# [EVDL] Is BMS needed for li-ion?



## EVDL List (Jul 27, 2007)

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## EVDL List (Jul 27, 2007)

Yeah you definitely do. Even if the mfg matches the spec initially, =

it's not going to stay that way, and nor does it address cumulative =

charge imbalance issues.

$1000 for a BMS? How many cells and AH per cell? There are different =

ways to make or get ahold of a BMS so maybe you could look again at what =

might be available.

Danny



> Daniel Cardenas wrote:
> 
> >I=92m selling electric scooters with out a BMS. Haven=92t had long term t=
> esting completed yet. My question is: Is a BMS a =
> ...


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## EVDL List (Jul 27, 2007)

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## EVDL List (Jul 27, 2007)

> Daniel Cardenas <[email protected]> wrote:
> 
> > Yes, but:
> > 1) I'm adding a large guard band to the voltage specs, both high and low. Max cell voltage is 4.25, I'm only going to
> ...


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## EVDL List (Jul 27, 2007)

I've seen folks sell products without a BMS with disastrous results. 
It never ends well. It often ends up in a lawsuit and/or a bankruptcy.

The self-discharge is very strongly influenced by temperature. The 
center cells are ALWAYS a different temperature than the end cells. 
Thus, no matter how closely matched the cells are off the assembly 
line, they will go out of balance quickly in a battery pack.

Once you slightly "wound" a cell by overcharge or under voltage, the 
pack goes out of balance very quickly. The pack won't last more than 
a few cycles.

I think the consumer will be willing to pay twice as much for a pack 
that will last 1500 cycles as they would for a pack that will last 10 
to 20 cycles. You will end up paying the price of the BMS, one way or another.

Bill Dube'

The fact that inexpensive-to-produce, practical, BMS electronics are 
possible is what makes these modern battery technologies possible. 
Chemistries that produce the best power and energy require the 
protection electronics. It is the engineering compromise that has 
been made, and you must accept that fact.

_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev


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## EVDL List (Jul 27, 2007)

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## EVDL List (Jul 27, 2007)

I saw this scenario plenty of times on the old
Visforvoltage.com, scootercommuterforum, etc.
"I just bought [insert name here] electric scooter. 
It doesnt work. I have contacted [insert name here]
via email but have not received a reply. Their
website seems to be old or not working, too. HELP!!" 
It kinda gets old after awhile. I hope to prevent
that same travesty with you. So here is what I have
observed.

Essentially, Customers are pretty simple when it comes
to electric scooters. They will drive it til its
dead. Then try to recharge it. It wont matter what
battery type it is. If the scooter doesnt recharge or
work like it should, they will return it. They will
not follow whatever prescibed maintenance or
conditions you may state, but will say that they did
when they try to return it. 

Overall, Quality is something lacking on many electric
scooters which if you want to build a lasting business
is something that you need to emphasize, especially if
you are small. Inshort, You need to include a BMS not
just for the customer, but for you. A BMS is an
assurance of quality even when the customer ABUSES the
scooter because a BMS will ensure that a customer is
doing the right thing by shutting off at a limit Which
will reduce the amount of returns and means maybe a
happy customer. This will lead to repeat business or
as in this day in age at least a good review on an
online forum. Also please keep in mind that this is
many times a persons first foray into EV's. How well
and how long your product works will resound for the
rest of their lives.

Although, You sound pretty convinced that you can get
by without it; even though we all know (including you)
that it needs to be included. Maybe, your flegdling
business can handle an influx of returns because you
failed to include a BMS or worse a lawsuit from a
fire, Go ahead and do what you think you can get by
with. And maybe the customer will monitor his pack
and not run it to ground.




> --- Daniel Cardenas wrote:
> > Yes, but:
> > 1) I'm adding a large guard band to the voltage
> > specs, both high and low. Max cell voltage is 4.25,
> ...


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## EVDL List (Jul 27, 2007)

Bill
I hope you haven't covered this before but what BMS do you use and how many cycles
do you expect from your battery pack?
Rick Prentiss

-------------- Original message from Bill Dube <[email protected]>: -------------- 


> I've seen folks sell products without a BMS with disastrous results. 
> It never ends well. It often ends up in a lawsuit and/or a bankruptcy. 
> 
> The self-discharge is very strongly influenced by temperature. The 
> center cells are ALWAYS a different temperature than the end cells. 
> Thus, no matter how closely matched the cells are off the assembly 
> line, they will go out of balance quickly in a battery pack. 
> 
> Once you slightly "wound" a cell by overcharge or under voltage, the 
> pack goes out of balance very quickly. The pack won't last more than 
> a few cycles. 
> 
> I think the consumer will be willing to pay twice as much for a pack 
> that will last 1500 cycles as they would for a pack that will last 10 
> to 20 cycles. You will end up paying the price of the BMS, one way or another. 
> 
> Bill Dube' 
> 
> The fact that inexpensive-to-produce, practical, BMS electronics are 
> possible is what makes these modern battery technologies possible. 
> Chemistries that produce the best power and energy require the 
> protection electronics. It is the engineering compromise that has 
> been made, and you must accept that fact. 
> 
> _______________________________________________ 
> For subscription options, see 
> http://lists.sjsu.edu/mailman/listinfo/ev 
_______________________________________________
For subscription options, see
http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

> Daniel Cardenas wrote:
> > 1) I'm adding a large guard band to the voltage specs, both high and
> > low. Max cell voltage is 4.25, I'm only going to 3.75. Min voltage
> > per cell is 2.5, I'm trying to stay above 2.75V. So if they are
> ...


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## EVDL List (Jul 27, 2007)

[No message]


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## EVDL List (Jul 27, 2007)

Well, actually people will be more willing to pay for the cheap one that 
self-destructs. When you wonder how "crap products" actually come into 
being, this is it. They'll soon be very angry at spending so much on a 
product that self-destructed in a month and trying to sue you. You 
won't be able to return much of their money because you already spent it 
all on those li-ion batts. In your desperation you'll try to weasel out 
of it by blaming the batt mfg for defective batts but only a few 
isolated people will believe that, the batt mfg will not sell to you 
again if they can avoid it and threatens to sue you for slander/libel. 
All the while believing you're not a scammer, you're a good man who 
wanted to sell a product at a price people can afford, just without the 
expensive bells and whistles, but in the end the customer's still left 
holding the bag.

All hypothetical, of course... but I don't see any way running lithium 
without a management system will ever work for long and the rest of the 
story is pretty much inevitable.

Danny



> Bill Dube wrote:
> 
> >	I think the consumer will be willing to pay twice as much for a pack
> >that will last 1500 cycles as they would for a pack that will last 10
> ...


----------



## EVDL List (Jul 27, 2007)

I don't think it would be so hard to use a PIC or uC to monitor those LEDs 
and do the voltage adjustments...

We have a very simplistic BMS on the KillaCycle at this time. Steve 
Ciciora calls it the "Opto-Bio-Mechanical Feedback" BMS. There are 
two LEDs associated with each parallel group of cells. You turn on 
the charger and watch the lights like a hawk. When a green LED come 
on, this means that parallel bank of cells is "full enough". If the 
red LED comes on too, it means that that bank is "completely" full 
and you need to turn down the knob on the charger. The operator turns 
down the knob when he sees any red LEDs, while waiting for all the 
green LEDs to come on. When all the green LEDs come on, the operator 
turns off the charger.





Thank you,

Dave Delman
1981 Electric DeLorean Project getting closer!
electricdelorean.com



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## EVDL List (Jul 27, 2007)

I agree that a BMS is required, but I don't think that it has to be
very expensive.

I'm designing a BMS for the lithium pack of my school's solar car. The
battery is arranged in modules of 16 parallel cells, with 30 of these
modules in series. The BMS monitors the overall current, the
temperature of every module, and the voltage of every module. We
haven't priced it out yet, but I expect the entire system to cost
under $800, and this is using more expensive circuits with
high-precision measurements (accurate to milliamps and millivolts).
Adding more series modules isn't a problem, either; I estimate about
$10 more for each additional series cell. I haven't priced everything
out, so I don't know the final price, though.

Due to race regulations, we can't include equalization in the same
system, so we are building it separately. The system we are designing
(using individual transformer windings for each cell) takes about $3
worth of transformers per cell in small quantities. It equalizes at
about 1 amp, which may seem slow, but it will have plenty of time to
equalize while charging.

I believe that we need a Li-Ion BMS for EV'ers that is affordable.
Something more like the Lithium equivalent of a Rudman Regulator than
a commercial BMS.

Another place to look is the Killacycle team. Can anyone working with
Killacycle's BMS let us know the rough cost?

-Morgan LaMoore

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## EVDL List (Jul 27, 2007)

>
>I don't think it would be so hard to use a PIC or uC to monitor those LEDs
>and do the voltage adjustments...

Until the controller comes on and then scrambles their 
little brains with 300 kW of RFI.....

How do they behave when the cell voltage goes below 2 volts? 
Or the temperature swings below -40? Or above 100 C?

Of course, it must draw less than a milliamp, on average.

This is not nearly as easy as one might first think.

Bill Dube'



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## EVDL List (Jul 27, 2007)

I was going to reply too but you summarized it very well Bill.

I'm on 4th generation of BMS design (will show it at EVS23) and
learn with every iteration how much deficiency each design has.

As Bill said It is very easy to have PIC on each cell with
12 bit ADC and some interface to the main controller, PIC will
report voltage allright. All as long as it's on your table.
Away from drive system noise, ran from nice bench power supplies, etc.

Go out there, build one you can put in a car and you will realize
the challenges. I can pretty much guarantee you what works in the lab
will not work in an EV. Trust me, done it.

One more thing - the goal is to make it work, and *then* to make it
as cheap as possible. If you manufacture in the US, not in China,
producing PCBs and stuffing them will likely cost you $3k-$4k alone,
that's excluding parts, and NRE (provided design was for free).
How do I know? I just finished one BMS like this for one serious OEM 
customer.

If you put money limit upfront and then try to build the best fitting
in that budget, may as well drop whole thing now. I'h hate to say
to you some months later "I've told you". Talk to someone who actually
built working Li BMS proven in a vehicle.

This is not to discourage you. This is just to make better calculated
decisions.

--
Victor
'91 ACRX - something different




> Bill Dube wrote:
> >> I don't think it would be so hard to use a PIC or uC to monitor those LEDs
> >> and do the voltage adjustments...
> >
> ...


----------



## EVDL List (Jul 27, 2007)

Isn't there a flexible BMS out there as a commercial product? Something
that can be set up with jumpers to work with _any_ battery technology? If
not, there really should be with all the reinventing of the wheel I'm
seeing.

-----Original Message----- 
I don't think it would be so hard to use a PIC or uC to monitor those LEDs 
and do the voltage adjustments...


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## EVDL List (Jul 27, 2007)

Hi All,

Lee wrote,

> One other option that I find interesting as an engineer. We recently
> talked about boost converters instead of buck converters for the motor
> controller. For a scooter, you can use a single large lithium cell and a
> boost converter to step this up to 36v or so. There is no need for a BMS
> with a single cell. 
> -- 

This sounds like a cool idea. It would certainly make the BMS and charging
simpler.

How about a Kokam 200 amp-hr HP cell.
http://www.kokam.com/product/product_pdf/high_power/PL-303_SLPB160460330H_200Ah_Grade.pdf

The 1 cell battery pack capacity would be 740 W-hrs. Boost it to 36 volts. 
You can
draw a peak of 800 amps * 3.7 volts = 2,960 Watts which is 82 amps at 36
volts or about 4 horse power. Constant draw would be 400 amps times 3.7 
volts = 1,480 Watts which
is 41 amps at 36 volts or about 2 horsepower. Some sag and some losses in 
the boost converter will bring down the numbers some.

Charger is simple. A Universal AC input to 4.1 Volt DC. BMS is just some 
sort of low voltage sensor to turn off the Boost converter when the voltage 
gets too low. Voltage isn't the best way to judge state of charge but would 
be cheap and accurate enough for a little 5 LED state of charge meter.

Form factor would work for a electric bike. 18" by 13" by 3/4". Weight would 
be 11.5 lbs. Be about a $1,000 for the battery though.

Cliff
www.ProEV.com






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## EVDL List (Jul 27, 2007)

What planet are you on?
Controllers capable of low standby current draw, milspec temp range, and
automatic reset on brownout have been around for, like, 10+ yrs easy.

No controllers that I know of are protected against latchup when Vdd
exceeds a maximum (like 6V or so) and cannot recover themselves in this
case. You may need some sort of circuitry on the input to prevent this.
There are many different ways to handle it. Also the controller may be
easy to find in milspec but the caps may be another matter.

Actually latchup is not guaranteed to come up as a problem. If you were
planning on taking the risk of going without a BMS then a hacked BMS
without latchup protection or milspec temp range may meet your low
expectations. You might make dozens and never see a problem in years. 
Or it may show problems in the first charge cycle. Truth is it's hard
to predict if it will actually be a problem and the proper protection
isn't really all that complicated or expensive anyways.

Danny

----- Original Message -----
From: Bill Dube <[email protected]>
Date: Sunday, November 25, 2007 9:44 pm
Subject: Re: [EVDL] Is BMS needed for li-ion?
To: Electric Vehicle Discussion List <[email protected]>

> 
> >
> >I don't think it would be so hard to use a PIC or uC to monitor 
> those LEDs
> >and do the voltage adjustments...
> 
> Until the controller comes on and then scrambles their 
> little brains with 300 kW of RFI.....
> 
> How do they behave when the cell voltage goes below 2 
> volts? 
> Or the temperature swings below -40? Or above 100 C?
> 
> Of course, it must draw less than a milliamp, on average.
> 
> This is not nearly as easy as one might first think.
> 
> Bill Dube'
> 
> 
> 
> _______________________________________________
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> http://lists.sjsu.edu/mailman/listinfo/ev
> 

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## EVDL List (Jul 27, 2007)

> [email protected] wrote:
> > What planet are you on?
> > Controllers capable of low standby current draw, milspec temp range, and
> > automatic reset on brownout have been around for, like, 10+ yrs easy.
> ...


----------



## EVDL List (Jul 27, 2007)

[No message]


----------



## EVDL List (Jul 27, 2007)

Hi Lee and all,


> One other option that I find interesting as an engineer. We recently
> talked about boost converters instead of buck converters for the motor
> controller. For a scooter, you can use a single large lithium cell and a
> boost converter to step this up to 36v or so. There is no need for a BMS
> with a single cell. 
>
>
This thought has crossed my mind before, and I find it very intriguing.
Although I question whether you would need no BMS at all. A blurb off the
ever-true (hehe) Wikipedia:

"Parallel arrangements suffer from the problem that, if one cell discharges
faster than its neighbor, current will flow from the full cell to the empty
cell, wasting power and possibly causing overheating. Even worse, if one
cell becomes short-circuited due to an internal fault, its neighbor will be
forced to discharge its maximum current into the faulty cell, leading to
overheating and possibly explosion. Cells in parallel are therefore usually
fitted with an electronic circuit to protect them against these problems."

No doubt you knew that Lee, I'm just wondering how you would mitigate that
problem. If there was a relatively simple solution to that problem, could a
*massive* 3.6V Lithium battery be made with the cost being sunk into the
boost converter? I'm wondering if you'd wind up with a lower total cost for
the entire system ... of course, you'd have the controller out of it too, I
suppose.

-Ryan
-- 
- EV Source <http://www.evsource.com> -
Professional grade electric vehicle parts and resources
E-mail: mailto:[email protected]
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## EVDL List (Jul 27, 2007)

> Lee Hart wrote:
> >> One other option... For a scooter, you can use a single large
> >> lithium cell and a boost converter to step this up to 36v or so.
> >> There is no need for a BMS with a single cell.
> ...


----------



## EVDL List (Jul 27, 2007)

>
>
> I suppose you could try dozens of small lithium cells, each with its own
> fuse in series, and then all in parallel. The fuse could be make of a
> metal that melts at a reasonable upper limit for the cell, so it opens
> if the the cell has too much charge/discharge current, or gets too hot.


Sounds workable.

A single *large* 3.6v charger could charge them all. A single *big*
> boost converter would step up the 3.6v to (say) 3.6v to 36v for a 36v
> motor.


Again, sounds interesting.

Seems like a workable scheme for a small vehicle like a scooter or NEV.


Why stop there?  How about a full-size EV? What's the limitations then?
Could it be cost effective?

-Ryan
-- 
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## EVDL List (Jul 27, 2007)

>
> Why stop there?  How about a full-size EV? What's the limitations then?
> Could it be cost effective?
>

The amperage would start getting rediculous... for a 100kW peak
demand, at 3 volts or so, you need to draw 33,000 amps. I think at
that point, the cost of the BMS starts being less than a power stage
that can handle that sort of amperage. Not to mention the wiring
required...

Z

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## EVDL List (Jul 27, 2007)

> Why stop there?  How about a full-size EV? What's the limitations then?
> Could it be cost effective?

Say you're cruising down the freeway at 15 kW. With a 3.7V battery,
this is 4054 battery amps assuming 100% efficiency. The wiring
resistance from the battery to controller would be killer; even 0.1
mOhm would drop your voltage by 0.4054, or 13.5% of battery voltage.
This would drive the input current even higher.

Also, the silicon for the boost converter would have to be rated for
full battery current and full motor voltage. If you have a max motor
voltage of 144V, this means using 10000 amp, 200V parts. (If you're
cruising at 4000 amps, you need more to accelerate, and you still need
a safety margin. 10 kA is probably on the low side!)

-Morgan LaMoore

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## EVDL List (Jul 27, 2007)

>> Seems like a workable scheme for a small vehicle like a scooter
>> or NEV.



> Ryan Bohm wrote:
> > Why stop there?  How about a full-size EV? What's the limitations
> > then? Could it be cost effective?
> 
> ...


----------



## EVDL List (Jul 27, 2007)

[No message]


----------



## EVDL List (Jul 27, 2007)

Not workable. Boost converters make the most sense at fairly low boost 
ratios when high power is involved.
Like for the single-inductor topology, say you need 10A @ 36V, 360W.
That's like 109A out of the 3.3V cell. But wait. This is probably like 
a 10% duty cycle on the output, so the output actually needs to get like 
3600W pulses during the on-time to meet the output specs. That's going 
to require huge inductors because the inductor needs to be wound with 
massive wire to avoid overheating (heat goes up with the square of 
current) and needs a large core to avoid saturating. The motor will 
probably not function well when run with low duty pulses far exceeding 
its design rating. We could put a cap on the filter's output until you 
see just how high the ripple rating has to be on that cap, and the 
capacitance needed to sustain the current to the motor smoothly during 
the 90% off-time.

We could go with a Cuk converter which should result in a smooth output 
without the surge problems- but we need 2 inductors, large ones too, 
and a capacitor capable of a phenomenal amount of ripple since 100% of 
the output current has to pass through the series cap. There are better 
topologies, they get more complicated and are not without limitations as 
well.

Well, it's technically possible, but when you consider the size, cost, 
ineffeciency, and limitations of such a converter, I don't see any way 
to justify this just to avoid a BMS. A BMS is nothing compared to 
this. A BMS could be made more cheaply than this.

Danny



> Ryan Bohm wrote:
> 
> >>I suppose you could try dozens of small lithium cells, each with its own
> >>fuse in series, and then all in parallel. The fuse could be make of a
> ...


----------



## EVDL List (Jul 27, 2007)

> Danny Miller wrote:
> > Not workable. Boost converters make the most sense at fairly low boost
> > ratios when high power is involved.
> > Like for the single-inductor topology, say you need 10A @ 36V, 360W.
> ...


----------



## EVDL List (Jul 27, 2007)

> Lee Hart wrote:
> >> and a capacitor capable of a phenomenal amount of ripple since 100%
> >> of the output current has to pass through the series cap.
> >>
> ...


----------



## EVDL List (Jul 27, 2007)

> Cory Cross <[email protected]> wrote:
> > Lee Hart wrote:
> > >> and a capacitor capable of a phenomenal amount of ripple since 100%
> > >> of the output current has to pass through the series cap.
> ...


----------



## EVDL List (Jul 27, 2007)

guys, isn't it worth it to draw the circuits on paper, scan and put 
online in a link instead of ascii graphics?
in my client (mozilla thunderbird) they look completely garbled but even 
if they didn't it tends to be a bit messy

Dan

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## EVDL List (Jul 27, 2007)

> Dan Frederiksen wrote:
> > guys, isn't it worth it to draw the circuits on paper, scan and put
> > online in a link instead of ascii graphics?
> > in my client (mozilla thunderbird) they look completely garbled but even
> ...


----------



## EVDL List (Jul 27, 2007)

On Mon, Dec 03, 2007 at 10:53:06AM +0100, Dan Frederiksen wrote:
> guys, isn't it worth it to draw the circuits on paper, scan and put 
> online in a link instead of ascii graphics?
> in my client (mozilla thunderbird) they look completely garbled but even 
> if they didn't it tends to be a bit messy

I've been most impressed with the ascii drawing I see on this list and 
I take this opportunity to ask how it is being done? Surely not with 
regular text editors?

I can't imagine how/why Thunderbird is screwing it up, perhaps 
mis-configuration? Even with multiple quoting, it comes through to me 
just fine.

I believe attachments are banned from this list? Properly so, IMHO. A 
scanned image would use far more bandwidth than the ascii art.

-- 
Willie, ONWARD! Through the fog!
http://counter.li.org Linux registered user #228836 since 1995
Debian3.1/GNU/Linux system uptime 182 days 7 hours 54 minutes

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## EVDL List (Jul 27, 2007)

> Lee Hart wrote:
> >> Cuk' converters do need larger capacitors, but not unreasonably so.
> >> Here's the simplest version.
> >>
> ...


----------



## EVDL List (Jul 27, 2007)

> Dan Frederiksen wrote:
> > guys, isn't it worth it to draw the circuits on paper, scan and put
> > online in a link instead of ascii graphics?
> > in my client (mozilla thunderbird) they look completely garbled but even
> ...


----------



## EVDL List (Jul 27, 2007)

> Daniel Cardenas wrote:
> > Dan Frederiksen wrote:
> >> guys, isn't it worth it to draw the circuits on paper, scan and put
> >> online in a link instead of ascii graphics?
> ...


----------



## EVDL List (Jul 27, 2007)

What lee said. AMEN

Although there is of course a limit in which ascii art fails you, but no
need to go bonkers if you don't need to. And it survives the archive.

I have mozilla thunderbird and it usually comes out just fine. Keeping
it under 40 columns always seems to help, which Lee is good at. The
worst case scenario is that occasionally I have to cut and paste it to
a text file and fix it up a tad. (which I do anyway before I save it to
me LeeHartAsciiArt Folder. Do you have yours yet?

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## EVDL List (Jul 27, 2007)

>
> > Why stop there?  How about a full-size EV? What's the limitations
> then?
> > Could it be cost effective?
> >
>
> The amperage would start getting rediculous...


Okay, this was one of those moments when a quick sanity-check would have
prevented me from asking a stupid question. Of course it's the amperage
that would kill the idea in a full-size EV.

-Ryan
-- 
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Professional grade electric vehicle parts and resources
E-mail: mailto:[email protected]
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