Electrified Downhill bike

[BrianG]

On the topic of charging: A simple way to do it would be to use a transformer that, when rectified and filtered, has ~5v higher voltage than the max voltage of the pack you are using, set up a constant current charge circuit (pretty simple), and then use a comparator IC that shuts off the charger at 3.6v/cell. It won't have the CV charge phase, but from what people say, very little charge is done at that point anyway.

So, if you are running 14s packs, that's 50.4v. Add 5v (to account for current source dropout) for 55v. The Y23695 3 in the link above would be ideal; wire the primaries in parallel and the secondaries in parallel, use bridge rectification, around 0.5F capacitance (can use an inexpensive car audio cap). The 40v AC output would be around 56v once rectified and filtered, and would provide over 10A charge rate. Add the CC source and cutoff circuits and be done with it. Balancing, if desired, can be done using multiple balancers, just overlap a cell on each so they are in sync.

Of course, this doesn't have cell count protection, but since it will be used just for a single application, it's fine.

[zeropointbug]

That sounds like a good idea actually... although half the stuff you said went over my head (source current dropout? )

I checked out the transformer, 15lbs is quite heavy, but it's not moving anywhere, who cares I guess. IF this were to be used I might want to get the BMS then as I takes care of balancing while being charged. 10amp charge sounds really nice, as it will be charged on a 720i at about 9amps, each side, so you have to do that twice. Looking for a dual output charger that can charge 7 cell packs... why not a 710i DUO, Hyperion?


Also, for another idea for emergency charging, say you are away from your charger and don't want to use your own effort to bike back (far away), I was thinking a small charging circuit built into the battery case. I was thinking just a couple amps at the most with a built in AC socket with protective rubber plug with a PC AC cord somewhere on the bike stored away.

[BrianG]

By CC dropout, I mean: The CC source circuit will consist of a voltage regulator configured as a CC source, and it has ~1.5v dropout, coupled with the V drop on the current programming resistor of 1.25v. The extra voltage is just a little extra to prevent the pass transistors from saturating when the cells are close to 3.6v (which would reduce the current) and to account for any deviation in supply voltage. Transistors by themselves could be used as a CC source, but their values do change with temperature and current, so a V reg is a better idea for consistent operation.

And actually, I made an error on the previous post. That 15lb transformer would actually be good for closer to 20A. So, you could either use that at 20A, or use a smaller (and lighter/cheaper) transformer for 10A. But as you said, the "base station charger" will probably stay in one place so moving it around is irrelevant. And for a small 2A on-bike charger, a similar idea could easily be made under 5lbs that will do the same thing.

Sure, you can use a Duo or whatever, but unless your charging station is located where the bike is (my R/C stuff is in my basement and my bike would be in the garage), it will be a PITA to move the charger, PS, and assorted cables around. Not to mention the charger/PS setup will be substantially more expensive.

[zeropointbug]

Okay, gotcha. Are you sure it could do 20A? I see 12.5 amps on the spec sheet for the 40+40 model. EDIT: Oh, the transformer must be center tapped? That is why you would get effectively double the current, correct? I am really gonna consider this idea for charging it.. btw this is for my brother so he is gone to University in fall so he will need his own charger, but for now it's the 720i.

The battery will be easily detachable from the bike by means of rare earth magnets built into the enclosure, and on the bike.

[zeropointbug]

Wait, isn't Sine = 0.7x something, so how do you get more than the average voltage when rectified and filtered? Shouldn't the voltage be pretty close to the AC voltage? Or am I missing some current formula that has to be factored in?

[zeropointbug]

Here is the donor bike, a 2 year old mountain bike from GT Bikes. 24 speed, alum. frame, magnesium fork, fr and rr disv brakes, so it should be a decent electric converter.

That's where I thought the batt and controller could go? Although the battery could be moved up and controller could move down and further back?

Each cell is 6.8 x150 x210mm, I still don't know whether I want to go 14 or 16s pack.

[BrianG]

Originally Posted by zeropointbug

Wait, isn't Sine = 0.7x something, so how do you get more than the average voltage when rectified and filtered? Shouldn't the voltage be pretty close to the AC voltage? Or am I missing some current formula that has to be factored in?

Transformers are rated in RMS voltage. But when you rectify and filter them, you get the peak value. 40v AC rms = 56.5v DC (with some ripple depending on capacitance used and current load). Minus around 1.4v for the bridge rectifier diodes gets you around 54v. Oh, and do use a bridge rectifier because instead of the 60Hz ripple, you actually get 120Hz which is easier to filter.

[zeropointbug]

0.71 = Sine Right, okay, I thought you still got the RMS voltage. Yeah, definitely would want bridge rectifier to get maximum power from it. Really gonna consider this as a charger setup for the bike, but later in the summer when he moves away and needs one. It would be nice to get a full charge from empty in less than an hour, and probably less than 1/2 hour regular use.

Was calculating some energy stuff, the bike will have a MPG equivalent of 1800mpg @~25mph. Also, with 16 cells, or ~750Wh, the battery will have nearly the same energy as 1 Cup of white rice, or ~650 Calories.

[zeropointbug]

Brian, do you think you could make up a simple circuit diagram sometime, no hurry, whenever you have time. Nothing fancy, just so I understand what I would be building before I actually did it, and so I have time to think of a good design/circuit setup and materials needed. Thanks.

[zeropointbug]

Alright, I sent a quote for 16 15Ah cells. That's over 50volts for most of the runtime for the bike. Should be able to do 50km or 31mph no prob with the FAST 9C kit. Buying the kit today, about $500 before taxes/shipping with the upgraded 40A controller and a 8 speed freewheel. That comes with a DP-Cycle Analyst which is nice.

[zeropointbug]

16 cells will be here in about 10 days time. $600 + $130 shipping and fees.

Working on designing a good termination plate and the enclosure now.

[Metallover]

Good deal. Looking forward to progress and pics!

[zeropointbug]

I am trying to find some large shrink wrap to cover ~520mm circumference around the pack. So ~260mm exact 'flat' shrink warp width would need to be maybe ~280mm wide to make it easy to get it on? Preferably something like 300mm?

Anyone know of a domestic place to purchase this at is all I want, I have found places that have it, but in China or UK, etc.

Thanks.

[zeropointbug]

bump

[Metallover]

You can get 20cm heat shrink at ebikes.ca. Go to the site, then the store, then search for "shrink".

Another idea is to use Cohesive Flexible Bandages.

http://www.jefferspet.com/ssc/produc...&pf_id=0029430

Wow that stuff looks pretty good.. I'll look for some locally for myself.