Battery energy and safety

[dmethvin]

Here are some interesting facts. Some of the new unicycles are going with really large batteries. A 840Wh battery holds 3 megajoules of energy. That's about the same energy in three-quarters of a kilogram of TNT or 1.5 sticks of dynamite. It is enough energy to run a full sized car for 1.5 miles at 40 mph. Still, a cup of gasoline is more than twice as powerful and weighs less, which is why it's so hard for electric cars to replace gasoline. 

We're all hoping we can travel with our EUCs, but remember that they don't let you bring gasoline or dynamite on a plane either. The difference between dynamite versus a battery is all about how the energy is released, but any high-energy storage device will have some failures. Let's hope they make those batteries really well!

[Cranium]

Very interesting information.  I knew about the energy density being very high in gasoline but haven't seen it put in a chart before alongside other seemingly unrelated items for comparison.

Wiki has a good energy density article too: https://en.wikipedia.org/wiki/Energy_density

They even threw in the energy of a Ham and Cheese sandwich!  Although I question the accuracy of this because even in what they referenced, it doesn't specify the type of bread used so how accurate could that really be?  

[Villac]

I think the battery restrictions are valid. The danger I see is the flammability of lithium: once exposed it is very hard to quench the fire. One good reason I have to keep my trainer EU is that it is below the limits for flight restriction.

[csmyers]

7 hours ago, dmethvin said:

Still, a cup of gasoline is more than twice as powerful and weighs less, which is why it's so hard for electric cars to replace gasoline. 

Very interesting statistics! However, we have to take into account that most internal combustion engines run at around 30% efficiency or less, and the rest of the energy is lost as heat. Electric runs closer to 90% efficiency (or more!)

[Cranium]

5 minutes ago, csmyers said:

Very interesting statistics! However, we have to take into account that most internal combustion engines run at around 30% efficiency or less, and the rest of the energy is lost as heat. Electric runs closer to 90% efficiency (or more!)

Lets take into account that a car is 14%-30% efficient and a EU is much more efficient but I doubt they are 90% since an electric car is only about 59%-62% but let's go with that 90% number anyhow.

For this I will use

• 20% efficiency for the gas car
• 32.4 MJ/L energy density of Gas
• 90% efficiency for the EU
• 2 MJ/L energy density for Lithium Ion Batteries

Gas Car =  32.4*20% =  6.48 MJ/L of energy to move you forward

EU =    2*90% = 1.8 MJ/L of energy to move you forward  

A gas car still has 3 times more power per liter than a lithium battery powered EU.  Incredible, isn't it? :)

 

[dmethvin]

5 hours ago, csmyers said:

Very interesting statistics! However, we have to take into account that most internal combustion engines run at around 30% efficiency or less, and the rest of the energy is lost as heat. Electric runs closer to 90% efficiency (or more!)

There are definitely a lot of factors involved here. Although batteries are more efficient they're also heavier and don't get lighter as you drain them. As you burn gas it disappears from the car and you no longer have to carry it around.  Engines tend to be heavier than DC motors though, so there's that. From the safety standpoint the energy in the gas is dangerous whether it's used efficiently or not, and a half-empty tank can be a higher risk since the fumes of gas-air mixture pose an explosive danger. If the tank is breached though I'd rather it be almost empty than almost full. Not sure if a discharged battery is significantly safer than a charged one?

[esaj]

6 hours ago, dmethvin said:

Not sure if a discharged battery is significantly safer than a charged one?

Maybe not "significantly safer", but apparently the thermal runaway temperature of Li-ions goes up as the voltage of the cell lowers, so it's less likely to catch fire / explode the lower the voltage (ie. the more empty the battery is). On the other hand, if the cell(s) get "too empty" (below 2.5V), they can get damaged, but from my understanding rarely cause any fire-danger, just permanently lowered total capacity and faster self-discharge. I recall some Battery University article stating that you should not attempt to charge any Li-ion -cells that have dwelled below 1.5V for a week or more, as then they can catch fire during charging (some electrochemical reaction happens inside the cell, causing metal buildups and internal short circuits start to form).