Building an EV motorcycle. Batteries: Need opinions.

[TurnNBurn]

Update

Got a stand for the rear wheel and the lower part of the chain still rubs. A chain tensioner is inevitable. I have ordered a skateboard wheel with a bearing. I'm going to have to rig up one of these

[puddleglum]

Originally Posted by TurnNBurn

Actually, I overlooked something. Battery management systems and chargers. If I'm going with Lifepo4 batteries, do they have integrated BMS or is there an external one you'd recommend?

I don't know if you have resolved what you are going to do with your BMS yet, but I would suggest that you don't really need an expensive battery management system with LifePo4 to keep them safe. There is a lot of debate over at "DIY electric car" about this, but from what I've read, I'm inclined to think that by bottom balancing the cells first and carefully selecting your charger cut off voltage, it is unnecessary. If I ever get to do my build, that is what I plan to do. I think a simple battery monitoring system may still be a good idea, but that can be done for under $100 on a small pack. I just can't see paying all that extra money for a system to balance the cells, when the systems have been known to fail and cause battery damage. That's just my opinion though.
So, what cells did you decide to go with?

[mechman600]

I visited a guy in Calgary a few weeks ago who did an FZR400 EV conversion. I know him because I sold him a Kelly charger on eBay. He told me that his lithium pack goes out of balance every single time he rides the bike. He thinks it is because the front cells are cooled by the wind, while the rear cells are heated by the motor, making their temperatures vary significantly. He made his own BMS for cheap, but he is an electrical engineer.

That being said, a lithium pack will normally stay balanced on its own for quite a while. But it will need a re-balance one in a while, which is time consuming. I think an EV should take care of itself with no tinkering required.

Also, why spend $10K on a battery pack and not $800-1000 on a BMS system? It seems like an insignificant amount for some insurance.

[puddleglum]

Originally Posted by mechman600

a lithium pack will normally stay balanced on its own for quite a while. But it will need a re-balance one in a while, which is time consuming. I think an EV should take care of itself with no tinkering required.

Also, why spend $10K on a battery pack and not $800-1000 on a BMS system? It seems like an insignificant amount for some insurance.

That is the argument from the battery management camp and was my thought too until I started reading the rebuttal from those who are proponents of bottom balancing and battery monitoring. The thing is, I'm not convinced any more that it is insurance as much as adding a risky failure point for the sake of convenience. If the BMS board (probably made in China) fails, it can kill cells. It requires the cells to all be slightly overcharged every cycle in order to balance them which is hard on the cells. It does provide a high and low voltage cut off, but you can do that with a cheap monitoring system.

[TurnNBurn]

Here's an update to my chain issue. I got a skateboard wheel and mounted it directly on the motor mount plate. It may seem like it's bending and there's too much slack, but that's because the chain isn't tense yet and I haven't shortened the chain. That'll be today or tomorrow.

After I remove the link from the chain and put the master link in and adjust the wheel to tighten it, I should be almost good to go. Motor is in, chain is on, I'm working on a mount for the controller. Throttle and grips are being installed today. I just need to figure out the battery situation and then wiring.

[Ryland]

I would support the outside of the roller skate wheel, even a long piece of steel strapping.

[TurnNBurn]

Originally Posted by Ryland

I would support the outside of the roller skate wheel, even a long piece of steel strapping.

You mean support it with another bracket to hold it in place?

[Ryland]

Originally Posted by TurnNBurn

You mean support it with another bracket to hold it in place?

Yes, just a steel or aluminum bar that puts tension on the outside of the skate wheel so it doesn't bend.

[TurnNBurn]

Originally Posted by Ryland

Yes, just a steel or aluminum bar that puts tension on the outside of the skate wheel so it doesn't bend.

The bolt itself is pretty well enforced with a very thick and long steel washer. Well, it's more like a shaft than a washer. The shaft is between the bearing on the wheel and the motor plate. I also have several lock washers in place.

I have a question regarding batteries again. My goals have changed significantly. After doing some highway driving here in vegas and being almost run off the road in my truck I doubt I'm going to want to take the bike on the freeway up to 65-75 MPH. However, since speed limits on city roads are typically 45-55 MPH, that is now my goal.

45-55 MPH for at least 40 miles. Once again I find myself lured into by lead acid, this time for new reasons. Not only are they cheaper, but I found out you can drain the acid and refurbish batteries from home for relatively low cost. Granted, this can only be done two or three times, but that's more life from an otherwise dead battery.

So my new question is with LifePo4 I was looking at 65 amp hour batteries. Those go for $77 a battery. That was back when my goal was sustained 65-70 MPH speeds. With my new goal, would 40 amp hour battery do the job? I'm still looking at a 72v setup.

[Daox]

I'd highly suggest sticking with lithium. Their cycle life is rated around 3000 cycles under ideal circumstances. Under those circumstances you can use up to 80% DOD. With lead acid, you get at most 800 cycles at 50% DOD. So lets do some math:

65Ah lithium @ 80% DOD gives 52 usable Ah.
40Ah lead battery @ 50% DOD gives you 20 usable Ah. In order to get 52 usable Ah out of a lead acid battery, you actually need it to be 104Ah. That is a big battery, I know because I have four of them on my riding lawn mower.

Now, you have the same usable Ah, but you still have a battery that weighs a TON more, and is only going to last at most 1/4 as long as the lithium. Also, refurbished batteries won't have near the rated capacity of a new cell.