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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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