1 - 14 of 14 Posts

I'm looking for a combination of battery packs that would ideally give me around 700 volts and between 50 and 90 kwh.

That's a discharge rate of 10 to 20 C/hour. At least the low end of that range is feasible for a few seconds (which is probably all you need) for at least the more suitable production EV batteries (particularly the plug-in hybrids, and generally those with good liquid cooling).I'm looking for a max 920 kw discharge.

If you want more than a few seconds of high-rate discharge at a time, you might be interested in the experience of the builders of the Tesla-motor Cobra-replica race car.

Capacitors are not as energy dense as batteries, so the physical space required becomes an issue. For your application, I'd try contacting Arlin Sansome (youTube) for the race batteries he has and use a larger energy dense pack to charge the smaller, but capable of 100C discharge pack. Lots of electronics and control systems to make it work. Possible, but expensive and definitely requires a large area for batteries.

Bill

The individual cells were 100C continuous, 5 ah, so you'd need 2 of these in parallel and 194 of these in series. And then another larger pack to get the range you want and BMS magic to make the two packs work together without damage to either. Again, possible, but not cheap. Also custom design for cooling on the high discharge rate cells.For batteries, I would need a 700 volt, 7.148 Ah battery bank capable of 200C discharge.

When you are calculating how much energy you get out of the capacitors, what voltages are you using? Obviously you start at 700 volts, but what voltage do they discharge to?You're right, I would only need such discharge times very occasionally: basically only when accelerating from 0-300 in 8.9 seconds, which is a fairly narrow use case. Would it be possible for me to use some sort of capacitor bank that I only charge up then to fulfill peak needs? ... Based on very preliminary calculations and a poor knowledge of how capacitors actually work, I would need a 700 volt, 36 farad capacitor bank.

For instance, if you could completely discharge the capacitors then the energy released would be

E = 1/2*V**2*C

= 1/2*700*700*36

= 8.8 MJ (= 2.4 kWh)

or about 1 MW for 8.8 seconds.

But that's not what is going to happen.

In reality the voltage will only drop as much as the battery voltage drops. If that's 10%, or 70 volts (to 630 V), then the energy released is

E = E1-E2

= 1/2*(V1**2 - V2**2)*C

= 1.7 MW (= 0.5 kWh)

or about 188 kW for 8.9 seconds

This all assumes that my basic physics is correct and the energy stored in a capacitor is equal to one-half of the product of the voltage squared and the capacitance. But I haven't had my coffee yet today.

That's 35 farads of 20 volt capacitor, or one farad of 700 volt capacitor. Just like battery cells, a series connection adds up the voltage but doesn't change the capacity (amp-hours for a battery, farads for a capacitor); a parallel connection adds up the capacity but doesn't change the voltage. For 36 farads at 700 volts, you would need 36 (in parallel) x 35 (sets in series) = 1260 capacitors at one farad and a maximum of 20 volts each. If 35 capacitors cost $3,000, this is a $100,000 capacitor bank... and it weighs about two tons.

This is why EVs are not built with capacitors for energy storage, especially low-voltage capacitors.

Of course the whole scheme works much better with high voltage capacitors - then you would only need one set in parallel. But they're not cheap, and it is still possible to use only a small fraction of the energy stored in them before the voltage drops too low.

1 - 14 of 14 Posts

Join the discussion

DIY Electric Car Forums

A forum community dedicated to DIY electric car owners and enthusiasts. Come join the discussion about electric vehicle conversions, builds, performance, modifications, classifieds, troubleshooting, maintenance, and more!

Full Forum Listing
Recommended Communities

Join now to ask and comment!