Re: (ET) When did they start making car batteries out of gold??

["David Roden (Akron OH USA)" <etpost drmm net>]

On 18 Aug 2013 at 20:35, Jeff Antonucci wrote:

> One of the few things I don't like about my ET is how the performance
> drops as the batteries discharge, and how it really slows down mowing up
> hill. 

If by "it really slows down mowing up hill" you mean with a reasonably 
full 
battery, that's more apt to be caused by excessive resistance in your 
wiring.  It could be high internal resistance of really decrepit 
batteries, 
I guess.

If by "it really slows down mowing up hill" you mean when your battery's 
about flat, you want to watch that.  If it's slowing down drastically, you 
may be overdischarging your battery.  You don't want to pull a lead 
battery 
below 1.75 volts per cell (31.5v total) under load, or you're risking 
reversing a cell.

Christopher alluded to the long life of NiCd batteries.  They have a rep 
for 
hanging around for many decades.  They're also quite a bit more tolerant 
of 
careless charging than lithium batteries, though I know of one person who 
utterly destroyed a car full of hard-to-find Saft STM5-180s (golf car 
sized 
NiCds) by putting them on a dumb charger and forgetting about them.

What he didn't mention is that NiCd batteries - not as fashionable as 
lithium right now, and probably harder to find - also have a very flat 
voltage on discharge.  That's a plus in that it keeps things running 
relatively well until the battery is nearly flat.  

However, one disadvantage of a flat voltage curve is that it's harder to 
judge by voltage how much charge you have left.  

This means that if you monitor voltage to guess state of charge (and I 
mean 
"guess"), you have less warning before you get into hazardous territory.  
With lead, you get plenty of warning, so you can usually finish up a few 
more passes across the lawn before trundling back to the garage.  With 
lithium, when you hit that drop in voltage, you really need to quit NOW, 
if 
you want to avoid damage to your battery.  (In fact, a good BMS will shut 
down the battery when it gets dangerously flat.)

If you want to plan your ET usage effectively, you really need to measure 
amp-hours consumed, instead of relying on voltage to tell you the SOC.  In 
addition to the BMS, an amp-hour counter is yet another conversion expense.

I've looked into this a bit - not exhaustively, so maybe you can beat 
these 
numbers with more shopping around than I've done.  However, I see prices 
around $1.30 per amp-hour per cell for typical Chinese LiFePO4.  (It's not 
clear to me how well supported these cells are; you seem to be at the 
mercy 
of your US distributor's policies.)

Nominal LiFePO4 voltage is 3.3 vpc (3.2v by some accounts), but typically 
you'll use 12 cells for a 36 volt battery (39.6v).  I guess you could 
scrimp 
and use 11, but all the premade 36v batteries I've seen use 12 cells.  
(The 
24v kits I bought for scooters, complete with BMS and charger, use 8 
cells.  
These are 10ah batteries and run a bit over $300 each delivered.)

So for just the bare cells, you're at about $1560 for a 100ah ET battery 
(the LiFePO4 boosters claim that their 100ah is equal to a lead battery of 
220 or more ah).   But we're not yet at the total cost for changing over.  

A Mini-BMS for 12 100ah cells will run you about another $200.  A typical 
amp-hour counter will add $200 more.  You'll also need a smart charger 
that 
knows how to converse with the BMS; add at least another $500.  By the 
time 
you add it all up and count in shipping, you're well over $2500.

Now, if they're right, that $2500+ should buy you a battery that lasts 
15-20 
years.  However, some folks using Chinese LiFePO4 (Calb, Winston, etc) in 
road EVs have reported failures of individual cells and BMS modules from 
time to time.  Maybe ETers will stress their batteries less and thus have 
fewer failures - or they might have just as many.  We don't know yet.

Bottom line: If a lead battery doesn't give you the run time you need, or 
if 
for some reason you need a much lighter tractor, then lithium may indeed 
be 
for you, expense aside.  If these things work for you with lead, then I 
recommend sticking with lead.