Electric Mobile - handmade electric car.

During reviewing the list of products of ThunderSky, I have noticed that  different models have different ratio of energy density/weight, or in physical units it is kW*h/kg. For electric vehicles, the higher this ratio, the better is the power source. For those who do not know yet, kW*h is the measure of work. In other words, if the electric heater with  1kW power worked within an hour, it has spent exactly 1 kW*hour of energy (but not the power!). It is wrong to write  kw/hour. Power can be measured in watts and in horsepowers.

Работу также можно измерять и в Джоулях, как в системе СИ. Из школьного курса физики вспоминаем, что 1 Дж = 1 Н * 1м. Однако, на практике было бы не удобно использовать единицы Джоуля ввиду необходимости оперировать большими значениями, поэтому условились измерять потраченную электроэнергию в кВт*ч.  Это и есть единица измерения совершённой  работы.

The best electric storage devices are those that have the highest ratio of kWh/kg. That is why we need batteries which can deliver more power at low weight.

The best electric storage devices are those that have the highest ratio of kWh/kg. That is why we need batteries which can deliver more power at low weight.

Let’s look at lithium-iron-phosphate batteries. We choose the average voltage of a cell at 3.2V. Depth of discharge is 70%.

It turns out that the best battery to use is the TS-LFP260AHA. TS-LFP60AHA has the worst performance. However, there are some nuances. For example, TS-LFP260AHA with allowed curents of 3C can be discharged by 780A current. Such current requires wires with huge diameter and mass. The use of powerful cells are not convenient because of losses on the wires, losses on power transistors, etc. It is easier to operate with high voltage and low current. It requires smaller wires, terminals, connectors, etc.

The Li-cell is not also guaranteed to pass 5000 cycles with charge/discharge current of 3C. But if we use them at the specified operating currents of 0.5C,  it is possible to reach 3000 cycles at 80% depth of discharge, or 5000 cycles at 70% DOD. Pay attention to the text of official instructions for testing of lithium batteries at http://www.thunder-sky.com/technical_en.asp?typeid=109. PDF file is http://www.thunder-sky.com/pdf/test-en.pdf. Look on page 6, section 6.6:

6.6. Cycle life test (80DOD %)

Place the cell under 20℃±5℃ condition and charge the cell with C3 (A) constant current.
When the cell voltage reach 4.0V, stop charge and place the cell for 1 hour.
Place the cell under 20℃±5℃ condition and discharge the cell with C3 (A) current until the discharge capacity reach 80% of rated capacity. There can be a 30 minutes to 1h interval between the charge and discharge of the cell. Repeat 100 times and the cell nominal capacity decrease rate should be less than 1‰ AH.

That is for first generation batteries. They have 0.3C nominal current. The second generation batteries (as in the table above) have 0.5C nominal current.

If you are going to buy LiFePO4 cells, you should calculate the necessary amount of cells to ensure that the nominal current will not exceed 0.5C at desired stable vehicle speed. This will ensure the long life for the battery.

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