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Re: (ET) New Member


On Wed, 2 Jul 2008, David Roden wrote:


On 2 Jul 2008 at 12:44, Michael S Briggs wrote:


I'm not planning on getting new lead-acid batteries, since there
*should* be some significantly improved Li-ion battery technologies
coming onto the market over the next few years - which would be great
not only for full electric vehicles, but also for ETs.


I think this isn't all that advisable, and here's why.

1. The main advantage of Li over Pb is much higher specific energy (Wh per
kg), so they are light for the amount of energy they store.  This is vital
for road EVs.  However, a tractor, like a forklift, THRIVES on weight.  And
for the way we use ETs, how often do you really need more energy than 6 
golf
car batteries can store anyway?
The main advantage of *current* Li-ion batteries over lead acid is higher energy density. Advanced Li-ion batteries though have many other advantages - in particular much longer cycle life and much lower internal resistance (higher net efficiency, no voltage sag under load, no significant drop in performance as the battery capacity drops, ability to rapidly recharge the pack, etc.). That is why my post focused on those issues, rather than the energy density. For an EV, the high energy density of Li-ion batteries is very appealing - for an ET though, as you point out it's not necessary. But, the other advantages are still very appealing (which is why I focused on them). Ni-CADs offer similar advantages (long cycle life and low internal resistance), but the main ones you can find (BB600s) are small so you need gobs of them (whereas the new Li-ions will be made in larger sizes intended for EVs), and they need frequent watering - presumably a pain if you have 90 of them. I've thought about going with Ni-CADs, but the new Li-ions should be preferable. As for weight though - note that the frame of the tractor itself is very heavy (as someone who stripped his ET down to the barebones while restoring it, I can attest to the fact that even without batteries, tires, etc., the thing is still damn heavy). If you are using "lifting" attachments (forklift, bucket loader, etc.), you presumably will still want more weight - but for basic mowing, the additional weight of lead-acid batteries should be completely unnecessary. 


2. Flooded Pb batteries can be charged with the dumbest gear in the world
and survive.  The GE charger is a perfect example.  ;-)  OTOH, Li needs a
smart, sophisticated charger that monitors >every single cell< or the
battery will be destroyed, possibly catastrophically.
Note that catastrophic failure only applies to current Li-ion batteries with liquid electrolytes (particularly if dendritic Li buildup on an electrode results in piercing of the membrane separating the electrodes, short-circuiting the battery internally - but that is primarily a problem with Li batteries moreso than Li-ion). Li-ion batteries do need a smarter charger (without the liquid electrolyte, you shouldn't actually need to monitor individual cells - no need), but with the low internal resistance of advanced cells, the result is that you can get a "roundtrip" (charge-discharge) efficiency over 90%, even with high loads (at low loads over 95% is reasonable). On the other hand, the massive internal resistance (thus the significant voltage drop under high current) with lead acid batteries results in a roundtrip efficiency probably averaging around 50-60% or so with an ET (a rough guesstimate). Ultimately that means significantly less electricity needs to be produced to power the tractor. 


3. Li batteries have a limited >calendar< life.  Cycles are not as big a
deal for them as for lead.  Because most ET owners cycle their lead
batteries about once a week, they usually get a good 10 years out of them.
It's possible that a Li battery would last only half that long, or less,
regardless of how much it's used.
Again, that applies to *current* Li-ion batteries. I'm talking about advanced Li-ion batteries with solid electrolytes and lithium iron phosphate cathodes. The limited lifespan of current Li-ion batteries results from electrolysis of the liquid electrolyte when the battery is overdischarged (due to the high electronegativity of the cathode), with the hydrogen produced filling up lithium intercalation sites within the cathode (to an extent also the anode, although the solid electrolyte interphase film that builds up on it limits that - but creates other problems (it significantly increases internal resistance). The use of solid electrolytes eliminates the formation of this film). Li-ion batteries with Lithium iron phosphate cathodes, improved anodes (such as carbon aerogel doped with silicon), and solid electrolyte (either lithium sulfide glasses or polymers) should be able to have lifetimes on the order of 10 years or more. I can dig up some articles on these if you'd like (most of the articles that go into detail are in various professional journals, such as the Journal of Power Sources, but unless you have a subscription to that, you can't access them. But, there have been some recent articles in other journals that are publically accessible). 


4. Lead batteries are reliable, proven, conservative technology.  They are 
a
commodity using an old principle that's public property, sold by the
millions. OTOH, Li batteries show great potential for uses where light
weight and small size are crucial.  However, they're new, unproven, and may

never< become a commodity, depending on how the licensing situation pans

out.
I'm not telling people they should abandon lead acid batteries - I'm just telling people what *I* am choosing to do, based on my knowledge of the state of Li-ion development. IMO, Li-ion batteries are a proven technology - and with the improvements that have been made in labs over the past few years, advanced Li-ion batteries will be significantly better than currently available ones. Unproven, sure. But as I am familiar with the science involved, I will personally be comfortable buying ones once they start being produced. The greatly improved cycle efficiency and recharge rate IMO are very appealing. 

Mike