Author Topic: (2x) 24V Li packs in series  (Read 7708 times)

Offline e-lmer

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(2x) 24V Li packs in series
« on: August 22, 2010, 05:17:14 AM »

I would like to get two of the GM 24V packs and wire them
in series to get 48V.

I have two questions.

Can I draw (1000W/48V < ) 21Amps from them without damage
to the battery packs?

Is there some way to tie the BMSs from both packs together and
balance charge at 48V instead of (2x)24Volts?

My goal is to mount one battery on each side of the bike rack for
balance and low center of gravity.


Offline MonkeyMagic

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Re: (2x) 24V Li packs in series
« Reply #1 on: August 22, 2010, 06:19:11 AM »
Hi Elmer

24v 16Ah GM battery has 60A max discharge current.

Running 2 in series will give you +~48v, 16Ah @ 60A max discharge current.
You can run both fine, and even dual drive if you wanted to.

You could also charge both of them from a 48v charger as the BMS in each battery will maintain the cells in the packs.

I wish we could change MP voltage settings on the fly, that means you could have had 24v for long range or 48v for speed !

:)

Offline Leslie

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Re: (2x) 24V Li packs in series
« Reply #2 on: August 22, 2010, 07:43:52 AM »
Polarity!

Yes you need some heavy duty barrier diodes to stop current from flowing through the bms the wrong way.  If one BMS cuts out too.


« Last Edit: August 22, 2010, 07:56:47 AM by 317537 »

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Offline e-lmer

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Re: (2x) 24V Li packs in series
« Reply #3 on: August 24, 2010, 05:24:20 AM »
Leslie

That's close, but I don't think that's right.

You have
0V-------------------------------------24V--24V--------------------------------------------48V

on the power plug side of the batteries.

and from the charger:
0---------------------------------------------------------------------------------------------48V

then inside the battery packs you have:
0-1.2-2.4-3.6.....24       with a wire from the bms to each of the cells for each of the packs,
with none of these connections exposed without opening the pack.


What I need is probably more along the lines of:

0V------------------------CHARGER/Controller---------------------48V
 |                                                                                           |
 |                                  |-------|                                             |
 |                24v             |         |                     24v                   |
 |                                  |         |                                              |
 | =====bms1======|         |=========bms2=======|
                     |                                               |
                     |------------Resistor?--------------|
                     24v                                          24v

It can't be as simple as connecting the high side of one BMS to the
low side of the other can it?  What happens when the two packs
are asymmetric?  There must be something to allow slow current
between the two packs to reach 24v equilibrium.




Offline Leslie

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Re: (2x) 24V Li packs in series
« Reply #4 on: August 24, 2010, 08:20:28 AM »
Thats pic is a print screen straight out off the Ping battery PDF. 8)

I know it looks odd but the nature of electricity is very odd.

Damage wise, the BMS don't care about the packs voltage as it slumps underload the BMS cares about the zero point polarity over all.  The barrier diode I believe works by containing the individual series pack polarities within each pack. If middle point of Resistance is allowed to slump past the proceeding pack cells, a cell has the chance to become under the polarity of cells outside its BMS.   No current runs through the diodes during slumps  as the positive output barriers any reverse polarity happening.

There is a difference between a negative voltage regulation and a positive voltage regulation in the BMS.  The zero point polarity shift causes a series BMS to attempt to bleed, switch and regulate negative polarity during large slumps when the pack is near flat..

The diodes in Pings config don't protect slumps, it protects the BMS from polarity shifts and establishes a ground point for the most positive point of any pack to T barrier any current flowing past the diode barrier and most positive point of the BMS.

They just fix the polarity points around BMS's, as the pack slumps, each pack cells polarities is contained within the diodes so the polarity shifts cant damage the BMS.  The Cells don't mind, just the BMS.

The Ping way is a good way as you don't have to discharge through silicon to get your juice.

« Last Edit: August 24, 2010, 12:24:38 PM by Bikemad »

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Offline Leslie

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Re: (2x) 24V Li packs in series
« Reply #5 on: August 24, 2010, 09:03:16 AM »
This is a sample of how I think the electromechanics are working with the Ping series pack diode config.  There is a whole lot more to understand it 100% but if I can understand it 25%, I can get the idea of the the other complexities involved.



The resistor will only will allow polarity to move past it but still divide the potential current flow as you suggest.  A voltage divider isnt needed for this application, The amount of current you would need to burn up to achieve polarity stability would equal the amount of current that was used to off set it.

The diodes need to be big and rated at the current draw you are doing with both packs, its huge surges that follow the polarity shift attempts past the diode that need to be blocked initially on heavy load times, max current loads really move the polarity fast, inrush could cause cells to discharge in reverse on very low cells, the BMS cuts out but the damage is being done none the less, wow it can get complex around the BMS.

The diodes wouldn't get too hot.  Not like a resistor would anyway.

Maybe the diode can stop a certain amount of slump on a single pack as they can stop the springyness in one pack from squashing the hell out of the lower energy pack, evening out the slump and just reducing the current flow still avoiding the higher energy pack from squashing the lower energy pack

Ounce the load has established inertia max current spikes and surges are not so dangerous, I believe the polarity settles into the pack accordingly.

The cells should be ok as its only 48v in the end, The BMS may not be so lucky.
« Last Edit: August 24, 2010, 09:35:07 AM by 317537 »

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Offline e-lmer

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Re: (2x) 24V Li packs in series
« Reply #6 on: August 24, 2010, 09:52:46 PM »
The diagram for series connections on the ping website
shows the diodes reverse connected across each battery
pack, with no actual current running through them.

I know that for the BMS to work, it must see that two
adjacent cells are at different voltages, and drain the one
at higher potential, possibly into the one at lower potential.

If you just connect + to - on two packs, you won't get the same
activity between the two packs, so one could be at 26 volts and one
at 22, but you would still get 48V out.  Somehow a circuit must
compare the adjacent cells of the two packs.

They also show the BMS as connected to each cell in the
pack in one drawing, then as external to the pack in the
one you originally posted.  That can't work either.

I think the diodes here are like the flyback suppressors
you put across an electric motor.  I don't see how it would
add to the battery pack here.  (but I can see the 0.7V reverse
current draining the pack while not in use.)


Offline Leslie

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Re: (2x) 24V Li packs in series
« Reply #7 on: August 24, 2010, 11:43:14 PM »
Diodes only leak current in reverse in the  100 ua range even at 1KV.  But this is not the problem.  Some electricity get through during the switching wich there shouldnt be any foir this purpouse, so for efficiency it works fine,


Yes what you say about the BMS is correct I believe.  

The diode works like a one way small capacitor that wont overcharge or better still is rated to not fail. The capacitance is tiny.


The way I show things isnt always the way I understand it. When I did my electronics course I could handle all of it but at the end the teacher told us everything we know is the opposite way around.

I had this post with holes and electrons ready to go.

Anode +
Cathode-

If you look at the lithium ion its easy to see thar it must lose an electron to make the a Li anode, and store it in the aluminium cathode. The insulator stops the leakage, and the electroltye promotes the exchange.  So yes the electrons are negative and what we call the +ions are the holes. But the electrons travel from negative to positive. to discharge.  

The diode here has capacitance, as at the positive (p) junction, electrons gather blocked by the depletion zone at the barrier and in turn push electrons out in the substrate on the negative side (n) side due to an electric opposing field, So holes don't move like our little electron friends because they are the atoms and they change ionic state depending on the action you perform upon them.


Charge a battery you don't actually fill it up like a petrol tank, all you do is pump electrons from the pos plates to the neg plates. By bombarding the negative plate with electrons you draw from the positvie plate.  You can charge the battery off a dynamo, the power allways goes to ground and that is magnetically induced bythe dynamo from the +anode.

The Li-ion battery is more like a capacitor with disimilar metals and uses chemicals to promote charge carriers.  Unlike chemical batteries which create a different moecule to charge and turn return to their original state during discharge.  This is why Li batteries charge so much faster as they don't need to make moleculse to achive there goal.

Li-ion

Atoms wont move unless the medium they are bonded into is destroyed. When you discharge the only partical that moves are electrons from the negative plate to the positive.  Not may people get that so we just telll them the power runs from pos to neg.

What put me back so much in elecrtronics is that when I learned verything one way it turned me upside down, I tried to work it the other way.  To this day I mstill get some stuff the wrong way around.  Just like my screeny.  Learning the correct way electricity works has been a better experience in the long run as It gives me better understanding on many physics subjects and learning them is not so impossible.




Exerpt from wiki  
Quote
During discharge, lithium ions Li+ carry the current from the negative to the positive electrode, through the non-aqueous electrolyte and separator diaphragm.[7]

During charging, an external electrical power source (the charging circuit) applies a higher voltage (but of the same polarity) than that produced by the battery, forcing the current to pass in the reverse direction. The lithium ions then migrate from the positive to the negative electrode, where they become embedded in the porous electrode material in a process known as intercalation.


During discharge, lithium ions Li+ carry the current from the negative to the positive electrode, through the non-aqueous electrolyte and separator diaphragm..


Ah ha!

So the current travels through the sepatator. So then why we need to link the cells up at all with conductors.  Yah right.   People cant see how the positive moves to the negative and the negative moves to the positive,  The cant see the holes are just positive imbalanced atoms that can gain an electron or lose one in the blink of an eye.  It will always have polarity to the negative charged cathode through a conductor.


I got no good info for this post from google, its all like the above, a little bit far fetched.  Im sure the engineers and pysicians understand this and I cant access the papers.

So if anyone wants to add to this for a better understanding of this science please do.


Im a very tired now as its damned 9:37 AM and I stayed up all night again. ::) I would love to discuss this more soon but my brain is behaving a little random.

Cheers and good night morning Duane.
« Last Edit: August 25, 2010, 12:17:23 AM by 317537 »

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