Need Help with Battery Purchase

[DerekG]

Looking for help with the purchase of a new battery pack. Love the one I have now 36v 10ah lifepo4, however, the cells inside are 2C and I would rather rebuild than try to make this work.

Want 35mph + on flats with 9C 2807 plus good acceleration (and thus a high C battery is required)(This thread will be related to the battery system for the new build. I will try to keep my thread in the general section devoted to torque-arm updates, controller questions, etc... no battery talk there )

Don't need a lot of capacity... I've never used more than 6aH or so on my rides of up to 25 miles and most rides don't even use 2aH. Shooting for 10aH capacity as that is lightweight while still providing range... maybe 15aH... no more because I don't have room.

Biggest thing with this battery is that I have to buy packs assembled b/c I don't have the free time to put them together(can buy either from here from one of you or off another reputable website. I would prefer the pack be in separate pieces so I can wire them up different ways to get what voltage / capacity I want. (ex 8 18v 5aH packs or 4 18v 10ah packs or 2 36v 10ah packs)

Exploring a number of options (Li-Po and A123 LifePO4 seem like the best though I guess a headway pack would work too??)

*Li-Poly (love the power density and high C ratings but batteries fragile which scares me... Don't mind putting down $200+ for a charger but if I crash I don't want a fireball... Even then I don't know... a fire in a dorm would be the end of everything (aka no more bike and probably worse) so no fire / smoke at all costs)

*A123 - sounds like one of the best still in power, safety, etc. However, I can't find decent sized packs on hobby websites, etc. Once again I would only need 72v at 8aH or so but I don't want to have to worry about building my own pack... a balance wire would probably fall out or something

*Headways- could work and at 10aH each I would only need a 1p pack (I would keep ouput to 4C cont with use of a 40A controller then), however, I like the idea of 2 or 4 smaller packs vs one large square/rectangle.

Oh yea, just a general thing... budget for the battery let's try to keep at around $1000 or so (with charger and bag)... I will have it with me so it getting stolen isn't a huge concern but I don't want to just blindly dump money into this... I''m only in college and not exactly making a fortune off to the side (Though if I was I would probably get a Bomber and be done with it ... not going to lie ) I figure I can sell my current battery with charger for around $400 or so (only 50 cycles can buy new from ebikekit for $650 plus shipping or so with the faster charger which I have) so I will only have to put down $600 or so (go figure this all started with a bike frame which cost me $40...)

Thanks in advance for the help... very torn with what I should do :-/

 

[dogman dan]

Uh, for starters, figure on getting a lot less for the used battery. Just the way it is, virgin is worth money, after that the buyer has to really trust your story about your past activites.

And you know voltage higher than 48v will be needed, so new controller time too. At 72v, you may only need a 20 amp controller depending on how steep a hill you need to climb. 35 mph out of a 2807 will take some volts, but a 2806 would get close to 32 mph at 48v.

I'd be looking at 5 ah 18v lipo packs, four of em. and mabye 4 chargers eventually since I hate waiting. Should still be pretty doable on a budget.

For 27 mph on a 2807, all you would need is 48v, and a headway pack from EV components would do nicely and have decent range. Better performance on hills and starts would come with a 35 amp, 48v controller.

For me, on my bike frames, 30 mph is my personal limit. Going down a big hill, I hit 35 and I start to grab brakes myself. With a real frame, like a DH bike perhaps, things would be different. I bet I'd love a bomber too!

 

[DerekG]

Thanks for the reply

In regards to selling my existing battery, if I should figure for a lot less than its worth more keeping the battery and using it as it is with the controller I have... everything stock still (because I still have a warranty on it and if I keep everything stock I know I will have a reliable battery for a while to take me to class which is all I need for that)

So that leaves me with looking for a "fun pack", something with not a lot of aH (at least 5 workable though because it goes fast ) but a high C rating and a high enough voltage to satisfy any need I have for acceleration, etc.

I guess this takes me back to Li-Poly or A123's. The Headways would be great if I were building a new pack for everyday use, however, this isn't the case and I feel I could get away with something a little smaller than a 72volt 10aH pack

Li-Poly's still scare me though... If a guy down the hall has a cellphone explode and the alarm goes off it's a big deal but it was just a cell-phone... no harm done. If I have a similar thing happen with a Li-poly pack then I can guarantee I'll be in deep crap. Not sure what the rules are regarding batteries but I don't want to have to deal with any sort of investigation... aka there is no room for error which puts a lot of pressure on me. Hence even with a $250 charger accidents can still happen. I guess if I bought hard-cased Li-poly packs that would reduce the chance of a puncture explosion, however, Murphy's Law isn't on my side

So looks like either hard-case Li-Poly packs + a good charger or an A123 Systems pack (would have to be 3P probably so I don't run them down too much)

Would you guys agree those seem like the best options on the table right now?

 

[dogman dan]

Oh yes, if you live in a dorm or something, lipo may be too scary. It's way improved now, but getting kicked out would be a catastrophe if it doubled your rent.

Charging A123's shoud be quite safe though. So go with some of them. I think you can get em tabbed, or in little packs for RC that could be stacked up till you have 72v 5 ah.

 

[DerekG]

Yea, I have a single right now but it's still in a dorm so yea... I talked to a University Rep about Li-Poly and he said it's fine... no issues so that's back on the table too I guess (though they would have to be hard cases for impact resistance)

Still, even with approval, it looks like an A123 Systems pack would make the most sense at 6aH (3P is what I would want... that way I could limit discharge to 5aH tops)

Now the next question is where can I purchase them / who would be willing to build one for me?

http://www.robotcombat.com/products/battery_build_li-ion.html was one place I found that builds custom packs.

http://www.radicalrc.com/shop/?cart=3516075&cat=199& was another place though I have a feeling it would be cheaper if I/someone here built one.

Still thinking about it all

 

[dogman dan]

I think hobby city is where the prices are.

 

[DerekG]

Good to know :_)

I'm PM'ing Stevo asking about the possibility of having a pack built for me using his used A123's... If I have a 6 or 8 aH pack even 50 amps peak would be less than 10C for it which should be fine as long as they are all genuine cells. Going to get a price from him although Li-Po is still an option too.

If I went Li-Poly I could get 8 Turnigy 5000mAh 6S 25C Lipo Pack although that is quite a few packs which would consequently mean a complicated way of connecting them all. Still not sure what to do :-/ I think I will wait to hear back from Stevo as the A123's would still be my first choice for safety reasons.