Re: (ET) 18650 Li Ion Batteries?

["David Roden" <etpost drmm net>]

Hi Keith,

You're talking about a LOT of cells to match the usable amp-hour capacity 
of 
a golf car battery.  And that's a HECK of a lot of connections, a lot of 
potential points of failure.  

To wit: a T105 golf car gives you around 130ah of useful capacity.  At 36v 
that's about 4.7kWh.  A typical 18650 laptop cell is around 3.3ah and 3.6v 
nominal, so it has about 11.9Wh of capacity.  You'd need 395 of them to 
match the golf car battery's capacity.  That's 790 individually welded 
connections!

But that's not quite all.  You also have to take precautions to prevent 
calamity (fire, expolosion) in the case of a cell failing shorted and the 
other cells in parallel with it dumping their charge into that short 
circuit.  The usual way is a fuse for every cell.  Now you have a lot of 
fuses to buy, and you've raised your number of  connections by about 50%, 
to 
almost 1,200.

I know, Tesla does it, and it works for them.  But: 

1. Their cells have very stringent quality control, and are closely 
matched 
in capacity and internal resistance.  Most LiIon cells we normal civilians 
can buy aren't nearly as well matched and of consistent high quality.  
Used 
ones are complete crapshoots.

2. Tesla battery packs are assembled by robots, which are pretty good at 
not 
making mistakes the way we humans do.  (Or at least making them 
consistently. :-)

The other question is: why?  

To save weight?  With a tractor, you WANT weight, especially when you 
start 
attaching heavy stuff like snow throwers to the front, or tillers to the 
back.  Lighten the tractor too much and you'll find yourself having to add 
ballast.  Old lead batteries make pretty decent ballast, by the way.  :-)

For longer life?  Golf car batteries already last 10 years in most ET use. 
 
They could last even longer with more sophisticated charging, and more 
even 
discharge (get rid of the lift and light taps).  With at least some kinds 
of 
lithium ion batteries, you're going to run into calendar life issues 
before 
cycle life depreciates them.

For less mess and maintenance?  For an ET, the better answer to this MO is 
lead gel batteries.  I've been using the same set of East Penn Deka 
Dominator batteries in my ETs for 20 years, and it's just been in the last 
couple that they're finally starting to decline noticeably in capacity.  I 
didn't bother to get rid of the taps, but I have been charging them with 
individual automatic chargers that are somewhat smarter than the dumb 
blunderbuss GE charger.  So no doubt that helped with the long life.

If you decide you still want to try lithium, I suggest that you look 
instead 
to salvaged modules from a wrecked Nissan Leaf or Chevrolet Volt.  You 
won't 
have the same hassles with an excessive number of connections, the 
consistency is generally good even with used ones, and they're usually 
pretty cost effective.  

Don't forget that in spite of what some folks will tell you, if you value 
your house and garage, for a lithium battery you need a well designed  BMS 
to properly and safely control charging and discharging at the cell level. 
 
That will be a significant portion of the battery conversion cost.


David Roden - Akron, Ohio, USA

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