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.​
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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.​
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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.​
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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.​
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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.

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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.​
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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.​
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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.​
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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.​
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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. ​
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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.​
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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.

“CUSTOMER REPORT VTT-CR-00089-20”​

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

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Conclusions and summary ​
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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. ​
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The whole report available by request.​