Lipo Lithium Battery

Electric transport (Lipo) VS thermal (gasoline): criteria for choosing a battery and comparative calculations

The performance of electric transport depends heavily on the quality of their batteries. It is currently the real Achilles heel which limits the development of electric propulsion, as well in road transport as maritime and… that air of course. We present to you a quick little state of the art of the performance of Lithium-Polymer batteries currently on the market with a few orders of magnitude to remember, some of which are compared with petroleum fuels (gasoline in our case).
In other words: what are the characteristics of a good battery for transport?

a) Have a good mass power capacity (kWh / kg)

A good kWh / kg ratio is the capacity to store energy per unit of mass… the best current Lipos are at 0.2 kWh / kg it is still low! In comparison gasoline provides 13 kWh / kg and with a correction on the respective yields on the energy cycles "from the tank or the battery to the wheel or to the propeller" gasoline still contains more than 20 times of energy useful per kg ...

useful energy for 1 kg of Lipo: * 200 0.8 160 = Wh / kg (80% yield)

Useful energy for 1 kg of gasoline: 13 * 0.3 = 3900 Wh / kg (at 30% efficiency)

Ratio: 3900/160 = 24 in favor of gasoline!

We can remember the order of magnitude of 20.

b) Have a good volumetric energy capacity (kWh / L)

The reasoning is the same as with the specific capacity. Currently the Lipo batteries are between 350-380 Wh / L (manufacturer data).

In essence, correcting returns is 8 times higher. Indeed, 1L of gasoline contains 9.7 kWh or 2.9 kWh useful / L at 30% efficiency.

Ratio: 2900 / 365 8 =

We can remember the order of magnitude of 10.

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Nevertheless, the power density of brushless electric motors (kW / L) as well as their specific power (kW / kg) are quite far greater than thermal (except in extreme cases…). This partly compensates (but far from fully compensating) these low energy storage capacities compared to petroleum fuels ...

c) Be able to provide a high discharge current (A) therefore an interesting power (W or kW)

It is the ability of the battery to give strong discharge current for a long time without getting damaged and keeping stable power. Being able to provide a strong discharge current is a major advance of lithium batteries compared to batteries of previous generations (especially lead batteries ... which, let's face it, are more than a hundred years old!)

A good current 6S 22.2V 5000mAh LiPo has a discharge current capacity of 20 to 30C, which means a maximum discharge current of 30 times its capacity or 30 * 5 = 150A. We obtain a power of 22.2 * 5 * 0.9 * 30 = 3000W, by retaining 10% of voltage drop at full load, hence the factor 0.9.

This battery for 800g which is quite interesting! But of course, this does not last very long after 2min13s the battery is empty.

The comparison with petroleum fuels is difficult, to increase the power as a heat engine, it suffices to increase the displacement and the flow rate of fuel and oxidizer consumed. The comparison here therefore makes little sense ...

d) Strong load current (A)

A Lipo charges between 0.5C and 2C (therefore between 2h and 30 minutes, regardless of the capacity ... only the power limitation of the charger will count) and the charge must be total to optimize the lifespan (the last% are important because done with weak current: look on your smartphone, it is they who advance the slower)

The load is thus at best, 15 times slower than the discharge.

Here too, the comparison with oil is irrelevant and makes little sense: the tank of a 50L car, i.e. around 500 kWh of "gross" energy, is filled in less than 2 minutes, i.e. more than 250 kWh. per minute or 900MJ / min (1 kWH = 3.6 MJ)… or 15MJ / s or… 15 MW! Even by correcting for the yields, this makes an enormous equivalent electric charge current ...

e) Good life: number of cycles or hours of operation

Lithium batteries are generally given for 1000 cycles… in reality it is generally less and is highly dependent on maintenance.

We lack a bit of info and real experience feedback ... for example I still use a smartphone that has exceeded 4 years and the battery capacity is still quite "good" (I would say around 50% of the capacity). initial)… it has therefore exceeded 1000 cycles (but cycles not necessarily complete) and can be operational.

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We generally consider an HS battery when it reaches "only" 70% of its initial capacity ...

It would be necessary here, to compare with the lifespan of a heat engine…. 15 years ago I was taught that a car engine is dimensioned to last 5000 h… ie 250 000 km at 50 km / h of average. I think that number has dropped since ...

f) The price kWh stored

Tesla has announced powerwall solar-photovoltaic / tesla-powerwall-profitable-t13891.html for $ 3500 for 10 kWh… or $ 350 / kWh (all included: with management, box…).

We can estimate that the only cost of the battery is 250 $ / kWh.
For the general public, the current best prices for small batteries (around 200Wh capacity) are at best 300 € / kWh.

It is the same price.

But the quality of a battery is not the other ... those of Tesla must at the top given their feedback.

On the other hand, gasoline is sold for less than 2 € for 10 kWh… well there is no picture as they say!

But gasoline can only be used once of course and it cannot be recharged in a 100% sustainable way with solar energy, for example… (well if but over millions of years…)

g) For this comparison to be complete, there would also be mention of the environment, CO2, embodied energy, recycling ... etc ... but these criteria are more subjective and difficult to measure and estimate and would therefore require much more study. push… both for petroleum fuels (indirect costs…) and for batteries…

More: discussion on the state of the art of batteries at the end of 2016 et forum on electric transport

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