jd Posted January 19, 2016 Share Posted January 19, 2016 I had an interesting experience yesterday and I'm hoping someone can help me make sense of it. My IPS 121+ went out with me for my first serious uphill ride and I collected some numbers I can't quite explain. My wheel has a nominal 350 Wh (Watt hour) energy capacity. I'm a pretty big guy (~100kg) and I went up a 75 meter (vertical) hill at a pretty slow pace, maybe around 6-8 km/h, total road distance about 1.5 km; parts of the hill are pretty steep. Initial battery state as reported by the IPS app was around 70%. By the time I got to the top of the hill, the app was reporting 35%. Assuming that the percentage reported by the app refers to the energy left in the battery and not something else, my wheel used about 35% of 350 Wh = 122 Wh of energy bringing me up the hill. The actual physical (gravitational potential) energy of moving 115 kg (me + wheel) up 75m is ~85 kilojoules, or about 24 Wh. Going up a pretty steep incline, with really strong power draws from the batteries, plus all the balancing and everything else using energy on the wheel, it seemed reasonable to see a 20% efficiency. To recap: 24 Wh: minimum energy required to move me up the hill. 35%: battery capacity change 122 Wh: 35% of 350 Wh, the nominal battery capacity, the estimated actual energy used to move me up the hill. 20% energy efficiency: seems low, but what do I know? Then what happened next surprised me. I rode my wheel back down the hill, expecting to get back some amount of the lost energy through regenerative braking. From somewhere I got a 50% efficiency number on regenerative braking, so I expected to get back a little bit of energy. And indeed at the bottom of the hill, my battery was back up to 52% charge. So I got back 17% of my battery, which makes sense -- I used 35% going up, and got back 17% -- about half -- going down. Except that would imply that my battery accumulated 60 Wh or energy going down a hill that only has 24 Wh of potential energy. Either IPS has managed a 250% efficiency in regenerative braking (someone call the Nobel committee!), or my math or physics is wrong. To recap: 24 Wh: maximum possible energy gained by rolling downhill 17%: battery capacity change (a gain this time) 60 Wh: 17% of 350 Wh, the estimated actual energy recovered by going downhill. 250% energy efficiency: huh? Anyone have any insight? Are the battery ratings optimistic? Is the charge meter in the app wrong or non-linear or something else? Perhaps the actual nature of battery chemistry means my battery underreported charge after a big power draw? If I assume that my wheel does recover 50% of downhill energy, and thus 24Wh / 2 = 12 Wh represents 17% of my battery, then its true (usable?) energy capacity would be only 70 Wh, a far cry from 350 Wh. (If this is common among EUs, someone must have noticed by now, right?) Thoughts? Explanations? Ideas? Errors? Link to comment Share on other sites More sharing options...
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