The Lee Hart alternative of this solution is to have only a single pack,
add a (very low current) tap in the center of the pack, connected to two
super-bright LEDs mounted in a very visible place (on the dash right in
front of you). Connect the two LEDs in anti-parallel so whichever direction
there is more than about 2V difference, one of these LEDs will light up.
Connect the other side of the 2 LEDs to a voltage divider that goes from
pack+ to pack- and which in rest outputs the same voltage as at the tap.
If you have an even number of cells/modules then this voltage divider is
simply two identical resistors in series across the pack so the center point
is at exactly half pack voltage. If you have an odd number, one of the two
resistors needs to be a little bigger than the other (or use 2 in series or
use a potmeter in between the two resistors so you can fine-tune the divider
to be equal to the tap.
Alternative could be a simple cheap zero-centered meter as often used in the
current meters (can measure positive and negative) and place that instead of
the two LEDs. Any imbalance between the two pack halves is indicated by the
LEDs or this meter.
Size the resistors so they allow for a small current, enough to light the LEDs
or move the meter - you can easily test this by deliberately tapping the pack
one cell too high/low and watching the result.
for example, if you have a 120V pack then you will tap this at the 60V point
and to get a 1mA through one of two red LEDs at that voltage when you have an
(arbitrarily) 6V deviation between tap and the average of the pack, then you
need a resistance divider of 10k + 10k Ohm. Note that this put a
constant drain of 6mA
on the pack - not a whole lot since it needs almost a week before it
drains 1Ah,
so even a moderate 100Ah pack would last almost 2 years from full to dead.
But it is something to be aware of and disconnect when you store an EV.
The approach with the needle meter is more sensitive since those
typically have
100uA full scale so you would use at least 10 times bigger resistance value
and this leads to 1/10th of the drain.
Regards,
Cor van de Water
Chief Scientist
Proxim Wireless
office +1 408 383 7626 Skype: cor_van_de_water
XoIP +31 87 784 1130 private: cvandewater.info
www.proxim.com
This email message (including any attachments) contains confidential
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received this message in error, please delete it and notify the
sender. Any unauthorized use, disclosure, distribution, or copying
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-----Original Message-----
From: EV [mailto:[email protected]] On Behalf Of Sean Korb via EV
Sent: Thursday, June 18, 2015 1:41 PM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] Bicycle battery
This sub-subject gets done to death pretty regularly but I like
gathering what I hear from both camps. Definitely going to use a
BMS but I really like the non-BMS workaround of getting *two*
expensive battery packs and using a really simple gauge that shows
their comparative voltage. If one is sagging more than the other,
pull over and find out what is wrong. If you have a smart BMS... it
takes care of it for you until you can get home
;)
sean
On Thu, Jun 18, 2015 at 3:58 PM, Cor van de Water via EV <[email protected]>
wrote:
> Paul,
> In your case, *you* were the BMS.
> But you were lucky that you noticed the lower end voltage and knew
> something was off.
> You were lucky that the charger finished charging without cooking the
> other cells to death You were lucky that the bad cell was a short and
> not a partial failure or you might not have detected it in time or
> faced a much more severe situation than a shorted cell.
> Anyone less knowledgeable than you about your EV (and that would be
> 99+ percent of all drivers) would not have caught this failure and
> continued until disaster, so your vehicle can be used by you alone,
> because you are an essential part of that vehicle! (namely, the BMS)
> It looks like you just confirmed the thing that you tried to disprove...
>
> Cor van de Water
> Chief Scientist
> Proxim Wireless
>
> office +1 408 383 7626 Skype: cor_van_de_water
> XoIP +31 87 784 1130 private: cvandewater.info
> www.proxim.com
>
>
> This email message (including any attachments) contains confidential
> and proprietary information of Proxim Wireless Corporation. If you
> received this message in error, please delete it and notify the
> sender. Any unauthorized use, disclosure, distribution, or copying of
> any part of this message is prohibited.
>
>
> -----Original Message-----
> From: Paul Dove [mailto:[email protected]]
> Sent: Thursday, June 18, 2015 11:13 AM
> To: Cor van de Water; Electric Vehicle Discussion List
> Subject: Re: [EVDL] Bicycle battery
>
> I had this issue early on in my vehicle and at the end of charge my
> pack voltage reflected this and I removed the bad cell. The cell was
> shorted but the rest of the pack was fine. I have no cell monitoring.
>
> Pretty sure it was caused by a loose connection. That said, apart from
> expense I have no problem with a BAttery Monitor that looks at every
> cell but prismatic cells have multiple cells in them in parallel so
> you can't monitor them individually.
>
> Still of the opinion it is an unnecessary expense.
>
> You haven't convinced me that cell discharge or drift between SOC is a
> real issue. I cat see it ever being an issue.
>
> Sent from my iPhone
>
> > On Jun 18, 2015, at 12:44 PM, Cor van de Water via EV
> > <[email protected]>
> wrote:
> >
> > Paul,
> > It is true that in series string the same *load* current goes
> > through all cells, but the principle of self-discharge is that it
> > occurs internal to the cell, so it is invisible to the outside world
> > except when you measure each cell (or blow them up due to overcharge
> > or
> under-discharge).
> > I have actually measured cells and seen the self-discharge.
> > As you say, it is a small effect in good cells.
> > But still, there is about 1:2 difference in self-discharge between
> > the cells I monitor and the differences add up over time.
> > It might not be a problem in the first year or even in the second.
> > Then in the 3rd year you try to squeeze an 85 or 90% discharge from
> > the cells and boom - one reverses (or more) and it is destroyed and
> > you might only find out from the fireworks when you try to charge it
> > the
> next cycle or when you drive it.
> > Oh BTW - one reason to monitor *all* and *every* cell is exactly the
> > issue with the infamous bottom-balancing without BMS approach:
> > Some of the cells that I have were abused and too deeply discharged.
> > Guess what happens? They become a resistor.
> > Some are "low" resistance which is OK when they resemble a wire,
> > others are still "in doubt" whether they want to become a piece of
> > wire
> or rather a heating element.
> > This has at least 2 disastrous effects if you do not detect this
> immediately:
> >
> > 1. The pack voltage has dropped and each cell now gets a much higher
> > finish charging so you might have been charging conservatively with
> > all cells in the string, but with some cells "removed", all the rest
> > is dividing up the difference and may easily be charged to
> > destruction
> now!
> >
> > 2. If charging does not harm the pack, what about discharging
> > hundreds of Amps through a resistor that is nicely embedded in the
> > pack, insulated from the outside by the cells around it? If it even
> > drops 10V across it at hundreds of Amps, you now have a
> > multi-kiloWatt heater
> inside your pack without much cooling. What do you think will happen?
> >
> > Just some easy illustration of the *need* for a cell-level BMS, from
> > practice by measuring what happened to a used set of Lithiums (that
> > indeed was run without BMS) and I have been monitoring since taking
> > it
> out of service. Learn from it or get your own experience, your choice.
> >
> > Cor van de Water
> > Chief Scientist
> > Proxim Wireless
> >
> > office +1 408 383 7626 Skype: cor_van_de_water
> > XoIP +31 87 784 1130 private: cvandewater.info
> > www.proxim.com
> >
> >
> > This email message (including any attachments) contains confidential
> > and
> proprietary information of Proxim Wireless Corporation. If you
> received this message in error, please delete it and notify the
> sender. Any unauthorized use, disclosure, distribution, or copying of
> any part of this message is prohibited.
> >
> >
> > -----Original Message-----
> > From: EV [mailto:[email protected]] On Behalf Of paul dove
> > via EV
> > Sent: Thursday, June 18, 2015 9:57 AM
> > To: Lee Hart; Electric Vehicle Discussion List
> > Subject: Re: [EVDL] Bicycle battery
> >
> > Excerpt:In theory, all batteries are identical. If wired in series,
> > you
> would therefore expect them all to charge and discharge equally. But
> in practice, there are differences. New batteries that are all the
> same brand, same model, same date code (and without "lemons" or
> quality control
> defects) will still have small differences. Each cell 's
> self-discharge rate, amphour capacity, internal resistance, and
> charge/discharge efficiency will be slightly different. This makes
> them drift to different states of charge. For example, cells with a
> higher self-discharge rate run down faster just from sitting. Cells
> with a lower amphour capacity get more deeply discharged on each
> cycle, which lowers their efficiency (so they need a bit more current
> to fully recharge). Cells with a higher internal resistance run a
> little hotter, which affects their efficiency and self-discharge rate.
> These differences tend to get larger over time. If not corrected, you
> can have a pack with some cells almost full, and some almost empty!
> > Ok, here's the deal this is fiction. Self discharge is so low
> > (milivolts
> over years) as to be non-existant.Secondly, if they are in series then
> all current passing through cells is equal. It is impossible to draw
> more current from one cell than another in a series pack. It can
> short and pass all current or it can open and pass no current but it
> cannot discharge faster or slower.
> > If it has higher resistance then it will cause cells to heat faster
> > that
> is all.
> > Please show evidence that your theory is possible.
> > The only person I spoke with having issues with new cells is when
> > they
> were placed in parallel with old cells. In that case the wires melted
> because the new cell was handling more of the current. You may damage
> cells if there is a big difference in resistance only if you exceed
> the max current draw from the cell. If one never approaches the max
> current draw this will not be an issue.
> >
> >
> > From: Lee Hart via EV <[email protected]>
> > To: Electric Vehicle Discussion List <[email protected]>
> > Sent: Thursday, June 18, 2015 10:53 AM
> > Subject: Re: [EVDL] Bicycle battery
> >
> > Paul Dove via EV wrote:
> >> That was a very good summary. Cell balancing is something that was
> >> done with other chemistry a and many have tried to apply it to Li
> >> Ion chemistries but it doesn't work with Lithium since cells do
> >> self discharge or drift.
> >>
> >> If one wanted to balance them it has to be done below 3.38 volts
> >> otherwise you are still charging the cell. Thanks for your input.
> >
> > Lithiums need balancing even more than other chemistries! You can
> > only get by without a BMS if the cells are so well matched that they
> > accidentally stay in balance. (Do you feel lucky?)
> >
> > Look at my own balancer at http://www.sunrise-ev.com/balancer.htm
> > I built it to *use*, not to sell. It doesn't load the cells, or
> > clamp
> the voltage at some arbitrary level, and doesn't have failure modes
> that murder cells.
> >
> > It basically does what you would do yourself, if you had the time
> > and
> inclination. It measures the voltage of every cell, and charges the
> ones that are low.
> > --
> > The greatest pleasure in life is to create something that wasn't
> > there before. -- Roy Spence
> > --
> > Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
> > _______________________________________________
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> >
> >
> >
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--
Sean Korb [email protected] http://www.spkorb.org
'65,'68 Mustangs,'68 Cougar,'78 R100/7,'60 Metro,'59 A35,'71 Pantera
#1382 "The more you drive, the less intelligent you get" --Miller
"Computers are useless. They can only give you answers." -P. Picasso
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