Individual cells that are designed for fast charging can be charged from near empty to near full in as few as five or ten minutes (for the very fastest cells). Vehicles like the Nissan Leaf can accept an 80% charge in 30 minutes from a 480V level 3 fast DC charge.
As you say, bike batteries are much smaller than car batteries and can pick up a significant charge even from a level 2 AC charger. The level 2 "chargers" (in truth, they're called supply equipment because all they do is supply AC current to the bike, the bike's onboard charger converts AC to DC and charges the battery in a controlled fashion) operate at 240V and typically provide 30A or more (6 kW), up to 80A (16 kW).
So why don't we see a 16 kW charger on the Empulse, since it supports J1772? That'd give the bike a 80% 30 minute charge..
1. Battery weight. Battery cells typically feature a tradeoff between power and energy .. which is to say, for a fixed weight and size budget you can either store more energy or discharge a smaller amount of energy faster. A power cell that can recharge very quickly (30 minutes) may not store as much energy as a cell that wants 1-2 hours to charge. Brammo has selected
very high energy cells for the Empulse, which may mean they're more limited in how fast they can accept charge.
2. Battery durability. Regularly charging at fast speeds (80% 30 minute charge) may cause the battery to degrade more quickly. Nissan claims 80% original capacity after 8-10 years for the Nissan Leaf if charged at AC level 2, and 70% original capacity after 8-10 years if regularly charged at DC level 3. These are projections based on lab tests; the real world may prove there to be a lesser or greater impact. Currently it's a moot point as few CHAdeMO chargers exist in the US.
3. Charger weight / size. A faster charger generally is larger and has to deal with more heat (90% efficient at 1 kW is 100W to dissipate, 90% efficient at 16 kW is 1.6 kW to dissipate..). Zero uses a 1 kW charger on their bikes. Brammo uses a 3 kW charger on their bikes. Mission Motors has a design for a
4.5 kW charger that shares a liquid cooling system with the motor .. wonder if Brammo does the same. In short, space is tight.
AC Propulsion has patents on a
motor controller (DC -> AC to power the motor) that can operate in reverse (AC -> DC) to charge the batteries. This would be
absolutely stellar for big-battery motorcycles that have very tight packaging requirements. The reductive system makes use of
IGBTs which might make it prohibitively expensive in the relatively low-cost bike market.
Oak Ridge National Labs (up in TN, a short quick hop and a jump away from me) has developed
a similar shared-component technology, and Tesla Motors originally licensed AC Propulsion's reductive technology but has supposedly since designed its own flavor.
When will we see something similar in bikes? I'd like to believe sooner rather than later...
Suppose you had 3 hypothetical designs.
1. Existing Empulse w/ 10 Ah energy cells, 36s9p 103V 90Ah
9.3 kWh, 470 lbs, 56 miles highway range, 3.5 hour charge time (16 miles/hour), $17k.
2. IGBT ACP Empulse w/ 8Ah power cells, 36s10p 103V 80Ah
8.0 kWh, 440 lbs, 48 miles highway range, 0.5 hour 80% charge time @ 16 kW (77 miles/hour), $19k.
3. IGBT ACP Empulse w/ 8Ah power cells, 36s11p 103V 88Ah
9.1 kWh, 470 lbs, 55 miles highway range, 0.5 hour 80% charge time @ 16 kW (90 miles/hour), $21k.
What do you think people would prefer?
