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  #11  
Old 06-16-2012, 08:45 PM
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Default Re: [EVDL] charger question - bypassing batteries

[quote] Mike Nickerson wrote:
> Hi Lee,
>
> I have similar cells, BMS and charger to Ben's. The system is not as
> intelligent as you describe. The charger decides on its own (from the
> current charging voltage and its built-in program) when to switch to lower
> current. It won't do that unless the cells are pretty well matched. The
> BMS doesn't have any communication with the charger about cells at the
> shunting level.

I too use a BMS controlled ac relay to cut my charger. That seems
simplest, safest and works with any charger or combination of
chargers.

However, I think the Elcon/TC charger has some other cut-back
capabilities though I haven't explored them. There is a CAN bus
option, but I don't know what that does. Then there is the little
signal connection. I just have my signal wires shorted together but I
think they can be used with the BMS to cut back the charging rate.
Sorry, but I know little more than the presence of those two things.

--
Willie, ONWARD! Through the fog!
http://counter.li.org Linux registered user #228836 since 1995
Debian3.1/GNU/Linux system uptime 88 days 15 hours 34 minutes

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  #12  
Old 06-17-2012, 12:06 PM
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Default Re: [EVDL] charger question - bypassing batteries

So in my case, all 50 of my Calb 130 Ah cells started at 3.2 volts. The charger was
setup by the factory for my pack (supposedly). It has a sticker on the side saying
it's for 180V max LiFePo4 pack. So I charge my pack and here's what happens:

* After charging for a few hours on 120V (about 7 amps into the pack), I have some
cells that start reaching 3.5 Volts.
* At this point, the mini bms starts shunting these cells. I'm still getting
around 7amps into the pack.
* If one of these cells hits 3.6 Volts, the mini bms shuts off the charger
(that's what's happening now).
* For me, I have 10 cells that are shunting when the mini-bms turns off the charger.
* Most of the other cells are at 3.4V (for Lithium, it's hard to know what the SOC
is from the voltage, but they should be pretty well charged).

So what I'd like to do is:
* Charge my pack until the mini bms shuts off the charger.
* Bypass the 10 early cells (I have a 2 gage bypass cable for emergencies)
* Start the charger again
* Allow the mini bms to shut off the charger
* See if I'm top balanced yet.

Can I damage anything if I hook up my Elcon 2kW charger (that's been "programmed"
for 50 cells) to 40 cells and let it charge? This is one question I still don't have
an answer for. Maybe someone out there knows.

I still could get a single cell charger to finish things off, but I'm looking for
a good way to get closer.

thanks!
-ben


[quote] Lee Hart wrote:

> On 6/16/2012 8:18 PM, Mike Nickerson wrote:
>> I have similar cells, BMS and charger to Ben's. The system is not as
>> intelligent as you describe. The charger decides on its own (from the
>> current charging voltage and its built-in program) when to switch to lower
>> current. It won't do that unless the cells are pretty well matched. The
>> BMS doesn't have any communication with the charger about cells at the
>> shunting level.
>
> Hmm... that's a problem. Do you know what its charging algorithm is? My
> thought would be to wire in a voltmeter and ammeter, and *watch* what
> the charger actually does. I often find that what the manufacturer
> claims does not agree with what it *actually* does!
>
> Until that data is available, let's assume it has the simplest algorithm
> that would barely work:
>
> a. Charge at maximum current until it reaches V volts.
> b. Charge at constant V volts until the current falls to I amps.
> c. Shut off.
>
> This works if the cells are all very well matched and top-balanced. At
> full charge, the charger assumes that (for example) V = 365v means you
> have 100 cells at 3.65v each.
>
> But if the cells are mismatched, some cells will go overvoltage while
> others are still undervoltage. The total may be 365v; but individual
> cells might range from 3.3v to 4.3v. 3.3v is nowhere near full, and 4.3v
> is well into overcharge!
>
> You can add a BMS that shunts some low current around any cell that goes
> over some "full" voltage (say, 3.65v). Let's say it can shunt 1 amp. But
> if the charger sees less than 365v for the pack as a whole, it can still
> be pumping out more current (say, 2 amps). It just overpowers the shunt
> regulator; 1 amp goes through the shunt regulator and 1 amp through the
> cell, so it *still* overcharges and overvoltages the cell.
>
> This is why you need some way to insure that the charger is at a low
> current when the regulators start to bypass. The charger should have an
> algorithm that automatically cuts back to a low current well before it
> reaches its voltage limit. Or, you need to add some other method to
> limit charging current when any shunt regulator is on. Then when *all*
> cells have reached their voltage limit, you can shut off the charger.
>
> If your charger has no such means to limit its current, you can add a
> relay that switches a resistor in series to limit the current. When the
> first BMS regulator turns on, it turns this relay off. The resistor is
> chosen to limit the current to whatever the regulators can safely
> bypass. (A light bulb works well as a resistor, because they naturally
> act as current limiters).
>
> If the charger doesn't already have a shut-off timer, you should add one
> of these as well. The simplest type is a 0-12 hour mechanical timer
> (Intermatic etc.). It fits in a normal electrical switch box, and has a
> knob that you twist to set the maximum time. It has 240vac 20amp
> contacts that automatically shut off the AC input to the charger at the
> end of the time.
>
>> The MiniBMS system and Elcon charger combination will work fine once the
>> cells are balanced.
>
> If the cells are balanced, the BMS shouldn't be doing anything.
>
> If the cells aren't balanced, isn't the BMS supposed to balance them? If
> it doesn't work when they are unbalanced, what is the purpose of this BMS?
>
> --
> *BE* the change that you wish to see in the world.
> -- Mahatma Gandhi
> --
> Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart at earthlink.net
>
> _______________________________________________
> | Moratorium on drag racing discussion is in effect.
> | Please take those discussions elsewhere. Thanks.
> |
> | REPLYING: address your message to xxx@xxx.xxx.edu only.
> | Multiple-address or CCed messages may be rejected.
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  #13  
Old 06-17-2012, 12:35 PM
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Default Re: [EVDL] charger question - bypassing batteries

You could just wait a few minutes and restart the charger. It should run
for a little bit and then get shut off by the BMS again. Each time the low
cells are getting closer to full. If they really are at 3.4v when the
charger it cut off then they aren't too far behind.

You might just want to put a resistor across the few high cells and then
run the charger again. That would likely be cheaper and you know your
charger won't mind.

[quote] Ben Jarrett wrote:

>
>
>
> So in my case, all 50 of my Calb 130 Ah cells started at 3.2 volts. The
> charger was
> setup by the factory for my pack (supposedly). It has a sticker on the
> side saying
> it's for 180V max LiFePo4 pack. So I charge my pack and here's what
> happens:
>
> * After charging for a few hours on 120V (about 7 amps into the pack), I
> have some
> cells that start reaching 3.5 Volts.
> * At this point, the mini bms starts shunting these cells. I'm still
> getting
> around 7amps into the pack.
> * If one of these cells hits 3.6 Volts, the mini bms shuts off the charger
> (that's what's happening now).
> * For me, I have 10 cells that are shunting when the mini-bms turns off
> the charger.
> * Most of the other cells are at 3.4V (for Lithium, it's hard to know what
> the SOC
> is from the voltage, but they should be pretty well charged).
>
> So what I'd like to do is:
> * Charge my pack until the mini bms shuts off the charger.
> * Bypass the 10 early cells (I have a 2 gage bypass cable for emergencies)
> * Start the charger again
> * Allow the mini bms to shut off the charger
> * See if I'm top balanced yet.
>
> Can I damage anything if I hook up my Elcon 2kW charger (that's been
> "programmed"
> for 50 cells) to 40 cells and let it charge? This is one question I
> still don't have
> an answer for. Maybe someone out there knows.
>
> I still could get a single cell charger to finish things off, but I'm
> looking for
> a good way to get closer.
>
> thanks!
> -ben
>
>
> On Jun 17, 2012, at 11:28 AM, Lee Hart wrote:
>
> > On 6/16/2012 8:18 PM, Mike Nickerson wrote:
> >> I have similar cells, BMS and charger to Ben's. The system is not as
> >> intelligent as you describe. The charger decides on its own (from the
> >> current charging voltage and its built-in program) when to switch to
> lower
> >> current. It won't do that unless the cells are pretty well matched.
> The
> >> BMS doesn't have any communication with the charger about cells at the
> >> shunting level.
> >
> > Hmm... that's a problem. Do you know what its charging algorithm is? My
> > thought would be to wire in a voltmeter and ammeter, and *watch* what
> > the charger actually does. I often find that what the manufacturer
> > claims does not agree with what it *actually* does!
> >
> > Until that data is available, let's assume it has the simplest algorithm
> > that would barely work:
> >
> > a. Charge at maximum current until it reaches V volts.
> > b. Charge at constant V volts until the current falls to I amps.
> > c. Shut off.
> >
> > This works if the cells are all very well matched and top-balanced. At
> > full charge, the charger assumes that (for example) V = 365v means you
> > have 100 cells at 3.65v each.
> >
> > But if the cells are mismatched, some cells will go overvoltage while
> > others are still undervoltage. The total may be 365v; but individual
> > cells might range from 3.3v to 4.3v. 3.3v is nowhere near full, and 4.3v
> > is well into overcharge!
> >
> > You can add a BMS that shunts some low current around any cell that goes
> > over some "full" voltage (say, 3.65v). Let's say it can shunt 1 amp. But
> > if the charger sees less than 365v for the pack as a whole, it can still
> > be pumping out more current (say, 2 amps). It just overpowers the shunt
> > regulator; 1 amp goes through the shunt regulator and 1 amp through the
> > cell, so it *still* overcharges and overvoltages the cell.
> >
> > This is why you need some way to insure that the charger is at a low
> > current when the regulators start to bypass. The charger should have an
> > algorithm that automatically cuts back to a low current well before it
> > reaches its voltage limit. Or, you need to add some other method to
> > limit charging current when any shunt regulator is on. Then when *all*
> > cells have reached their voltage limit, you can shut off the charger.
> >
> > If your charger has no such means to limit its current, you can add a
> > relay that switches a resistor in series to limit the current. When the
> > first BMS regulator turns on, it turns this relay off. The resistor is
> > chosen to limit the current to whatever the regulators can safely
> > bypass. (A light bulb works well as a resistor, because they naturally
> > act as current limiters).
> >
> > If the charger doesn't already have a shut-off timer, you should add one
> > of these as well. The simplest type is a 0-12 hour mechanical timer
> > (Intermatic etc.). It fits in a normal electrical switch box, and has a
> > knob that you twist to set the maximum time. It has 240vac 20amp
> > contacts that automatically shut off the AC input to the charger at the
> > end of the time.
> >
> >> The MiniBMS system and Elcon charger combination will work fine once the
> >> cells are balanced.
> >
> > If the cells are balanced, the BMS shouldn't be doing anything.
> >
> > If the cells aren't balanced, isn't the BMS supposed to balance them? If
> > it doesn't work when they are unbalanced, what is the purpose of this
> BMS?
> >
> > --
> > *BE* the change that you wish to see in the world.
> > -- Mahatma Gandhi
> > --
> > Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart at earthlink.net
> >
> > _______________________________________________
> > | Moratorium on drag racing discussion is in effect.
> > | Please take those discussions elsewhere. Thanks.
> > |
> > | REPLYING: address your message to xxx@xxx.xxx.edu only.
> > | Multiple-address or CCed messages may be rejected.
> > | UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
> > | OTHER HELP: http://evdl.org/help/
> > | CONFIGURE: http://lists.sjsu.edu<http://lists.sjsu.edu/mailman/listinfo/ev>



--
David D. Nelson
http://evalbum.com/1328
http://2003gizmo.blogspot.com
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  #14  
Old 06-17-2012, 07:15 PM
EVDL List EVDL List is offline
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Default Re: [EVDL] charger question - bypassing batteries

If the charger never reaches its 80% indication
because one of the cells gets too high and the
BMS shuts the charger off, then I would expect that
it will help to set the charger voltage *lower* until
it is the *charger* that shuts off the charging, not
the emergency shutoff from the BMS because a cell goes
over limit.
In fact, with the charger at (much) lower voltage
you would allow any high cells to be shunted for a
while by the BMS so all high cells are pushed to lower SoC
until the point that you might be able to turn the
charger up a little when after a long period of shunting
all cells stop shunting because the total pack voltage
is divided more equally and all cells stay below the
shunting voltage. If that happens then you can turn the
charger up just a tad until you reach the point that
the lowest cell is almost shunting while all other cells
are barely shunting. That gives the lowest voltage and
the lowest waste of energy while making sure that all
of your pack will get balanced if you charge long enough.
It has the risk of over-charging cells at too low current
so it is wise to not charge to this point very often,
only when you absolutely need the max energy from your pack
(very long trip) or an occasional balance charge, once
a month or so. At all other charges you might want to
shut off the charger when it reaches 80% SoC or so to
get the longest life from your pack.

Hope this clarifies,

Cor van de Water
Chief Scientist
Proxim Wireless Corporation http://www.proxim.com
Email: xxx@xxx.xxx Private: http://www.cvandewater.com
Skype: cor_van_de_water XoIP: +31877841130
Tel: +1 408 383 7626 Tel: +91 (040)23117400 x203

-----Original Message-----
From: xxx@xxx.xxx.edu [mailto:xxx@xxx.xxx.edu] On
Behalf Of Cruisin
Sent: Saturday, June 16, 2012 5:45 AM
To: xxx@xxx.xxx.edu
Subject: Re: [EVDL] charger question - bypassing batteries

Change the algorithm in the charger to the next higher voltage and
recharge letting the BMS control the charger. The charger is shutting
off too soon.

--
View this message in context:
http://electric-vehicle-discussion-list.413529.n4.nabble.com/charger-que
stion-bypassing-batteries-tp4655708p4655722.html
Sent from the Electric Vehicle Discussion List mailing list archive at
Nabble.com.

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  #15  
Old 06-18-2012, 03:35 PM
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Default Re: [EVDL] charger question - bypassing batteries

[quote]Ben Jarrett wrote:
> So in my case, all 50 of my Calb 130 Ah cells started at 3.2 volts.

That means they were initially somewhere between 20% and 80% state of
charge.

> The charger was setup by the factory for my pack (supposedly)...
> for 180V max LiFePo4 pack.

With 50 cells, that's 3.6v per cell; a reasonable "full" end point.

> * After charging for a few hours... some cells reach 3.5 Volts.
> * At this point, the mini bms starts shunting these cells.\
> I'm still getting around 7amps into the pack.

It is desirable for the charger to cut back its current. But it sounds
like it just stays at 7 amps. The mini-BMS can't bypass 7 amps, so it
can't stop these cells from continuing to charge. Very little balancing
is going on.

> * If one of these cells hits 3.6 Volts, the mini bms shuts off the
> charger (that's what's happening now).

It's basically saying "I can't slow down the charger or stop these cells
from overcharging, so I'll shut the charger down completely. It saves
your batteries; but does little or no balancing.

> * I have 10 cells that are shunting when the mini-bms turns off the charger.
> So what I'd like to do is:
> * Charge my pack until the mini bms shuts off the charger.
> * Bypass the 10 early cells (I have a 2 gauge bypass cable for emergencies)
> * Start the charger again
> * Allow the mini bms to shut off the charger
> * See if I'm top balanced yet.

This would be a tedious and error-prone way to do it. It would be easy
to make a mistake and leave it on too long. 180v across 40 cells is 4.5v
each, which would murder them!

You have a BMS; you just need to get things set up so it actually works.
Find some way to get the charging current down low enough so the
mini-BMS can bypass it to stop the "full" cells from charging any more.
The low cells can then continue charging for long enough to bring them
into balance.

For example, stick an ordinary 120v 100 watt tungsten light bulb in
series with the charger output. It will limit the current to under 1
amp. Then the BMS can run for long enough to finish charging the low
cells, without overcharging the high cells and having to shut down the
charger too early.

> Can I damage anything if I hook up my Elcon 2kW charger (that's been "programmed"
> for 50 cells) to 40 cells and let it charge?

The charger won't be hurt; it will current limit at its maximum charging
current (7 amps). However, it will still be trying to drive the pack up
to 180v. The voltage will keep rising until it does! That would destroy
your CALB cells.

If you left the BMS connected, it would still shut off the charger when
any cell reached 3.6v. But you'd still be getting no balancing action,
so you haven't corrected the problem.

Here's an analogy. Suppose you're trying to fill gallon milk jugs with a
garden hose. The goal is to exactly fill each one; not leave it 80%
full, or overfill it so you flood the floor.

You can't do this by running the hose at full blast, and then suddenly
shutting it off completely. As the water nears the top, the bubbling and
swirling water will foam out the top and make a mess. Or, if you shut
off before any water foams out the top, then when the air bubbles out,
you're left with a jug only perhaps 80-90% full.

You need *slow down* the hose to a trickle as it nears full. Then you
can see where you are, and stop when you *just* reach full. That way,
you can precisely fill each jug to the same level.

--
First they ignore you; then they mock you; then they fight you; then you
win.
-- Mahatma Gandhi
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart at earthlink.net

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  #16  
Old 06-18-2012, 06:15 PM
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Default Re: [EVDL] charger question - bypassing batteries

When I set up my 40 CALB 100Ah cells with miniBMS, it took me almost 2
weeks of charging to get the whole pack to the point that all cells were
above 3.5 v (all red LED's lit) at the same time. I used a variety of
chargers from an adjustable power supply (one that Dimitri recommended)
which can charge individual cells, or small groups of cells, a 12v charger,
and a 24v charger for groups of cells. I would use a 12v DC 50 watt bulb
and a 12v load tester to drain the first ones to light up while charging.
Once they all reached that point, I haven't had to do anything. I just
periodically check end of charge to make sure that they still all light up
before one hits the shut off point. It has been a little over 6 months, and
I haven't seen any real variation yet; cells are all within 0.01 of each
other.
Michael B

[quote] Lee Hart <xxx@xxx.xxx> wrote:

> Ben Jarrett wrote:
> > So in my case, all 50 of my Calb 130 Ah cells started at 3.2 volts.
>
> That means they were initially somewhere between 20% and 80% state of
> charge.
>
> > The charger was setup by the factory for my pack (supposedly)...
> > for 180V max LiFePo4 pack.
>
> With 50 cells, that's 3.6v per cell; a reasonable "full" end point.
>
>
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  #17  
Old 06-18-2012, 06:55 PM
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Default Re: [EVDL] charger question - bypassing batteries

[quote] Ben Jarrett <xxx@xxx.xxx> wrote:

>
> So what I'd like to do is:
> * Charge my pack until the mini bms shuts off the charger.
> * Bypass the 10 early cells (I have a 2 gage bypass cable for emergencies)
> * Start the charger again
> * Allow the mini bms to shut off the charger
> * See if I'm top balanced yet.

You don't really want to do that...you would have to have a different charger to charge 40 cells vs 50 cells (or to reprogram the charger's max voltage - which I don't think you can do to yours, but you can on a Manzanita Micro PFC charger).

Just charge all the other cells that are low. Or, manually "shunt off" current on the high cells to get them lower. However, I haven't done the second option, so I'm not sure what the best way to do it is (maybe a light bulb).

corbin

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  #18  
Old 06-19-2012, 01:35 PM
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Default Re: [EVDL] charger question - bypassing batteries

The last time I installed my cells, I charged them each individually up to =
3.60v with a PowerLab 6 charger. I installed them all, drove a bit, then=
charged them with my vehicle charger. My charger shuts off at 3.5vpc av=
erage, and there were 10 or so MiniBMS red lights that were not on at end o=
f charge (on mine, the lights come on at ~3.47v). I figured they had to =
be pretty close since I had manually charged them all to 3.60v, and just le=
ft it be. About 10 discharge/charge cycles later, I checked. The same=
cells were still not lighting up the MiniBMS lights. At that point I ma=
nually brought them up with a power supply one cell at a time, and found th=
ey required less than 1ah to get up to shunting (typical was about 15 minut=
es at 2a). I don't think the MiniBMS is able to handle much balancing at=
all (at least in my case when I'm only charging to 3.5vpc), but at least i=
t will make up for any cell balance difference due to it's own power draw.
:-)

Now all 48 lights are on at end of charge. I've also noted that over the=
last few months, the same lights come on in the same order every time, and=
the same cell is always the last one to light up. Not a lot of balancin=
g going on I don't think.

fwiw, it's easy to balance them with a power supply. You don't have to d=
isconnect anything, just set it to 3.6v open voltage, alligator clip it acr=
oss the single cell, and turn up the current to charge until the red light =
on the MiniBMS comes on. You are now at 3.47v, monitor the cell till i g=
ets to where you want it to be (in my case 3.5v), then shut off the power s=
upply.

Check again next charge cycle and repeat as necessary.

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| Please take those discussions elsewhere. Thanks.
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  #19  
Old 06-19-2012, 03:55 PM
EVDL List EVDL List is offline
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Default Re: [EVDL] charger question - bypassing batteries

[quote] Lee Hart <xxx@xxx.xxx> wrote:
> ...
> It sounds like there is a fundamental problem with how the mini-BMS and
> this Elcon charger work. Each one is assuming that the other is doing
> the balancing; so in fact *nobody* is!
> ...

My impression is that the mini-BMS is more suited to keeping a pack
balanced than balancing an unbalanced pack.

If you start out balanced and set the charger to taper off to constant
voltage near 3.4-3.5 V/cell average, the mini-BMS should be able to
keep the cells balanced even with a 'dumb' charger that doesn't know
about the BMS. If you start with a few amp-hours difference, you'll
need to put a lightbulb in series and charge for many hours to do the
initial balance.

-Morgan LaMoore

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  #20  
Old 06-19-2012, 04:15 PM
EVDL List EVDL List is offline
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Default Re: [EVDL] charger question - bypassing batteries

FWIW,
A top balancer that really works automatically, every time you charge means
you never have to do anything to the batteries but use them to move the car.


In essence this means as most of us have been saying, a balancing system
with LiFePO4 is a good idea.


Sincerely,
Mark Grasser
=



-----Original Message-----
From: xxx@xxx.xxx.edu [mailto:xxx@xxx.xxx.edu] On Behalf
Of David Ladd
Sent: Tuesday, June 19, 2012 2:31 PM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] charger question - bypassing batteries

The last time I installed my cells, I charged them each individually up to
3.60v with a PowerLab 6 charger. I installed them all, drove a bit, then
charged them with my vehicle charger. My charger shuts off at 3.5vpc
average, and there were 10 or so MiniBMS red lights that were not on at end
of charge (on mine, the lights come on at ~3.47v). I figured they had to=
be
pretty close since I had manually charged them all to 3.60v, and just left
it be. About 10 discharge/charge cycles later, I checked. The same ce=
lls
were still not lighting up the MiniBMS lights. At that point I manually
brought them up with a power supply one cell at a time, and found they
required less than 1ah to get up to shunting (typical was about 15 minutes
at 2a). I don't think the MiniBMS is able to handle much balancing at all
(at least in my case when I'm only charging to 3.5vpc), but at least it will
make up for any cell balance difference due to it's own power draw.
:-)

Now all 48 lights are on at end of charge. I've also noted that over the
last few months, the same lights come on in the same order every time, and
the same cell is always the last one to light up. Not a lot of balancing
going on I don't think.

fwiw, it's easy to balance them with a power supply. You don't have to
disconnect anything, just set it to 3.6v open voltage, alligator clip it
across the single cell, and turn up the current to charge until the red
light on the MiniBMS comes on. You are now at 3.47v, monitor the cell ti=
ll
i gets to where you want it to be (in my case 3.5v), then shut off the power
supply.

Check again next charge cycle and repeat as necessary.

_______________________________________________
| Moratorium on drag racing discussion is in effect.
| Please take those discussions elsewhere. Thanks.
|
| REPLYING: address your message to xxx@xxx.xxx.edu only.
| Multiple-address or CCed messages may be rejected.
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