battery SYSTEM

As for most electric bikes and trikes today, VELOKS make use of lithium cells in its batteries. Today, there are only a handful of Lithium cells producers. Key producers are Panasonic, Samsung, LG, SONY and SANYO. Most other brands are based on OEM versions produced by these manufacturers.

Lithium cells comes in many shapes and sizes, but most common for electric bikes are the cylinder shaped 18650 cell and more rarely the cylinder shaped 21700 cell. The first two digits refer to the diameter (18 mm and 21 mm respective, while the 3 last digits refer to the height (65mm and 70 mm respectively).

Each of these types of cells has a nominal voltage of 3.7 volts, while the energy (expressed in Ah = amperage hours) varies depending on chemistry and size. Similarly, the continuous maximal discharge current and maximal change current will vary deepening upon chemistry and size. Cell types that VELOKS uses or have used are:

Critical criteria for selecting these cell types were: capacity (>=2.9Ah), energy density (>= 240 W/Kg), price, quality manufacturer, availability, charge current (>= 0.5C), discharge current (>= 10A), and charge cycles (>=500).

VELOKS is producing its batteries in house from scratch, this allows us to make use of the best battery cells on the market, and make quick changes as the technology evolves.

When we started in 2015, we used Panasonic NCR18650PF, as these were and are very good cells for the price. Following that we switched to LG INR18650MH1, which are similar in price, but has more capacity. Since spring 2019, we have used the INR18650-35E and INR21700-50E cells from Samsung, as these now are priced reasonable, and have higher capacity. Lately we have added high current cells in order to support our new 4000w and 6000w MK3 models. 

Creating batteries from individual lithium cells is done by connecting X number of equal lithium cells in parallel, creating an “aggregate battery cell”, with an X * cell capacity,  then connecting Y number of these “aggregate battery cells” in series until the desired nominal voltage is reached..

This type battery configuration is specified as for example 16S17P, meaning 16 aggregate cells in series (16 x S), where each aggregate cell has 17 lithium cells in parallel (17 x P).

The configurations that we are using is always 16 “aggregate battery cells” in series to create a 60 V nominal battery (actually 59 Volt more precisely, but this is normally referred to as 60V). For the various battery capacities, we use the following configurations:

Once a given configuration has been established, it is important to validate that the characteristics of the battery meets the requirements.

For our trike and motor design, we need a battery this is capable of delivering from 250 watts up to 6000 watts continuously, and are able to handle regeneration and charging requirements.

The voltage of lithium cells varies from 4.2 V fully charged to 3.0 V fully discharged, so the actual voltage of the battery will charge from 67.2 volt fully changed down to 49 volts fully discharged. So, in order to generate for example 3000w continuously  at any state of charge, the current required will vary from 3000 / 67.2 = 45A to 3000 / 49 = 61A continuous current.

The lithium cells that we use are all able to generate 10A continues discharge current each, so for the smallest capacity of 1.5 kWh we have 7 of these is parallel, which gives a maximum continuous current of 7 * 10A = 70A, which is sufficient to fulfill this requirement. If we take the largest battery of 5.1 kWh of capacity, this is 17 * 10A = 170A, or more than 2.8 times more than required at 3000 Watt.

The Battery Management System (BMS) installed in VELOKS batteries provides protection for:

    - Max. discharge current
    - Max. charge (Regen) current
    - Min. and Max. voltage for battery
    - Min. and Max. voltage for each cell
    - Battery Short circuit
    - Min. and Max. charge temperature
    - Min. and Max. discharge temperature
    - Improper cell balance


The default configuration is as follows::

    - For the standard current battery, the max. discharge cur. is 60A.
    - For the high current battery, the max. discharge cur. is 120A.
    - The max. charge (regen) cur. is battery type dependant.
    - Min. and Max. Battery voltage protection is set to 68V and 46V respectively.
    - Min. and Max. Cell voltage protection is set to 4.25V and 2.85V respectively.
    - Min. and Max. discharge temperature minus 20 degrees Celsius and plus 60 degrees Celsius respectively
    - Min. and Max. charge temperature 0 degrees Celsius and plus 60 degrees Celsius respectively
    - When more than 0.005v cell diff, balancing occurs between 65.6 volt and 67.36 volt.

The BMS can monitored and configured wireless via Bluetooth by VELOKS.

Now let’s talk about how to actually put the battery together, and let’s discuss how to do this in an optimal way. The areas to consider are:

      - Size & Shape of battery
      - Cell interconnection
      - Current flow
      - Mounting cells & battery
      - Battery box design

Size & Shape of the battery

The easiest and most optimal shape of a battery is a rectangular shape, where the cells are parallel in one direction, and in series is the orthogonal direction. This is because this shape will allow for the most optimal current flow between the cells, as well as the most compact design.

The issue of this shape, is that it might be difficult to accommodate for certain types of bikes, where space is limited or constrained in other ways.

VELOKS has been design from ground up for electric drive, and the battery shape and placement was a major design criteria. This has allowed us to use the optimal rectangle shape, and even has allowed us to vary the width of the shape to a large extent with no negative impact.

More to come....

Charging lithium ion cells is quite simple and requires a charger that can handle constant current (CC) during the initial stage and constant voltage (CV) during the final stage of the charging cycle.

During charging, the maximum charge current must not normally exceed 0.5C (which corresponds to the current needed to discharge a fully charged battery in two hours).

Charging beyond 0.5C is possible, but cooling of the battery is required. Examples of this is the so-called super charging used by TESLA for their electric cars, but this is obviously beyond the scope for e-bikes and e-trikes.

The charger must not exceed the maximum allowed charge current of the battery.

Also charging at temperatures below zero degrees celsius must be avoided as this will destroy the lithium ion cells. This protection is handled by the Battery Management System (BMS) which is described later.

For lithium ion cells, the maximum voltage is always 4.2 V per cells. For the VELOKS batteries, where 16 cells are connected in series, the max. charge voltage is 16 X 4.2 volt = 67.2 volt.

Similarly, we need to determine the maximum charging current for the battery. For the configurations we use, we can calculate that maximum charge current as:

As can be seen, the standard charger of 8A that we provide is a safe choice for all batteries. For all but the smallest battery the 18A is also a good choice.

Regeneration is similar to charging and must follow the same rules as charging. The difference is that the voltage and current originates from the motor, which through the motor controller charges the battery, while at the same time providing mechanical resistance (braking) to the wheel in relation to the regeneration (charging) current

The motor controller controls the voltage and current to battery so that it won’t exceed, in the case of VELOKS batteries, 67.2 volt, and the current is limited depending upon the battery capacity and the state of charge.

The maximum regeneration current as a function of the battery capacity and voltage is specified for each battery in terms of max. regeneration power:

In practice the maximum regen power is configured to not exceed 1500W for the rear wheel drive version of MK3 to not loose traction. For the front wheel drive and all wheel versions there is no traction issues, and they can the maximum regeneration power.

The controller must also limit the current sent to the battery when the battery is close to fully charged. This is done by starting ramp down of the regeneration current at 66.5 volt battery voltage, ramping down to zero at 67.2 volt battery voltage.

Each of the selected lithium cells are designed to retain 80% of their capacity after 500 charging cycles. But this requires that each are not treated outside of their specifications, i.e. we must not exceed: max. discharge current, max. charge current, min and max operating temperatures, and storing batteries long term only between 80% and 50% SOC.

One charging cycle is defined as a complete discharge from full to empty followed by a charge to full again. If you discharge your battery from full to 50% empty and then recharge to 100%, that counts as half a charge cycle.

The lifespan of 500 cycles is valid when charging the battery to 100% capacity every time.