## Sunday, 27 October 2013

### Petrol and electricity

I was thinking about how the Lipo batteries that I'm using, compare with petrol. After a bit of googling and calculation, I came to:

1 kg of Lipo battery, fully charged = 150 watt-hours
1 kg of petrol, converted into power by an internal combustion engine = 3000 watt-hours

So that's why petrol is a more portable store of energy than a battery.

Another thought - how long does it take to charge a Lipo battery? The ones I'm using are 5 aH and 4s. That means they'll give you five amps at 14 volts for an hour. For discharing, they're rated at 20C. That means that you can get that energy out in three minutes, if you wanted to, by drawing 100 amps from it.

For charging, they're rated at 5C. That means that you can charge it in 12 minutes, by giving it 25 amps at 14 volts.

When I have four batteries to charge, I can charge them in parallel. So if I can supply 100 amps at 14 volts, I can charge all four in 12 minutes. My power supply isn't able to deliver so much current, but if I wanted to, I could build one that would do it.

So, if you have an electric car made from batteries like this, how fast could you charge it? If the charger could supply enough current ... 12 minutes. Which is about how long it takes me each time I fill up with petrol, including the wait for the pump, and the payment.

One of the argumants against electric cars just curled up and died - they don't have to take a long time to recharge.

But isn't it dangerous to have so much current flowing? No. Now if you have 100 volts, that would be dangerous. But 14 volts isn't dangerous, because if you put a finger on the positive and a finger on the negative, very little current would flow. Even though the charger is able to supply 100 amps, the ohmic resistance of your body would mean that only a tiny current would flow. And 14 volts isn't something you'd even feel. And that means that you don't have to take the kind of careful precaustions that you have to take with handling petrol. Which means that outlets for charging could be all over the place.

I was talking to some folks at the Worthing bridge orgy yesterday, and I made three technology forecasts for the near future.

1) Self-driving cars. I really really want one, and even more, I really really want everyone else to be in one. Self-driving cars will *not* tailgate, will *not* change lanes without checking that it's clear. The death rate from cars would plummet, and it's worth it for that alone. The biggest obstacle right now, I think, is, when there's an accident, who's to blame? The human occupant, the car manufaturer, the software writer? As soon as that is sorted out, I expect to see a huge number of cars offering a self-drive option.

2) Lighting using LEDs. Incandescents are very wasteful, 90% of the power is lost as heat. Fluorescents flicker, and the low-energy bulbs in some of our rooms, take so long to come to full brightness,  I lose patience! But LEDs are very economical to run, don't flicker and come to full brightness immediately. The main obstacle is cost, but this is semiconductor technology, and we all know what happens with costs.

3) Electric cars. See my discussion above.