charging parallel Lipo packs

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miro13car
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charging parallel Lipo packs

Hi!

This question would be especially to Xyster who ride his bike with large Lipo battery, or others , please say your opinion.
I want to buld 10s2p pack out of E-moli Lipo cells.
I want to tap every cells and use balancer with charging.
I don`t know if I heard correct , but is it better to charge each string seperately?
I want to use switch mode charger which would recognise end of charge 42V and shut down .
Xyster, do charge each string separately?
Any opinions?

Miro

NickF23
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Re: charging parallel Lipo packs

HI there miro13car,

I'm guessing that your making packs from individual cells. The info below does not apply if your using milwaukee drillpacks as separate units with the inbuilt bms.

li-ion batteries should be charged indivually in a series string. You can have 2 or more cells in parallel with one charger, but you mustn't charge 2 or more cells in series without a battery management system that insures that no cell can go above a certain voltage.

here's some basic info here

http://www.mpoweruk.com/lithiumS.htm

You should be able to either buy an lithium charger from a radio control model stores - these are expensive or like xyster buy many individual chargers.

miro13car
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Re: charging parallel Lipo packs

Thank you Nick F23.
Thank you very much for answer.

Yes, I want to buid from separate E-moli cells.
I got your point.

When you have string of 10 cells connected to Lipo charger you a have no way of saying what are individual voltages at the end of charge - that`s without balancer.
I want to use Tunder Power Lipo programable charger and Tunder Power balancer combo. They communicate, talk to each other when string is being charged and balancer keep an eye on each cell voltages, Because it is tapped to every cell. IT tells charger to disconnect when cell reach 3 V.
Balancer can be used for balancing without charging of course.
Yes,
BMS checks every cell during charging like balancer.
So I look at charger, balancer and BMS.
Over discharge is unlikely on my 500W limited motor in Turbo mode, I use Normal 250W most of the time. So overcurrent is no concern.
BaterrySpace sells BMS for 37V Lipo packs, 27A limit, but with LV cutout as low 2.5V/cell , I don`t know why so low. I guess it can be modified to 3V by changing precision resistors but I am not sure.
It is commonly known that LV cut out for E-moli should be 3V.
TF balancer I want to use monitors individual voltages and have beep warning when any cell reach 3V. I am not sure if I can use it during ride and can I HEAR beep??

So I look at charger, balancer and BMS with 2.5V cut out.
Yeah
it gets so complicated if you cannot have decent capacity cell to build only series pack.
That you have to paralell cells.
3ah is not enough for E-bike application. Can I trust Kokam or others if there are cobalt based? Reading horror stories of houses and cars of RC people burnt down with regular Li-Poly. This not the case at all with A123 and E-moli cells. That`s why theay are so attractive.
So now I guess when I look at this with 2 strings of 10 pack you cannot charge this with one charger+balancer at hte same time.
Bottom line is you must monitor every cell during charging for voltage so you need 2 balancers at the time and two chargers, because 2 balancers cannot connect to one only charger. On the top of this you need BMS described above for each string separetely.
That is crazy:
2 chargers
2 balancers
2 BMS
to buy just for 6Ah pack to get close to 1 hour charging time.
Am I correct , or I am not correct??
Well , I can use Watss Up meter for low voltage pack monitoring and depower manually.
What do you think?
OR use double time charging strings individually. All advantaqe of E-moli fast charging is not so attcactive any more.
Anybody , any thoughts I would really appreciated.
Miro

CM

NickF23
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Re: charging parallel Lipo packs

I've not actually ever seen an emoli cell, my knowledged is only based on what i've read so do crosscheck it elsewhere.

I think your make everything way too complicated. the good thing about emoli cells is that they aren't damaged by deep discharge as with normal lithium cobalt. This makes the bms much easier. All you need is either 10 individual chargers like xyster uses or a fancy charger that monitors each cell voltage at cutsoff when the cell is charged. Its ok to charge them in parallel, so you only need 10 chargers or a 1 fancy charger that charges a 10 series string monitoring individual voltages.

basically something like this, though it looks a bit pricey

http://www.tmenet.com/xtrema.htm

In otherwords a balancing charger.

I think a lot of "balancers" that are sold for RC are discharge balancers, meaning they keep the cells balanced as they discharge. This is nice if you have a plane or a car as it might give you an extra few seconds to win a race but not really necessary for an ebike. It can be useful to check your cell voltages once in a while to make sure one cell isn't weak but this is the same as for lead acid or nimh/nicad etc.

miro13car
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Re: charging parallel Lipo packs

Thank you for answer, Nick.
Xtrema charger you gave me link to doesn`t have balancing capabilities, just looking at the picture it cannot connect to each cell individually=cannot monitor each of 10 cells.
Sure going out of balance doesn`t happen right away from what I read, especially that E-molis
will have very easy life with my motor at continous 14-15A only.
But the main function of TunderF balancer I mentioned is to work in tandem with TunderF charger informing him about voltage of cells so charger can terminate.
Obviously you can charge only with charger and forget balancing , check manually voltage of every cell can you imagine makeshift of connect/disconnect wires.
I want to carry the pack with charger., I don`t have to carry balancer , but it is must if you want cells to last.
Miro

CM

Fechter
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Re: charging parallel Lipo packs

I think you can run cells parallel as long as they are connected at every cell. This would require taking apart the packs and re configuring the cells. Make a string of sub-packs where each sub-pack is a number of cells in parallel. Each sub-pack will have the same voltage since the cells are in parallel.
You could have pairs (or more) of parallel cells strung together in a single string and use a single balancer BMS and charger.

NickF23
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Re: charging parallel Lipo packs

Miro,

My mistake by linking that charger. Yeah your right if your going to use a standard lithium pack voltage charger you need a balancer to the charger will know what to do. But you'd only need one. The other option is use 10 cheap battery space individual chargers, like xyxster. check that emoli cells have the same final cutoff voltage as lithium cobalt. I think they do but maybe not.

miro13car
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Re: charging parallel Lipo packs

Thank you Fetcher and Nick.

I would have to think everything over.
But I almost decided to charge each string separetely. Why? Charging 2 strings of 10 cells together or separetely is going to take about the same time! TForce charger can feed max 5 A for single 10 cells string according to specs , w 2 strings parellel /good idea or not, safe or not/ is going to feed 2.5 A into each string according to elec. laws. , that`s about double time to charge.

All I know is that I must try to mach cells as close as possible or at least to check if they are matched even before building any pack. I need 20 cells.
Have to make a search of this forum, or does anybody know how to exactely mach Lithium cells?
Any help appreciated
Miro

CM

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