Charge Shuffling for Li cells

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antiscab
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Charge Shuffling for Li cells

Post by antiscab »

an elcon 2kw charger with 417v 5.5A limits costs ~$750

I suppose it depends upon what you consider expensive

Theres a kit for sale over on DIY that lets you roll your own relatively easily for ~$500 in the 10kw region

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Matt
2017 Renault zoe - 25'000km
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2000 prius - has 200 x headway 38120 cells

Nevilleh
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Charge Shuffling for Li cells

Post by Nevilleh »

My $NZ1220 = $A950 and for that you get an 1800 watt charger, a bms and a high powered balancing system. I thought that would compare quite favourably. Mind you, that's about the cost of doing it myself, but I think its worth going a bit further.

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Richo
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Charge Shuffling for Li cells

Post by Richo »

Ok if I had a 2.4kW AC/DC 48V PSU then each "pair" of headway cells had a 10W 48V/5V Isolated DC-DC then I use a Li-po charger IC to perform the CC-CV.
I would be up for ~$4200.

I'm sure there is a "sweet spot" for charger size.
So using the larger TS or SE cells it may work.

You can get off-line fly-back IC's now that do it all - just need the right transformer made.
Your pricing sounds about right.

So the short answer is NO but the long answer is YES.
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Johny
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Charge Shuffling for Li cells

Post by Johny »

While you are speaking of cost I'll bring it up again....
Don't forget that there will have to be some kind of smart decision to alter the individual charger "kick-in" voltage when in mobile shuffling mode otherwise it will drain the pack (at 85-90% efficiency) trying to get every cell up to 3.65V. So that complexity adds cost too - probably based on what weber/coulomb and Neville have already but cost nonetheless.

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Charge Shuffling for Li cells

Post by Nevilleh »

My plan is that the bms controls that stuff and it is user programmable for any voltage in the range 2.5 - 4.0 V. I intend using my present bms module and including it on the pcb that carries the cell charger so you get charger, bms and balancing system all in one.

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PlanB
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Charge Shuffling for Li cells

Post by PlanB »

Nice concept squire. I'm a bit off topic here but I've become besotted with the 130 Wh/kg capacity of pouch cells. Will your design be specific to the big traction cells with bolts on top or is there a way to accommodate the floppy bag cells too?

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PlanB
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Post by PlanB »

Nev you can buy potted 20w 48v-5v isolated DC DC converters for ten bucks in qty. Could you just drop one of these onto a PCB with your BMS micro & a wireless chip (bluetooth or zigbee for reporting cell status to a host)?

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Charge Shuffling for Li cells

Post by Johny »

But if they are potted you can't change the voltage.
Last edited by Johny on Thu, 15 Dec 2011, 10:32, edited 1 time in total.

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PlanB
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Charge Shuffling for Li cells

Post by PlanB »

Something like this maybe?
With a 4A diode in the output to stop battery backflow & 10% output reduction via a digital pot on the trim input via your BMS you'd have a 4.5-0.7v=3v8 output which would suit lithiums fairly well?

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Charge Shuffling for Li cells

Post by Nevilleh »

PlanB wrote: Nice concept squire. I'm a bit off topic here but I've become besotted with the 130 Wh/kg capacity of pouch cells. Will your design be specific to the big traction cells with bolts on top or is there a way to accommodate the floppy bag cells too?

I'll make the pcb to fit cells with "bolts on top" but really, they could connect to anything. I don't know what terminals the pouch cells have.
Off the shelf converters already have a diode which will stop current flow back from the cell and an un-potted one would be fairly easy to change to whatever voltage your cells demand ie 3.6v for CALB and 4.2v for lipo. The converter needs to be current limited, not "hiccup" mode and easily adjustable for o/p voltage - and efficient! Also, 20watts only gives 4 amps or so and I'd like a lot more - I have designed mine for 9A which is about 32 watts of output - 40 w input at 80% eff which is 1800 watts for 45 cells ie the max load of my cheap 48v converter.
I have done a preliminary design and made a transformer, just need to assemble the rest of the bits and fire it up to see what happens! I'm not too sure how it will work charging a cell rather than driving a resistive load as I haven't been able to find any application notes doing just that, so it comes down to make one and see.
Last edited by Nevilleh on Fri, 16 Dec 2011, 00:44, edited 1 time in total.

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Richo
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Charge Shuffling for Li cells

Post by Richo »

Johny wrote: But if they are potted you can't change the voltage.


Yep that's why I said use a li-po charger chip to handle the CC-CV.
May not suit the larger power systems tho.

At least if a flyback transformer is made the feed back can adjust the voltage and current out in one system.
So the short answer is NO but the long answer is YES.
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Richo
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Charge Shuffling for Li cells

Post by Richo »

Nevilleh wrote: . I don't know what terminals the pouch cells have.


Usually they are just a nikel plated tab.
So a wide rectangle PCB could be made.
The tab could be soldered onto large square pads on each end.

That's how I'd do it if I decide not to use cylindrical cells.
So the short answer is NO but the long answer is YES.
Help prevent road rage - get outta my way!

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Charge Shuffling for Li cells

Post by Nevilleh »

Its hard enough making one for "bolts"! Maybe you can do a board for tab cells if/when I get it going?

How about "conductance is supportive" Image
Last edited by Nevilleh on Fri, 16 Dec 2011, 09:53, edited 1 time in total.

Tritium_James
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Charge Shuffling for Li cells

Post by Tritium_James »

The pouch cells are a mechanical nightmare without spending a LARGE amount of time and money testing. A one-off project is unlikely to get it right, to the point where your pack is reliable for a decent lifetime.

Nearly all the solarcars use them, and they nearly all have problems, with the tabs fatiguing and cracking. It's hard to support everything properly so that vibration etc doesn't stress the tabs.

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Charge Shuffling for Li cells

Post by PlanB »

Jeez you're a party pooper TJ, my motor is a dud my battery choice is no good. Sigh, it's so hard to get all the little duckies to line up. PlanA was lookin' so much better, sob, bring back the Dodge circuit EV.
Hey Nev. My battery idea (pre TJs last post) was a bunch of 3S3P 18650s in parallel to make 12v 60A modules that I can series. I can buy 12v 30A mains power supplies quite cheaply, 1 supply per 5 modules for a 10 hr charge. I was thinking I could loop 240v~ flex around the battery pack to each power supply. How far from removed from a battery charger is a regulated output 12v chop mode supply?

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Charge Shuffling for Li cells

Post by Tritium_James »

Yeah it's a real bummer because the pouch cells are generally fantastic. I think they're best left to the likes of GM/Ford/etc unfortunately...

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Charge Shuffling for Li cells

Post by Nevilleh »

PlanB wrote: Jeez you're a party pooper TJ, my motor is a dud my battery choice is no good. Sigh, it's so hard to get all the little duckies to line up. PlanA was lookin' so much better, sob, bring back the Dodge circuit EV.
Hey Nev. My battery idea (pre TJs last post) was a bunch of 3S3P 18650s in parallel to make 12v 60A modules that I can series. I can buy 12v 30A mains power supplies quite cheaply, 1 supply per 5 modules for a 10 hr charge. I was thinking I could loop 240v~ flex around the battery pack to each power supply. How far from removed from a battery charger is a regulated output 12v chop mode supply?


Hear that faint sound in the background? Its a bunch of violins playing specially for you Image I'll send you a packet of tissues.

First off, if you charge and balance as a 12V pack, you aren't doing individual cells. Prob'ly OK if you balance them individually before assembling the 12V packs, but they can still drift off. Better than no balancing at all! But if you're going to spend all that money, you should do it on a cell by cell basis.
Any SMPS with current limiting works great as a Li battery charger. The charge characteristic requires a constant current (CC) until the voltage reaches the correct level; 3.6 for CALB, about 4.0 seems right for Sky Energy and 4.2 for LiPo, and the supply holds that voltage (CV)until the current drops to a certain level, usually about .1C. You can then consider the thing charged.
Before I had a "proper" charger, I had 5 Meanwell 27V 6A supplies hooked up in series with their outputs screwed up to the max voltage - about 32V each - giving just about 162V which is 3.6 v per cell as required. This setup delivered a bit over 6A as a CC and then 162 V as the CV part. Eventually I found some cells going well over 3.6v while others were still about 3.4 and so not good! Now I have the bms connected and cut off charging as soon as one cell reaches 3.6v.
I also connected 4 Sky Energy cells up as a 12v battery and stuck a 12V charger on them, but found one cell at about 4.3V while the others were still about 3.8V when charging was done. Again, not good.
I really think that if you want to treat your Li cells the best way for long life, you must monitor each cell and balance them properly. Everything I've read indicates that over voltage on charging is a surefire cell destroyer, even more so than taking them down too low when discharging.
This is why I am looking at the idea of individual cell chargers, it seems like a pretty foolproof way to do it.
Also, if you run 240v wiring around your car, you have to follow the regulations for that sort of wiring with proper fuses, markings and all that bs. I heard on the grapevine that Australian regulators are considering making 200V the max allowable voltage in EVs too. Whether that just applies to "home builds" or not, I don't know, because all the commercially built EVs such as Mitsubishi and Toyota all use higher voltages than that.
Anyway, running a 48V bus to power all the chargers avoids all the safety regulations and any "bunny" can do it with only a small chance of blowing themselves up, a very necessary consideration.
Last edited by Nevilleh on Sat, 17 Dec 2011, 02:46, edited 1 time in total.

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PlanB
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Post by PlanB »

Tissues would be good, this EV gig is tough.
I see your point with looking after individual cells with the high dollar value traction batteries. I've dismantled quite a few laptop type battery packs (commodity 18650s), typically 3P3S(11.1v 6Ah) or 2P3S(11.1v 4Ah). All these packs have over & under voltage protection but no balancing. I don't know if the manufacturers choose which cells to parallel when they make up the packs but they seem to get away with 3 in series without balancing.
Re your charge shuffling when the car is running. I get the idea of continually topping up the weaker cells but where will you draw the line? Sooner or later cells will need replacing as the pack ages how do you plan to identify cells that are past nursing & need euthanasia?

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Post by Nevilleh »

PlanB wrote: they seem to get away with 3 in series without balancing.

how do you plan to identify cells that are past nursing & need euthanasia?


Maybe you could get away with 3S modules, I don't know. It would certainly be easier to do a 48 - (3 x 3.6) 10.8 converter rather than a 48 to 3.6 one and you'd only need 1/3 as many. They wouldn't be much larger either. Something worth exploring perhaps and maybe when days stretch out to 32 hours......

The bms will identify cells that need euthanising. It will control all of this stuff and record cell voltages and this is where it may be possible to only charge 3-cell modules, I haven't really worked out what the software might do as yet, it still relies on the operator (me) to look at the numbers its giving out and make -hopefully - intelligent decisions.
Basically, the bms identifies a low cell and switches on its charger and this can happen at anytime, or be programmed to only happen when not driving, I don't know what is best. Prob'ly when not driving, I would think. It would not be hard to add another serial port to my controller and have it output a data log to a laptop so you could review what's been happening.
Any cell that is consistently low and needing lots of balancing current would be ready for replacement.
I haven't thought through all the ramifications of this at all and if anyone has any useful thoughts, let's hear 'em!

And yes, this EV gig is tough - else everyone would be doing it
Image
Last edited by Nevilleh on Sun, 18 Dec 2011, 01:29, edited 1 time in total.

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