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Old 08-22-2019, 11:48 PM   #14 (permalink)
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spyder2 - '00 Toyota MR2 Spyder
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Originally Posted by mpg_numbers_guy View Post
Yes, it mostly does just produce extra losses at higher RPM, but more power is available at lower RPM than otherwise, and for max regen braking you'd want to keep it in the highest gear possible down to 1000 RPM to avoid engine friction when slowing down. As I said this would be counterproductive if the alternator was going to be used normally, but if it's just going to be used for regen as the OP seemed to indicate, then it might be worth checking out at least.

Why 24V? The car runs on 12V....15V max if you want to keep things fully charged.

Cost is also a factor - $570 is a decent chunk of change. Once you start getting up there in price you might as well have a hybrid that has full regenerative braking and whatnot. Part of the economy of not having a hybrid is to avoid having expensive batteries in the car, whether those are HV batteries or expensive lithium 12v batteries.
A custom pulley is going to be a good chunk of change too, and the diameter on the alternator pulley is already small. Spinning the alternator faster for a tiny bit more regen doesn't sound like it's going to end up very economical to me if you end up having belt slip issues. Also the belt efficiency will drop, and the alternator fan will produce more losses.

24V because it instantly doubles the power output from the alternator. I'm not sure if stock rotor field windings can handle double the field current, but if they can there's no issue at all. If they can't, you might need to downshift one gear to get the best out of the alternator (you'd be increasing the regen to engine braking ratio anyways so it's fine).

Of course you can get away with less than 500 on the battery too, 30Ah is pretty generous. 20Ah would probably be totally fine, since not all the alternator current is going to the battery. It depends how often you want to cut the alternator and how long you want the battery to last.

Ultracapacitors to absorb the charge would let you use 12V LiFePO4 batteries to exclusively power the car, but the ultracapacitors also cost money, and you still need a DC-DC converter.

Obviously the cheapest solution is to just use the stock alternator with a 12V battery, but that barely saves any fuel.

Last edited by serialk11r; 08-22-2019 at 11:53 PM..
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