# [EVDL] LiFePO4 w/out BMS



## EVDL List (Jul 27, 2007)

Hi Mike,

The vehicle we are presently performing our BMS testing on has a product
from "Hot Juice Electric" that is performing battery balancing for us until
we have battery balancing integrated into our component. You can find out
about it here:

http://www.hotjuiceelectric.com/product_beq1.html

~mark





> m gol wrote:
> >
> > I was wondering what sort of "short-term" issues will happen if I used
> > a pack of LiFePO4 batteries without a BMS?
> ...


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## EVDL List (Jul 27, 2007)

And, to answer your question, the LiFePo4 batteries are very sensitive to
overcharging. When overcharged, they have a tendency to swell very quickly. 
I would not recommend attempting to charge without some sort of bypass
circuit. Unless you're willing to charge your batteries individually, and
monitor them while they are charging and when one is full move on to the
next.

The active zener diode approach works... it just generates a lot of heat if
you're dumping a bunch of amps into your batteries when they (some of them)
start to bypass. Once some of the cells start to reach their full charge
state, then you need to drop your charge current down, unless you don't mind
the heat output from the zeners.

~mark




> m gol wrote:
> >
> > I was wondering what sort of "short-term" issues will happen if I used
> > a pack of LiFePO4 batteries without a BMS?
> ...


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## EVDL List (Jul 27, 2007)

> m gol wrote:
> > I was wondering what sort of "short-term" issues will happen if I used
> > a pack of LiFePO4 batteries without a BMS?
> >
> ...


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## EVDL List (Jul 27, 2007)

Do you have a like to that?



> Steve West wrote:
> 
> > The ThunderSky mailing list recently discussed a very basic (2
> > component) voltage clamp that provides very basic protection: "a PNP
> ...


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## EVDL List (Jul 27, 2007)

Mark,

How is the device your working on different. If your going to shunt 
current at a certain voltage the energy has to go somewhere. If not 
into heat, than where does this energy go?

I have given this a lot of thought, and it would be nice to not waste 
the energy that is being shunted during the finishing / equalization 
charge. The thing I came up with is individual, isolated chargers 
doing the equalization charge. You could have a bulk charger charge 
the pack until one cell hits 3.65V, and then switch to the individual 
chargers to charge all the rest to 3.65V. These individual chargers 
could be low current, constant voltage chargers (small and cheap).

This would eliminate most of the inefficiency during equalization.

Roger



> mark at evie-systems wrote:
> 
> >
> >
> ...


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## EVDL List (Jul 27, 2007)

> Roger Heuckeroth wrote:
> > You could have a bulk charger charge
> > the pack until one cell hits 3.65V, and then switch to the individual
> > chargers to charge all the rest to 3.65V.
> ...


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## EVDL List (Jul 27, 2007)

You will pay a lot of money in hardware to save a tiny amount of energy.

A by-pass resistor circuit costs a few dollars. An isolated charger 
is going to cost at least $20, but more like $40. You are going to 
have to buy 50 to 100 of them.

You will also have to haul all this extra bulk and weight down the 
road. This would likely offset any energy you might save.

Bill Dube'

At 06:40 AM 10/2/2008, you wrote:
>Mark,
>
>How is the device your working on different. If your going to shunt
>current at a certain voltage the energy has to go somewhere. If not
>into heat, than where does this energy go?
>
>I have given this a lot of thought, and it would be nice to not waste
>the energy that is being shunted during the finishing / equalization
>charge. The thing I came up with is individual, isolated chargers
>doing the equalization charge. You could have a bulk charger charge
>the pack until one cell hits 3.65V, and then switch to the individual
>chargers to charge all the rest to 3.65V. These individual chargers
>could be low current, constant voltage chargers (small and cheap).
>
>This would eliminate most of the inefficiency during equalization.
>
>Roger
>
>


> mark at evie-systems wrote:
> >
> > >
> > >
> ...


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## EVDL List (Jul 27, 2007)

Good point regarding the added weight. How about a module with a 
single transformer that has a single primary and multiple isolated 
secondary coils followed by simple, low current rectifier and voltage 
regulator circuits. You could service multiple cells with each module.

I know this is a bit out of the box type thinking. Almost everyone 
else is going with voltage clamp / current shunt type circuits. I 
just threw this out there for consideration.





> Bill Dube wrote:
> 
> > You will pay a lot of money in hardware to save a tiny amount of
> > energy.
> ...


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## EVDL List (Jul 27, 2007)

If I understand their documentation correctly, China Hipower do charge 
individual cells. No voltage clamping.
Actually, multiple secondary coils in a transformer are very common. 
Before the advent of switched power supplies that was the standard way 
to obtain multiple voltages in a power supply.
Bas.

Roger Heuckeroth schreef:
> Good point regarding the added weight. How about a module with a 
> single transformer that has a single primary and multiple isolated 
> secondary coils followed by simple, low current rectifier and voltage 
> regulator circuits. You could service multiple cells with each module.
>
> I know this is a bit out of the box type thinking. Almost everyone 
> else is going with voltage clamp / current shunt type circuits. I 
> just threw this out there for consideration.
> 

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## EVDL List (Jul 27, 2007)

Hi Roger,

Let me preface this by saying "I don't know". And, let me qualify that by
saying I have a theory that I haven't been able to disprove on paper. But,
consider this;

If your cell is charged, ideally you would want to remove it from the
circuit. To do this you would disconnect the terminals, and short-circuit
the two interconnect wires. That would provide you a "bypass" for that
cell, with no energy loss (ignoring the losses in the wiring). The
"energy", in your question, would go to the next cell.

If your charger is set to current limit, then it would develop the voltage
it needed to produce those amps. If it is set to 15 amps, and you had
120volts worth of cells, then the charger would output around 120volts (plus
a little extra for the waitress). Actually, the voltage is irrelevant, so
just 'go' with me on this one.

If you bypass one of those cells, the way I described above, then the
voltage on the charger would drop in order to maintain the current limit of
15 amps. The result would be a lower overall power output from your
charger.

What I (believe I) have is a shunt that will behave intrinsically as I
described above... ie; it will act like a "short" between the interconnect
wires, while at the same time not short out the battery. I'm not expecting
this to be a zero-loss shunt, but instead a very low-loss shunt. I believe
this shunt will be able to pass 20 amps easily, and will be able to operate
at (approx) 80% efficiency. I also believe I will be able to add this to my
existing BMON (battery monitoring) circuit with an additional 3 or 4
low-cost parts, keeping the overall cost of the system low, but allowing for
rapid high-current charge mode.

This circuit, coupled with a "programmable" charger, would allow for a very
rapid charge system, for a very low cost. It would be light weight as well. 
The idea would be that a charger could be connected to the pack ends, and
set for max current (within the limits of the pack, of course) and as the
cells become charged they begin to bypass. There will be losses in the
bypass circuits, even as efficient as they are, and as more of them kick in,
the charger current can be reduced (automatically) to lower those overall
energy losses.

The key to this system is a low-loss low-cost bypass circuit. I think I
have one. But we'll have to wait and see, I guess.

So, back to your question, yes the 'energy' that was going in to the cell
that is now bypassed has to go somewhere. And it does... it goes in to the
next cell. When that cell bypasses, the energy continues down the chain to
the next cell. Once all of the cells are in full bypass mode, then yes, to
answer your question, the energy that's being pumped in to the system
(initially 15 amps) has to go somewhere... and it will... it will finally go
off in heat. But, by then all of the cells will be charged, and the charger
can just be shut off.

I hope that explains it.

~mark




> Roger Heuckeroth wrote:
> >
> > Mark,
> >
> ...


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## EVDL List (Jul 27, 2007)

And, let me add that I might just blowing a bunch of smoke. In fact, if you
see a big mushroom cloud over Texas, you can say; "Hey, I knew that guy! He
was nuts!"





> mark at evie-systems wrote:
> >
> > Hi Roger,
> >
> ...


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## EVDL List (Jul 27, 2007)

Hi Mark,
mark at evie-systems schreef:
> If your cell is charged, ideally you would want to remove it from the
> circuit. To do this you would disconnect the terminals, and short-circuit
> the two interconnect wires. That would provide you a "bypass" for that
> cell, with no energy loss (ignoring the losses in the wiring). The
> "energy", in your question, would go to the next cell.
> 
That would be true if lithium cells wouldn't require a constant current 
- constant voltage charging regime: after they reach the target voltage 
by charging with constant current they are not fully charged yet, so 
from then on you'll have to keep the voltage constant and keep charging 
(with decreasing current) until the charge current drops below a certain 
threshold. Then the cell is fully charged.
If you disconnect before that the cell is not fully charged.

> The key to this system is a low-loss low-cost bypass circuit. I think I
> have one. But we'll have to wait and see, I guess.
> 
Well, you also need a low-loss constant current supply. That means 
something akin to a switching power supply, but modified to keep current 
constant instead of voltage.

Bas Doeksen

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## EVDL List (Jul 27, 2007)

OK. I don't understand how you can short out the charging circuit 
without shorting out the cell, unless you put a transistor in between 
each cell. If you did that the transistor would have to handle the 
full discharge current of the battery pack.

Then again I'm sure your a lot smarter than me when it comes to these 
things.




> mark at evie-systems wrote:
> 
> >
> > Hi Roger,
> ...


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## EVDL List (Jul 27, 2007)

Well... like I said... it looks good on paper. I'll keep you posted.




> Roger Heuckeroth wrote:
> >
> > OK. I don't understand how you can short out the charging circuit
> > without shorting out the cell, unless you put a transistor in between
> ...


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## EVDL List (Jul 27, 2007)

On Thu, Oct 2, 2008 at 2:56 PM, mark at evie-systems


> <[email protected]>wrote:
> 
> >
> > Well... like I said... it looks good on paper. I'll keep you posted.
> ...


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## EVDL List (Jul 27, 2007)

Muwhaahaahaa... we shall see, eh?

If what I am planning works, it will involve just a few low-cost parts added
to the existing BMON (Battery Monitor) module that we're finishing. No
interconnect breaks or anything, just a shunt. If it works, we should be
able to control the charge being applied to the cell very accurately, and
very efficiently.

My goal here is to be able to provide battery balancing in parallel with
fast charging coupled with low energy loss (no mini-heaters). Those of you
in the great white north might want to stick to the active zener though,
just to keep your cells warm 

We have to finish the monitor units and the display, get the website open...
then we'll shift attention to this charge-bypass manager.

~mark




> Ben-149 wrote:
> >
> > On Thu, Oct 2, 2008 at 2:56 PM, mark at evie-systems
> > <[email protected]>wrote:
> ...


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## EVDL List (Jul 27, 2007)

What's your timeline on developing this system?



> mark at evie-systems wrote:
> 
> >
> > Muwhaahaahaa... we shall see, eh?
> ...


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## EVDL List (Jul 27, 2007)

Two weeks.

I always quote "two weeks" for any development project schedule. That way I
don't get too discouraged too early, and the customer always feels it's
almost right there... isn't that what they did on "The Money Pit"?? 

We are finishing final testing on the monitors. If you have not already
seen some of our (Blair Witch) videos they are here:
http://www.youtube.com/user/eviesystems

We are planning to open the site on or about the middle of this month
(October). When that happens, we will have, probably, just the monitors and
a large-format LCD. While the hardware on these components will be
proprietary, the software will be open-sourced. That way, if someone has a
mind to it, they can add/change/delete functionality to suit, and,
hopefully, they will be willing to share their efforts.

Depending on interest and time availability we will begin immediate
development on the bypass function. When and if that becomes available
(assuming we don't blow ourselves up), we will offer an "expansion" module
that will fit on the end of the existing BMON module, allowing someone to
add bypass functionality at a minimal cost. We will also offer an an
integrated Monitor/Bypass module as well. We'll probably spend a good solid
four weeks on the bypass function once we get started on it.

All of these modules will be available for any battery chemistry. That does
not mean one size fits all, but one module fits one battery chemistry, and
there are modules available for all battery voltages.

The cost at this time is completely unknown. However, we are trying to keep
this just as frugal as possible without loosing functionality. We are
trying to target $10 per cell for the monitor modules, but have no idea at
this time if that even remotely feasible. Cost is a big issue here, for us
and for the rest of all ya'll, we are well aware. Cost is going to depend
largely on order volume as well. So, we shall see. We should be able to
answer all this in the next two weeks, and have some better looking videos.

We started all this over three months ago, and I thought it would take about
two weeks then 

~mark




> Roger Heuckeroth wrote:
> >
> > What's your timeline on developing this system?
> >
> ...


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## EVDL List (Jul 27, 2007)

I would be most interested in not just the monitor, but the monitor 
with the bypass function (if you don't blow up). Also, do you know 
shat type charger you would recommend to work with your system?




> mark at evie-systems wrote:
> 
> >
> > Two weeks.
> ...


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