Re: (ET) Wanted, information on installing Lithium cells in my EGT 200

["David Roden" <etpost drmm net>]

On 1 Jun 2013 at 16:31, Barry wrote:

> I am not using a BMS.

As you probably know, not using a BMS is controversial.   I don't know of 
any reputable manufacturer of any lithium-battery product - laptop, power 
tool, road EV, whatever - that doesn't include at least a rudimentary BMS. 
 
However, a few EV hobbyists think that BMSes actually make matters worse.  
Some even go so far as to blame the BMS for the fires in a tiny handful of 
conversion EVs.  

>From what I can tell, most or all of these folks are, in effect, acting 
>as 
their own manual BMS.  

A popular manual approach seems to be "bottom balancing." Please correct 
me 
if I'm wrong, but as I understand it, in this method you find the weakest 
cell in the battery, and then manually adjust all the other cells' state 
of 
charge so that all the cells reach flat at about the same time.  This way 
you can see the big drop in voltage when it's time to quit using the 
battery.  (If just one cell went flat, you'd be apt to miss that decline 
in 
a full-battery voltage monitor, and you don't have a BMS watching every 
individual cell.)

Part of this scheme seems to be that you can never charge the battery to 
full.  The bottom-balancers argue that the percentage of range they give 
up 
by doing this is trivial.  This makes some sense, since the weakest cell 
in 
the battery is always your limiting factor.

With this scheme, when you use the EV, you have to carefullly monitor 
battery voltage.  When it falls to what you consider to be "discharged" 
voltage, you have to stop using the EV.  

There's no "wiggle room" here.  You stop immediately, or risk destroying 
your battery by over-discharge.  Unlike with a good BMS, which monitors 
individual cells and shuts down battery output as soon as any one goes 
flat, 
there's no hardware keeping you from thinking "I'll just drive one more 
mile, there's a charger in the plaza ahead," or for an ET, "I'll just mow 
one more strip of grass."  You have to be watchful and disciplined.  As I 
said, YOU are the BMS.  Your battery's life is in your own hands.

Of course you hope that your battery monitor gives you plenty of warning 
before you reach that critical discharge point.

I'm keenly interested in this because thus far I have only a little 
experience with lithium secondary batteries in EVs.  I've used only 
"prefab" 
units where the battery comes pre-configured, with BMS and charger.  I've 
had excellent results with that setup.  But it tends to be pricey.  It's 
definitely more expensive than what you did, buying individual cells and 
configuring your own BMS-less battery.

I said above that commercial lithum-battery products nearly always have 
BMSes, but I HAVE seen a few cheap commercial products that don't.  

For example, I bought a cut-rate, made-in-Taiwan rechargable crank LED 
flashlight a few years ago, thinking it would be good to keep in the car.  
I 
also got a second one to keep in my computer room, for lost-screw 
excursions 
under the desk.

The car light was first to quit.  It didn't even last a year.  It'll put 
out 
light as long as you crank it, but holds no charge at all.  I suspect that 
summer temperatures in the car killed the battery.

The computer room light lasted for about 3 years of intermittent use - 
which 
from what I've read is about where even a well-cared-for lithium laptop 
battery starts to decline.  This, though, was a far steeper decline than 
any 
laptop battery I've ever used.  The light is essentially useless.  It 
barely 
holds any more charge than the car light did.  

Both of them are sitting on my workbench.  I suspect that these are like 
ones that I've since read about on the web, and when I open them up 
(assuming I find the time to do so) I'll find that the manufacturer just 
dropped in a lithium battery where a NiCd had been in previous models.  My 
guess is they did this to avoid cadmium disposal responsibility (I think 
there are laws now about this, at least in the EU).  

If I'm right, this was definitely a high stress lithium test, and no 
surprise, lithium flunked.

Don't misunderstand me, I'm not suggesting that your lithium experiment 
will 
turn out like my flashlight did!  But I'll be very interested to hear how 
well your battery is standing up 5 years or so from now.  I'll also be 
keen 
to learn whether your battery monitor consistently gave you enough warning 
to get you back to your charger before the battery hit the danger zone, 
and -
 if it didn't - whether you were able to resist that "I'll just mow one 
more 
strip" temptation, and willingly pushed or towed the tractor back to the 
garage.  ;-)

Good luck, and please keep us posted on how this is working out.  I 
suspect 
there are others here who might like to try something similar.  


David Roden - Akron, Ohio, USA

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