Ninja 250 conversion

for your first, i'd say go direct drive... you can always modify later. A good torque converter (CVT) would cost over 150 for the power band you'd need.

Not many people have used a CVT, and the 250CC is plenty light for that etek motor. Maybe people have used the etek with no problems. Just keep it cool, thats all.

Direct drive is what most motorcycle guys do, I've only seen one or two that did a CVT and I don't know if they got results that were that great to warrant the extra fabrication and equipment needed. Not sure how much extra weight they are either.

look on EV album for CVT's.

not sure, I got 14 18Ah SLA's and made a pack of 72V 36Ah...

just do some measurements on what room you have in the frame.... go from there.

You'll be fine with a direct drive setup. I would recommend an Etek-RT, even though it is slightly heavier. The RT has a lower rpm/v constant, and should be easier to gear down. Also, the RT is officially rated to run up to 72v, and it has a higher continuous current of 125 amps vs. 48v and 100 amps for the Etek-R.

Thanks or the reply. Is there a particular DC SLA battery to use (Brand)? I was thinking of 4-5 12v 33ah. Or even the smaller Walmart Deep Cycle, if I coule fit (4). They have an 18 month warranty.

I highly recommend using sealed lead-acid of absorbed glass matt (AGM) type. Flooded batteries will deliver poor performance in a small size, and also have the potential to leak acid and corrode stuff. If you want to use flooded, then it's best to get large batteries, I would guesstimate at least 50 ahr.

I think pretty much the best AGM battery on the market for cost and quality are B&B batteries. Some easier to get brands could be Werker, and Powersonic which both seem to be sold by Batteries Plus. Also, Interstate Batteries, and MK Batteries which you might be able to find from an industrial battery supplier. All of these are fair quality, but make sure and get AGMs and not Gels because Gel batteries won't perform nearly as well. Gels are an option I guess, but you'll probably want to get very large batteries if you went with them like if you went with floodeds.

BTW, while your at it, you also may consider Lithium iron phosphate (LiFePo4). These will probably run you about $2,000 for a 60v 40 ah pack, but the weight would be so low in comparison, and they should last much longer than lead-acid.

Sorry if I'm rambling on. The important thing is to consider your goals when deciding on a battery. If you just want to get your project moving now on the lowest budget possible without much concern for battery life, then something from walmart might be just the thing.

You're correct, the motor may draw close to 200A.... but remember power is power. That 200A might only be at 0-1000rpm during acceleration. So lets say the first 25% throttle. Using only 25% PWM. When the controller turns the FET's on, it uses power from the cap bank inside the controller AND battery power. When the FETs are off, the batteries charge the caps back up. So when you're on, the caps are in parallel.

I will only be drawing like 80A from my pack, but the current coming out on the motor side is close to 350A.

But I would still agree, 33Ah and only 5 of them is a bit small (1980Wh). Those bats would be fine, for about 15 miles. you only get 80% of the bats, so roughly 1500W, say you can cruise a lightweight ninja around 100Wh/mile. 15 miles is on the high side. 33Ah would be fine, but I'd put more of them in there. If you're limiting yourself to 72V on the controller, I'd step up to something above 40Ah.

I'm going to change my pack over to either lifepo (prices coming down) or 10 30Ah SLA's for 120V for a 3600Wh pack. Right now, I've got about 2500Wh.

Hmmm, interesting.....I don't think the controllers cap bank capacity will have any significant contribution to the motors heavy current load. I also think Kirkoffs current law would say 80 amps out of the battery and 350 amps into the motor is not possible...gotta equal zero...no?

Nope, it doesn't.

Kirkoff's current law only applies to circuits where the voltage is fixed. Once you start converting, the game changes. If you PWM 100V at 50% the average voltage is 50V. If you have a cap bank on it (all controllers do)... when you gate the FET's, it'l discharge the caps AND bats for 50% of the fixed frequency (say 15khz). So, its got all the rest of that 50% off time to recharge the caps. The batteries see a CONTINUOUS draw of current, but the controller is only switching at 50%. The motor loop (with "freewheel) diodes keep the motor loop going. When you switch on, the power goes to the motor, when its off, the freewheel diodes let the current keep flowing. Kind of like giving it gas, then letting off, then giving it gas, then letting off at 15khz.

That 350A motor side and 80amp battery side was measured on my bike last week.

If you don't believe me, go measure it on your bike, I think you'll be suprized. Use a decent DC clamp ammeter.