Lithium ion still has a pretty steep early adopter tax, and they're not as forgiving as lead-acid (Overcharging lead-acid batteries ruins your $2k pack. Overcharging lithium ion batteries will ruin your $20k pack and can easily burn up your car and your house).<p>But they're very much coming down in price. In RC-plane sized packs, lithium polymer packs (with a 5C discharge rating) are down to about $500/kWh retail. A minimum viable pack for a very light car might be five kilowatt-hours (and might only weigh 70 pounds, a little over a tenth the weight of lead-acid).