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



no. the bms monitors every cell thats part of the reason a good bms is so 
expensive you have a pcb for each cell that cost  $16 - $30.  it is still 
a good idea to monitor the charge cycle.

Ciao for now,
DaveB


On Jun 1, 2013, at 4:00 PM, John Jeno <jjjeno hotmail com> wrote:

> Refer to evtv.me for detailed LIPOFE analysis. 
> As to BMS systems as far as I know they do not monitor each cell.  
> Rather they monitor each battery, which is comprised of many cells. So 
> in every case you are the BMS in the end. 
> 
> Please keep us informed as to you're experiences with this pack. I need 
> to lose mass as well. 
> 
> Sent from my iPad
> 
> On Jun 1, 2013, at 5:31 PM, "David Roden" <etpost drmm net> wrote:
> 
>> 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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>> 
>> 
>> 
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