Monday, November 19, 2012

Most cost-effective currently available lithium battery source and/or configuration

From the AtomicZombie bike building forum:

"Well, another spell in the repair dock gave me more time to think about possible add-ons (or more correctly build-ins) to the Timberwolf languishing in what I laughingly call my workshop. I've been giving a lot of thought to various possible additions to Brad's basic design, including a geared mid-drive, but mostly about how best to incorporate electric assist right from the get-go.

The logical first idea is to use an electric hub motor on the front. This has the advantage of simplicity, but on the other hand, it puts the motor at the part of the trike where ground contact weight is lightest. I don't know how much of an issue this will be, but its fair to say that the majority of the weight in any of the Deltas is carried at the rear, and so traction at the front wheel is a question mark.

The second idea is to use one, or two, Currie-style external motors either directly driving one or both rear wheels, or else connected into the chain/derailleur drive train at some point between the pedals and the rear axle.

However, the biggest single consideration is batteries. I have the wreck of an e-scooter that will, I hope, become a tadpole recumbent trike at some point; its all-up weight without passenger is over 200 lbs, and of this probably 2/3 is lead-acid batteries. I just can't see building a Timberwolf, which with rider will mass somewhere between 135 and 150 kg., and then adding a great whacking load of lead-acid batteries. Even if the tires would stand up to the load, it would be a tremendously heavy vehicle.

So, we then come to the lightest currently available battery technology, the various lithium-based types. These have an astonishing power-to-weight ratio, but they are new technology, and as such there's a lot of mis-information floating around. There's also the question of cost: what form of lithium cell offers the best "bang for the buck" in terms of watt-hours stored vs. dollars (not forgetting overall weight).

It's a basic truism that for a given power rating, using a higher voltage means a lower current draw. Since with most types of batteries the rate of current draw or discharge is a big factor, using a higher voltage would allow the use of cells with less amp draw capability.

The brick wall that I keep coming up to, and banging my head against is this: What form of lithium pack is the lowest cost for a given current draw, and what form of lithium battery is the most flexible in terms of series-connecting the packs to achieve higher output voltages. I have read various "opinions" and "reports" until I'm drowning in hyperbole, and I'm no closer to the answer: what's the best available form or configuration of lithium battery to start with.

For example, there are a wide variety of lithium cells used in RC model aircraft, robots, etc. There are any number of different lithium cells available for commercial-grade power tools. There are lithium batteries available in various ratings for laptop computers - and the list goes on. The common denominator is that they're all expensive, and so if one is going to bit the bullet and invest in a lithium battery system as an integral part of the build, it would make sense to try and maximize the power storage capacity and ease of recharging for a given dollar expenditure.

Once upon a time, I had a shingle that said I knew something about electrical engineering, and I suppose I still do know some parts of it. But I'm having tremendous difficulty in separating fact from hype in discussions about lithium technologies, and even more difficulty in finding out how best, and from whom, to source the batteries for the least cost without buying junk.

I've considered other options, from lead-acid, NiMH, Nickel Cadmium, etc. - and all are expensive if purchased new. So although lithium technology is expensive, any discussion of cost vs. weight vs. energy storage capacity has to take into account that, unless you stumble across a free or very cheap source, the battery pack is going to be the largest single expense in an electric drive system for a bigg-ish delta trike. Even if motors are purchased new, controllors likewise, the battery is going to be the big expense, according to all the research I've done so far."