Electric drive made simple

L7 Drive multi-platform power electronics technology is the only system in the world that enables using a parallel connected very low voltage Li-ion battery pack to power vehicles or devices up to 48V and 8 kW

Case: Battery motor drive​

Traditional Li-ion series connection

L7 Drive Li-ion system​

L7 Drive innovative approach

We have concluded that, connecting Li-ion cells in parallel has many benefits over the traditional series connection. Parallel connection of battery cells is nothing new, it has been used for increasing the battery capacity for a long time. Regardless, increasing the voltage has always been based on connecting the cells in series. Previously, there has been no other way that could have done the voltage the boosting efficiently and profitably.​

L7 Drive has developed a technology that makes this possible. Our patented power electronics technology can utilize one large Li-ion cell or any number of parallel connected cells. If you want to know why this is important, please go ahead and read further below

The problem with Li-ion batteries is that they are like humans. They function best when kept in room temperature and are not totally depleted or overcharged. And they are all just a little bit different, even though they look very similar.​

One Li-ion cell has a nominal voltage of 3,2V – 3,7V (depending on the exact chemistry). To power anything bigger than a smartphone or a tablet you need a higher voltage. Only way you can get a higher voltage from Li-ion batteries is to connect them in series. Connect 10 cells together in series (10 x 3,6) and you’ll get 36V, which is good for your e-bicycle for example.​

But now it gets more complicated. Because the cells are connected in series, they can not be discharged or charged individually. If the system demands 1 Ah worth of current, each cell must provide as much, whether it has it or not. If it has not, the cell will be damaged beyond repair. If the pack is charged with 1Ah, each cell must receive it. If one of the cells has no space, it will heat up and may catch fire if the charging continues regardless.​

To solve this problem there is an electronic device called BMS (Battery Management System). It must be wired into each individual cell in order to monitor its voltage and operate the whole system based on the individual voltages of the cells. The voltage of a cell correlates with the State Of Charge (SOC), but it is only accurate at the low and high end of the SOC. To know exactly how much capacity is in the pack, the system also must calculate how much current is charged in or taken from the pack. It would also be good, if the BMS knows the temperature of each cell, or at least the average temperature of the pack, since the cells are very sensitive to overheating and, especially during charging, freezing temperatures.​

When charging, the BMS must control the charger and stop charging when the first of the cells reaches the highest allowable voltage. If the other cells lag considerably behind, the BMS must be able to discharge the first cell alone to be in balance with the rest and then continue charging until all the cells reach the target voltage together. Only then the pack will be in balance and, in theory, all the cells will have the same amount of energy stored in them.​

The inner resistance of the cell determines how efficiently the cell can take or release current. Unfortunately, there are always small variances between seemingly identical cells. Also the temperature affects the inner resistance of the cell. Before long, the cells drift in different state of charge even if not particularly abused.

When discharging, the BMS must stop the discharge when the first cell reaches the minimum allowable voltage and preferably warn the user some time before this happens.​

If the BMS is equipped to take care of all these functions, it will become quite complicated. Complicated also means quite expensive. That’s why cheaper Li-ion battery systems usually omit the BMS balancing function and only base the capacity monitoring to the pack’s total voltage. This results in a pack that loses its capacity faster than the individual cells inside. Also the user will have to get accustomed to the capacity indicator to suddenly go from ab. 40% to empty within a couple of minutes.​

Also, there is a sustainability issue with a simple BMS system; Because the capacity loss is in most part caused by imbalance of the SOC within the cells in the pack, it means that a majority, if not all, of the cells are still good. If a battery pack is discarded, these good cells go to waste.​

In our experience, a simple BMS causes premature capacity loss of the battery pack and a complex BMS is the most common point of failure in a Li-ion battery equipped system. That is why we did away with it.

It is relatively easy to keep one Li-ion battery cell working well. It only has to be operated within certain voltage, current and temperature limits, and it will last for at least as long as the manufacturer’s specification states. If you connect multiple cells in parallel, the pack still behaves as one individual cell, since the differences in voltage balance themselves, much  the same way as a liquid in communicating vessels.  As a bonus, also the current load will divide between the cells according to individual cell’s current properties, therefore it is possible to have more variety in cell quality within the pack. Our tests with an independent laboratory have confirmed these properties.​

This results in a battery pack that is easy to assemble and can include any number of cells. In series connected systems capacity is always tied into voltage steps (e.g. 36V system requires 10 x 3,6V cells), but with L7 Drive system this limit does not exist. There’s no electronics inside the battery pack, which makes it very robust and reliable. Since the battery cells are always in balance and handled according to their individual characteristics, the pack lifetime will increase to the same level as one individual cell.​

The remaining problem to be solved is the voltage. Connecting cells in parallel does not raise the voltage of the pack regardless of the number of cells. This is where the L7 Drive’s patented power electronics technology steps in. ​

We are able to raise the voltage from 3,2V all the way up to 48V steplessly, bi-directionally and more efficiently than any other DC-DC conversion technology. Because the system is bi-directional, it is possible to charge the battery through our device with any voltage between 5V and 48V. The charging power can also be variable, as I the case with photovoltaic cells. We have included MPPT software in the charging circuit, so connecting any variable voltage DC power source directly to the charging port is as efficient as possible.​

When used as an electric motor drive, a motor control bridge is added to the system creating a rotating magnetic field for the motor, and this is controlled together with the DC-DC conversion creating a motor control unit that is compatible with all types of electric motors. Because of the way the system works, our motor controller is able to send more current to the motor during the lower speed phase (i.e. acceleration) compared to other motor drive systems. In theory, L7 Drive is capable of constant power throughout the speed curve of the motor. Practical applications do limit this capability, but it gives a boost to the acceleration when needed.


By experimental research, the functionality and the feasibility of the module construction consisting of parallel-connected cells was studied. ​

Conclusions and summary ​

The construction of the parallel-connected battery module was found workable in many ways. Cell balancing is not needed, and the weak cells do not limit the performance of the whole module. These are clear advantages when compared to a module with series-connected cells. The mechanical structure is sturdy, and the module is quite simple to assemble. If the baseplate is cooled with a fan, it is possible to load the battery with a continuous rate of C/2 cycling. The structure is also sturdy enough to pass the vibration test according to ECE R100 Annex 8A. ​

Unlike the more commonly used serial connected battery module, this concept has no need for multiple cell voltage monitoring or balancing, and the capacity of the module is not limited by the weakest cells but is always the sum of all cells despite their capacity diffrences. The outcome of the lifetime test was that the cells are naturally balanced and their temperature raises no more than 7 °C above the ambient during continuous C/2 cycling, making the difference in the lifetime between a single cell and a module negligible. ​

The whole report available by request.​

Platforms and Hardware

L7 Drive StarGazer

L7 Drive StarGazer platform is a modern, cloud connected telecom backup energy solution. It consists of three main components; parallel connected backup battery with dedicated power electronics, communication interface and a real time cloud service.

L7 StoreGaze

L7 StoreGaze battery and power electronics module includes a Li-ion battery pack and L7 Drive G600 Power electronics unit. Battery cells are connected parallel and the voltage is raised to 48V by L7 Drive patented power electronics. This unit will manage the energy flow from the battery and to the battery. The output of the G600 converter is connected in parallel with the output of a grid powered AC-DC power supply that feeds the loads. This allows a full control of energy flow between the different sources. The power for the loads can be mixed in any ratio between the grid and the battery.


L7 SmartGaze

L7 SmartGaze is a communication unit that transfers CAN data from the battery hardware module to the cloud over the ethernet connection. In addition to the main function, L7 SmartGaze has a set of features that facilitate operating telecom industry equipment under variable conditions, like temperature and moisture sensors, cooling and heating control, input for a door sensor and an optional remote locking function.​


L7 CloudGaze

L7 CloudGaze is an AWS (Amazon Web Services) based cloud solution for monitoring and controlling a backup battery network. All the data gathered from the individual batteries is available in real time and can be used for different O&M purposes. All the error and alarm messages can also be addressed to different stakeholders. ​
The connection between the battery and the cloud is real time and always on. This enables completely new ways to use the battery capacity for secondary purposes when not needed for the primary function. L7 CloudGaze includes a VPP-engine that can combine individual backup batteries into groups that can act as Virtual Power Plants to balance power consumption in the electric grid and generate revenue to the battery network owner in the process.


L7 CanGate

L7 CanGate is a wireless device for securely connecting any CAN interface enabled device or vehicle with a cloud backend. In addition to CAN messages, data from different wired (I2C) and wireless (BT LE) sensors can also be sent to a cloud. The device has built-in support for GPS and other location technologies, making it ideal for implementing different fleet management use cases for vehicles and other moving objects. It operates in real time and is thus faster than other products used for similar purposes.​


Powertrain and DC-power hardware

L7 Drive 250 series

L7 Drive 250 and 2000 series power electronics units are versatile DC converters enabling robust and long-life parallel connected Li-ion battery pack to be used in multiple vehicle or DC-power applications with or without a motor control function. They can be supplied with an optional communication hardware that makes them fully connected with our real time cloud service for various remote control and monitoring functions for fleet operators or other use cases. ​


L7 Drive 2000 series

L7 Drive 250 and 2000 series power electronics units are versatile DC converters enabling robust and long-life parallel connected Li-ion battery pack to be used in multiple vehicle or DC-power applications with or without a motor control function. They can be supplied with an optional communication hardware that makes them fully connected with our real time cloud service for various remote control and monitoring functions for fleet operators or other use cases. ​


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