battery to make it as close as possible to the other batteries in the same string.

The SOC balancing of each element is performed during charging and discharge of the string through the use in an adequate electronic device.

This device is called BMS (Battery-Management-System) and must be connected in parallel to each cell-battery that compiles the string.

In fact, the BMS automatically handles the input / output current flow from each battery reducing it in the case of a lower SOC battery and increasing it in the case of a battery with SOC higher.

This balancing tends to balance the SOCs of all the elements that make up the string increasing (in some cases even considerably) the autonomy and average life of the battery pack.

The BMS is equipped with a port for charger input, a power output port (on
which battery pack provides energy) and finally a door on which inputs are present connect to each battery cell (figure 2).

BMS

The various battery cells of each string are connected in parallel so it is sufficient to connect each BMS line to a single battery to balance the same batteries placed in the same position on the other strings (figure 3).

By making the parallel connections between the various batteries, each ‘parallelo’ branch of the battery pack will assume the same SOC; balancing, through the BMS, the SOC of each battery of each string, the battery pack as a whole will assume a uniform SOC.

Therefore, it is of utmost importance to insert a BMS inside the battery pack; this must be done following the diagram shown in Figure 4.

CONNECTIONS

The charging of the battery pack and the power supply of the converter compete with two different ports (figure 3).

The charge-battery connection will be carried out on the “charging-port”.

The charging current must have a value of 1/10 and 1/2 of the battery capacity.

In our case, having our parcel a capacity of 12 A / h, charging current can be between 1.2 and 6 A / h.

The load, that is, the converter that will then power the motor, must be connected to the “dischargingport”; on this door will flow all the current required to move the engine at various speeds.

RECHARGER

 

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