Re: (ET) New Member

["David Roden" <etpost drmm net>]

On 2 Jul 2008 at 15:31, Michael S Briggs wrote:

> ... 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.). 

AGM lead batteries fit that description pretty closely.  They have very 
low 
internal resistance and can produce some ferocious currents.  Look here :

http://www.evdl.org/pages/plasmaboy.html

But it doesn't matter.  To characterize the internal resistance of even 
golf 
car batteries as "high" is simply incorrect.  They can deliver thousands 
of 
amps.  That's far more than an ET can ever use, especially if you're just 
mowing the lawn.


> for basic mowing, the additional weight of lead-acid batteries should
> be completely unnecessary. 

Agreed.  I take it you never plow or move things with your tractor.  In 
that 
case lighter is better.


> 
> 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 

The round trip efficiency of a lead battery is about 90%.  Most of the 
losses are from heat and electrolysis in the absorption phase.  From 20% 
to 
80% SOC, the efficiency is very close to 100%.  Of course it's not healthy 
to skip the absorption phase in batteries used the way ET batteries are.

NiCd is somewhat lower, 80-85%.  

The greatest share of the inefficiencies exist in the charger and will be 
essentially the same for other chemistries.  These can be minimized, but 
the 
cost to do so is usually more than most users are willing to pay.


> Ultimately that means significantly less electricity needs to be
> produced to power the tractor. 
> 

The difference is negligable, IMO.  See above.


> Again, that applies to *current* Li-ion batteries. I'm talking about 
> advanced Li-ion batteries with solid electrolytes and lithium iron 
> phosphate cathodes. 

It's easy to make claims about something that isn't available yet!  I've 
been following EVs since the late 1960s, and the "miracle battery" has 
always been about 2 years away.

As the song says, there are liars, damn liars, and battery salesmen.   ;-)


> 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. 

Go for it!  Be sure to report back.  We'll see you in 10 years.  ;-)

It sounds like you're an electrical engineer interested in lithium for its 
"newness" and able to deal with the challenges of integrating it into an 
existing device.  It IS fun to be on the leading edge of technology, but 
not 
too helpful for the majority of users who just want to keep the grass cut 
and the driveway clear.  

Non-engineer ETers who'd like to try lithium in their ETs right now, with 
absolutely minimal hassle, can buy Valance U-Charge Lithium Ion Phosphate 
drop-in replacements for marine batteries.  A BMS is built in.  They're 
available in a group 24 size, which fits our battery boxes perfectly 
(they're a bit shorter than golf car batteries).  They're 12v 100ah, so 
they 
have about the same kWh capacity as golf car batteries.  They weigh only 
about 35 lb, compared to a golf car battery at 62-65 lb.

http://www.valence.com/products/ucharge_overview.html

Several Solectria EV owners have been using these batteries and seem to be 
satisfied with them.  They report appreciably improved range.  No word yet 
on service life.  Also, I think all of them are using the earlier US 
production; I read that Valence recently moved production to China (but 
the 
prices didn't decline at all, AFAIK).

To try these nifty little guys, you'll need deep pockets.  Six of them 
will 
set you back $9,000.  Sorry, no, I didn't add an extra zero to that price. 
 
Nine grand it is.


David Roden - Akron, Ohio, USA

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