Passive Balancing waS Yesterday

Due to their chemical-physical properties, lithium cells require a battery management system (BMS) to make the maximum cell capacity usable and to keep the cells of a battery at the same voltage level. Almost all BMSs for Li batteries use passive balancing. Excess energy of single full cells is wasted in heat to bring other cells to the same charge state. The efficiency of the passive balancing is thus 0%! Over the years of operation the individual cells drift apart in their properties. As in a chain the weakest link defines the load capacity, the weakest cell is also responsible for the ever-decreasing capacity in the battery cell of a battery.

As far as the state of the art ...


And now the future comes to your home! Our self-developed BMS Joulie-16 uses Active Balancing. Here, the charging states of all the cells of a battery are permanently balanced among each other, that is, the energy of the somewhat fuller cells is used to charge the less full. The efficiency of the balancing thus increases to over 92%, compared to 0% in passive balancing. At the same time, active balancing increases the service life of the Li cells by more than 30%, since the cells hardly drift apart in their properties over the years. Another positive side effect is the increased energy yield from the charged battery: with passive balancing the discharge depth (DoD) should not exceed 70% in order to allow a long battery life. With our active balancing, 90% DoD and more are possible without any loss of the service life, ie over 20% more energy at the same nominal capacity.


Joulie-16 also allows communication between the battery and inverter via CAN interfaces for common inverters, as well as the remote monitoring of the battery via the Internet independent of the inverter. Here, all relevant data such as voltage, charge state, temperature etc. are recorded and evaluated for each battery and each cell of a battery. Thus, for example, it is possible to identify a single prematurely aged cell of a battery and to initiate its prophylactic interchange, thus avoiding a possible failure of the system.


In the case of lithium cells, too, we rely on the commercially available standard lithium iron phosphate (LiFePO4) and use lithium-iron-manganese phosphate (LiFeMnPO4). Doping with manganese increases the lifetime and performance of the cells. The conception and cell chemistry of our Li cells is considered to be intrinsically safe, that is, the cells can not burn in the event of a problem. The batteries of laptops and mobile phones are, by the way, not intrinsically safe!

Lithium cells are almost completely reusable and contain no environmentally hazardous substances in contrast to lead batteries.


In cooperation with the University of Ansbach, the LiFeYPO4 cells used by us were subjected to an extreme test. The cells were thereby discharged approx. 33 times per day to 70% DoD at 1C charging / discharging current. The test was terminated successfully after 10,000 cycles. The results confirm an exceedingly high load-bearing capacity, reliability and the enormous quality of the cells used for our memories.

Equivalent to LiFeYPO4 cells are the LiFeMnPO4 cells. Doping with Mn also increases the lifetime. Both cell types are similar.