200 cell balancer?! help!

[FairlyFaded]

Hey guys! Im thinking about putting together a 200 cell 18650 lithium ion pack for a small ev, and need some way to balance it. One way I can think of is to simply split the pack into 10 or 12 parts and balance each sub pack with say, a few of these http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=5653&Product_Name=Smart_Balancer_2-5_Cells.
every 10 charges or so. Do you guys have any better ideas? Maybe i could make a large balancer capable of doing more like half of my pack in one go? Any help would be great!

Michael

[mjsas]

You must have a lot of money. The Toyota Prius has a way of balancing the battery pack so you could start there. They sell used from $250 and up.

[lutach]

I'll be watching this.

[BushmanLA]

Hey guys! Im thinking about putting together a 200 cell 18650 lithium ion pack




Yeeesh!

I would defiantly go the route of breaking it down into sub groups.

Whats the final configuration of the cells?

Does anyone know what the standard method of cell balancing is? Are they just sampling the voltage with an AD and then opening up some fets to let the high voltage packs be connected to the lower voltage ones?

[AndyKunz]

Talk to your local TI rep. You want information on the HCE PowerLAN devices.

Analog has comparable stuff, but it's under NDA right now. You can get a copy (I did) but you have to jump thru a few hoops. Best to talk with their local rep.

Andy

[FairlyFaded]

Thanks for the replies guys!

First, about the prius, im hoping to be able to balance this pack for under 250$, if that doesn't work out ill give it a shot!

BushmanLA, I think going with sub groups would be the cheapest method. I am going to go with a 12 cell series 17 cell parallel pack. That will give me 36-50.4 V pack, with 37.4 AH. That would make a 1660 watt hour pack aprox, good for around 5kw (at 3c, no biggy). Thats around 6hp... Yummm...

Now, tell me if this would work.... Could I split the pack up into 12 seperate chunks, each chunk wired in parallel -- each parallel chunk would then be charged at 4.2v every so often to bring them all into balance? Would that work? I would think it would as any straggler cells would catch up with the rest of the cells if they were all getting the same voltage. That way i could easily balance 17 cells at a time, with something as simple as a 4.2v cc/cv power supply! Please say that it would work! :P

Andy: I'm going to be honest, i dont really know a word of what you just said. TI rep? HCE PowerLAN? NDA? Im sorry for my ignorance! would you care to explain? Thanks again for all your time! Let the ideas keep flowing!

Michael

p.s. 204 cells X 2.60 = $530, a good bit of money, but compared to a car, I don't think its too outlandish : )

[vintage1]

I had a long discussion with a friend of mine on how to do this sort of thing.

WE decided that parelleling cells to get the amps, and balancing that block, and then putting them in series was the basic way to build the pack.

We also decided that every block should have a charge and discharge circuit bolted on to it. And a serial link to a master CPU so the whole pack could be monitored to spot 'bad cell blocks'

[BushmanLA]

TI is Texas Instruments.

If you get in touch with them apparently they have a line of products called powerLAN that can help you with this this project.

NDA means Non Disclosure Agreement. It is an agreement, in this case between the buy and seller, that the buyer will not tell anyone about the specs of the device etc. This is done for some parts because the seller doesn't want the competition to have an easier time making competing products I guess.

[Almaz]

I can't help you with your problem but why can't you just use 4 motorcycle batteries 46v. Easy to charge and balance and cost 100 times less. You'll get the same power but just a little heavier. I'd love to see some pictures of your monster project.

[Zlatko]

Hi FF,

A while back I got some cheap 18650 cells rated at 2400mA ... after I tested them, they were only 1100mA ( disharged at 1C ... theoretically 2.4Ah ... they couldn't go above 1.3Ah on the discharge ) ... got my money back.
I hope that you have tested your 18650 cells that they actually provide you with the expected 2.2A capacity and 6.6Ah discharge rate.... to avoid disapointment....

Cheers

EDIT...
I've done some reseach on the EV stuff and realistically you still can't go past the Trojan T105s for best bang for buck ...
But, if you only want about 1.7kWh from 48V get 4 x 12V 60Ah+ batteries
if you drive them at 2-3C discharge you will only get about 50% capacity ( hence the 60Ah+ ) ... something like the Trojan 24TMH ( 12V 85Ah ) or the Crown 24DC36.
Naturally ... if weight is not a problem ....

[FairlyFaded]

Vintage: How did you decide to go about balancing the parallel blocks? Could you do it as I said by charging it up at 4.2v? Or would you have to use a balancer of some sort?

Bushman: Thanks a ton for the clarification. I have done a little research into the TI stuff, it looks like I should try and build my own balancer system before going that route because of cost.

Almaz: I too was under the impression that SLA batteries would be only a bit heavier, but way less money. This is however, not the case. They are about 5x heavier, which for my project makes a pretty huge difference. This pack is going into a Super Pocket bike (a large pocket bike that is street legal). The all up weight of the bike should be about 60 lbs and a 130 lb rider. In order to get the 1.6kwh's from SLA id be looking at a 100 lb pack, instead of a 20 lb one. As far as cost goes, a good SLA pack is at least 50-60$ a piece, now I need 4, thats allready getting to be over half the cost of the lithium ion one.

Zlatko: I am buying the cells directly from the company, it sounds like yours where either miss labeled (they could have been the 1100-1300 mah high discharge form of 18650s) Or very old. You point out the big flaw in SLA batteries and that is that they loose so much capacity when discharged. a 4x60ah pack would be far to heavy for what i need : / .

[Zlatko]

FF,

I'll be interested in "some" genuine 2.2Ah 3C discharge 18650s at $2.60 each :).

Can you share the details of the company?

Cheers

[David T]

If you are looking for ideas still, you could also do 'per cell' charging instead of 'balancing' to level them off. This would require any number of individual chargers daisy-chained together. Each charger would need to have its own mains transformer to provide proper isolation. Each simply needs to be set to 4.2v and have appropriate current limiting (eg: using L200's). You can then charge them in batches depending on how many individual chargers you make.
David.

[AndyKunz]

Bushman: Thanks a ton for the clarification. I have done a little research into the TI stuff, it looks like I should try and build my own balancer system before going that route because of cost.

I guess I missed something - the TI PowerLAN is going to be inexpensive by the time you get 200 cells hooked up.

The other option might be the LTC6802 family.

Andy

[vintage1]

If you are looking for ideas still, you could also do 'per cell' charging instead of 'balancing' to level them off. This would require any number of individual chargers daisy-chained together. Each charger would need to have its own mains transformer to provide proper isolation. Each simply needs to be set to 4.2v and have appropriate current limiting (eg: using L200's). You can then charge them in batches depending on how many individual chargers you make.
David.

This is how I envisaged it. You split your automotive battery into cell packs. Each one has its won charger fed fom wahetevr power sopurce it can get.

I envisaged (UK/Europe) having a single phase (240V) supply and possibly a three phase (440v) , and possibly just a DC..the SMPS chargers would autodetect and run at a rate appropiate. Each charger charges its cell block.

In the UK it might be appropriate fr example to limit draw to 3KW on a single phase, but allow very rapid charging off 3 phase..up to maybe 50KW or more.

If a cell block becomes bad, some form of solid state relay switches it out of circuit and replaces it with a short..so you lose its voltage in teh final pack.

A serial bus monitors all this.

The electronics is simply replicated - say - 200 times. In practice you would probably make up blocks of 3s packs or something, and hope they stayed in balance withing themselves: the cost/benefit is between replacing more cells than you need to and spending a load on extra electronics.

You need some sort of system that utterly guarantees the moment the chargers start, each cell is isolated from the drive ESC's etc...and e.g. an interlock on the charging socket so that you cant drive the vehicle away still plugged in to something!

But overall I think a modular approach is indicated. Each cell block should be voltage and current limited on charge, and undervoltage and overcurrent and oer temeparture limited on discharge. i.e. as you get near to a flat battery, or a super hot battery the cells start either flipping themselves out of circuit, (last resort) or sending a signal to the drive ESC's saying 'that's all I can supply mate!' The latter being a 'limp to next charging station' sort of mode.

So, you end up with a modular pack, three phase and single phase charge power on a bus, and a serial link talking to all of the cells.

It sends signals like 'I am getting low on voltage' 'I am too hot' I am pulling too much current' etc..allowing things like fans to be switched on, and acceleration to be down graded..but in extremis the block's themselves should simply shut themselves down to protect themselves.

In the event of a major crash (> 2g say) the cells would have accelerometers that would need a manual reset. Those would switch the cells out of circuit completely. That means at least you would be dealing with 200 3 v cells and no voltage on the final terminals. And no chance of exploding batteries due to wires being shorted outside of the pack assembly.

[AndyOne]

A while ago I built some very large li-ion batteries and we did indeed put a balancer on each of the series element in the pack all 70 of them. This is the only way of ensuring all the cells stay in balance. There was also an optically isolated signalling system back to a master controller that had a massive MOSFET switch arrangement capable of handling 40A at 300V, it was really just a scale-up of what you get in a cam-corder pack.

Andy.

[tvdude310]

...has anyone asked someone from Tesla Motors how they did it?