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Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Hi all. So I finally bit the bullet and purchased an electric bike mod kit. Fitted it on the front fork and its slightly different in size but this isn't a big problem.
However, the brake pads are way out of alignment so the front brakes no longer work. This might not be a big problem as my plan was to use regenerative braking i.e. dump the energy through two diode bridges into a drive circuit to charge the 24V battery and lighting system.
Obviously the problem is going to be that damned control box- it appears that the drive circuit is designed badly (cheap chinese junk) so i will probably need to use a 10A dual NC/NO relay (one per winding) so I can either engage the drive or the brakes.
It also has four extra thin wires which appear to be a speed controller and position feedback similar to a standard 3 phase motor, so driving it should be fairly simple.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Well, the first obvious problem is that the batteries are just going to have hard time with you dumping your (mv^2)/2 into them repeatedly in few seconds. That way your 'braking' circuit would only be useful for recovering small amounts of energy from each braking (while dumping the rest) or charging over a longer period of time, like while going downhill.
The solution they use in electric cars are large capacitor banks that store braking energy and then relatively slowly recharge batteries over an inverter.
Considering the large amount of energy you'll need to store (100kg at 20km/h -> 1.46kJ) you'll need some rather large capacitors to make much use of regenerative braking circuit. And even then batteries need to stand lot of stress in order to be charged quickly enough between brakings.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
I don't know what the spec is but if we are talking about 24V batteries and 10A relays then the battery must be able to deliver something like 240W. If you bring yourself from 20 km/hr to as stop in a few seconds that's about 500W. The battery will probably cope with being over run by a factor of 2 for a few seconds. The efficiency will suffer but I think the battery will live. The real problem with regenerative breaking is that, if the eleectronics fail, you have no brakes.
Registered Member #347
Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106
A battery can absorb quite a bit of charging current when it's not near a fully charged state. For example, I did a test with a 50AH sealed battery, and it could take a 100A recharge current while at 50% depth of discharge, reaching 14.5V during the test. So if this scales with capacity, you should be able to dump a current of about 2x the AH capacity of you battery, for a few seconds to tens of seconds at least.
An electric bike I built has regen, and you can literally go for a complete ride without ever touching the mechanical brakes. It runs 4x 12V 17AH batteries in series, with a 300A drive/150A regen motor controller connected to a 48V PM motor.
The motor controller basically uses the motor as the inductor in a boost converter, to boost the generated voltage high enough for the batteries to absorb current. This allows regen braking right down to almost zero speed. The controller will also cut back regen if the battery voltage gets too high, if you need more braking than the controller can provide then you just squeeze the brake lever further to engage the mechanical brakes.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Interesting ideas. I spoke to someone today who suggested I added a tilt sensor which increases the pulse width to the batteries as the gradient increases, so that the speed remains constant and the batteries stay charged.
Adding a capacitor across the batteries would seem a good idea, to lengthen their useful life (there's virtually nothing in the controller apart from some silicon, not even a voltage monitor, 21-24V according to the label) so it will also run from a pair of 11.1V Lipo packs in series.
I will probably also add a failsafe which shorts two phases through an unfused normally closed relay and resistor and disconnects the motor drive, in case something else breaks. (so no matter what it will stop) Would also seem essential to add a series connected circuit protector on the battery itself just in case.
Interesting to note that using a switching converter I can keep the voltage constant with variable current (might even be able to charge a second pack this way) and the kit seems to include various switches and sensors to cut off power automatically when I stop pedalling.
Registered Member #1497
Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801
Since it hasn't been suggested yet, perhaps a supercapacitor could be used depending on how much kinetic->electrical energy can be gained via braking.
A supercapacitor (or any capacitor really, just a supercapacitor due to its higher energy density compared to regular electrolytics) will charge up faster, and you could do current limited charging or immidiately expend the energy in the acceleration stage right after stopping your bike.
Perhaps even one of those 12-24V audio capacitors that are 1F could act as a good energy storage device incase you really needed that kick to get you going. Label a toggle switch 'nitrous' and leave your friends wondering what exactly it does.
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
Conundrum wrote ...
Interesting ideas. I spoke to someone today who suggested I added a tilt sensor which increases the pulse width to the batteries as the gradient increases, so that the speed remains constant and the batteries stay charged.
Adding a capacitor across the batteries would seem a good idea, to lengthen their useful life (there's virtually nothing in the controller apart from some silicon, not even a voltage monitor, 21-24V according to the label) so it will also run from a pair of 11.1V Lipo packs in series.
I will probably also add a failsafe which shorts two phases through an unfused normally closed relay and resistor and disconnects the motor drive, in case something else breaks. (so no matter what it will stop) Would also seem essential to add a series connected circuit protector on the battery itself just in case.
Interesting to note that using a switching converter I can keep the voltage constant with variable current (might even be able to charge a second pack this way) and the kit seems to include various switches and sensors to cut off power automatically when I stop pedalling.
Lithium is a different beast.. I wouldnt want to put more then 2C into a Li-Poly pack, maybe up to 5C if it was a pack meant for rapid charging, much more then that and I would be worried about degrading the pack. LiFePO4 batteries on the other hand I wouldnt worry about so much, I would definitely look into a supercap, and smps/boost converter to drain the supercap and charge the batteries. Not exactly cheap though...
Registered Member #1497
Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801
My only thought is safety at this point, do you want to be going 20-50km/h, only to brake, and have your LiPo's burst into flame between your legs under your crotch.
Get a nomex jockstrap :D
Really though, the lipo's won't take too much charging, and while there may be some tolerance for faster charging, in order to dispose of excess energy during say a downhill slope where energy can't go into the batteries fast enough, either a big resistor, or a capacitor would be good...
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Regenerative braking?
