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02-09-09, 09:05 AM
#24
safe
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The Differing Battery Size Scenario

What if we vary the battery size?

Freewheel pack is set at 30 lbs (so change the online calc to read 150 total lbs)

Regen pack is set at 15 lbs (so change the online calc to read 135 total lbs)

The freewheel pack has 800 Wh.

The regen pack has 400 Wh.

Now we do the calculations again:

Downhill (5% negative slope for 10 miles)

Freewheel - 10 miles / 26.8 mph = 0.37 hours (net power gain/loss is zero)

Regen - 10 miles / 15.00 mph = 0.67 hours (behind by 0.30 hours)

Recaptured Energy = 0.156 hp * 746 watt/hp = 116.4 watt

116.4 watt * 0.67 hours = 78 Wh * 0.7 (motor losses) = 54.6 Wh

54.6 Wh * 0.9 (battery losses) = 49 Wh (this is the energy recovered)

Uphill (5% positive slope for 10 miles)

Freewheel - 10 miles / 13.5 mph = 0.74 hours

Power Needed - 0.476 hp * 746 watt/hp = 355 watt / 0.7 (motor losses) = 507 watt

507 watt * 0.74 hours = 376 Wh

Regen - Must do 10 miles in 0.74 hours - 0.30 hours = 0.44 hours (or less)

10 miles / 22.94 mph = 0.44 hours

Maximum input power - 0.938 hp * 746 watt/hp = 700 watt / 0.7 (motor losses) = 1000 watt

1000 watt * 0.44 hours = 440 Wh

...but we get to subtract the "savings" so the actual value is:

440 Wh - 49 Wh = 391 Wh

-----------------------------------

But what happened to all the regen savings we were hoping for?

If the motor is limited in power (1000 watts input) then the regen bike will go as fast as is possible to catch up. If the freewheel bike goes just fast enough to stay ahead of the regen bike then it only uses as much as is needed to win.

It's a "reverse calculation" to do... the freewheel "Hare" gets to be lazy only going as fast as it needs to be. The regen bikes motor gets "maxed out" and can't catch the "Hare" because the "Hare" can use up more energy if it needs to. (after all it has a battery twice the size)

Not only that, but the freewheel bike has a pack with double the energy so we got to the end of the race and the weight difference didn't help the regen bike very much.

Freewheel - 800 Wh pack and uses 376 Wh = 53% in reserve.

Regen - 400 Wh pack and uses 391 Wh = 2% in reserve.

.

Last edited by safe; 02-09-09 at 09:45 AM.