Chinese pack + new cells

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careca   10 mW

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Chinese pack + new cells

Post by careca » Aug 16 2019 5:38pm

Hi, i've been reading about mixing cells in a pack and i understand that the series string cannot vary in voltage, for example.
But im confused about this particular problem.
My idea was to add 1 or 2 parallels to my current pack, which is a 20S7P with chinese generic 2.6Ah cells.
These cells have a very low discharge rate and i need more discharge rate, so i thought about the possibility of adding 2.6Ah cells like the samsung 25r's.
What do you think about this?
Thanks.

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MadRhino   100 GW

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Re: Chinese pack + new cells

Post by MadRhino » Aug 16 2019 7:09pm

This will not improve your discharge rate, only the max power availability ( by 1/7 if you add 1 p)

The limit of a chain is its weakest link.
Make it fool-proof, and I will make a better fool.
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markz   100 GW

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Re: Chinese pack + new cells

Post by markz » Aug 16 2019 7:20pm

When connecting battery series strings into parallel, the voltages have to be within 0.10V or lower, otherwise you will have a massive current drain from one cell to another. You will have to snip off each series string ( say you got 10S10P, get each down to 10S1P)... then add your additional 18650 can(s) to each series string, then be absolutely sure that each string are all withing 0.10V of each other, then start connecting the parallel wire to make your 10S10P.

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careca   10 mW

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Re: Chinese pack + new cells

Post by careca » Aug 17 2019 12:50pm

I see, they will try to equalize the voltages when connected in parallel.
But would it work safely, despite the discharge rate difference?
And im struggling with trying to understand the @MadRhino's remark about the weakest link in a chain.
From my understanding of it, since i would connect another parallel, that string would not be limited by the weakest link, would it?
I mean, if i made a 20S1P with just these new cells, i would have a discharge rate equivalent to 20A, how does adding weaker cells hinder the performance of these stronger cells? If you could make it clear i would appreciate.
Best Regards.

markz   100 GW

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Re: Chinese pack + new cells

Post by markz » Aug 17 2019 5:38pm

Well if you have a 1.5Ah 18650 can, in with the rest being 2.5Ah then the 1.5Ah will be drained and fail.
Same goes for if you have a 2.5Ah at 25A and you have another different 2.5Ah but its 8A discharge, and you are asking each 18650 can do do 15A or 20A, then that 2.5Ah @ 8A discharge will fail too.

The weakest link will fail.
And you will be walking home.

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careca   10 mW

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Re: Chinese pack + new cells

Post by careca » Aug 17 2019 5:58pm

Ok, thanks, one last question if i may, can i mix:
https://www.imrbatteries.com/sanyo-ur18 ... p-battery/
with
https://www.imrbatteries.com/samsung-25 ... e-battery/

The issue was i ordered 30 sanyo, and now they're discontinued, the seller went out of stock, so,
in order to build the battery i intend, 20S7P, im thinking about mixing those with the 25R which is available.

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MadRhino   100 GW

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Re: Chinese pack + new cells

Post by MadRhino » Aug 17 2019 6:15pm

Yes of course you can. The cells are close enough, same chemistry, same C rate.

The only downside is the 2600 mah cells will only be used at 2500 mah to match... the weakest link.
Make it fool-proof, and I will make a better fool.
Current bikes
Street: https://s20.postimg.org/ewrvugywt/Session_04_2015.jpg
Dirt: https://s20.postimg.org/lbqwr55ml/IMG_0157.jpg

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careca   10 mW

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Re: Chinese pack + new cells

Post by careca » Aug 17 2019 6:38pm

Alright, many thanks! BTW your sig links are dead. ;)

Tommm   10 kW

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Re: Chinese pack + new cells

Post by Tommm » Aug 17 2019 6:57pm

Terrible idea to mix and match cells in the same pack, even if the actual stats (not the advertised ones) match, they will still age differently. If you can't afford it to go out in a blaze, just don't do it.
Make another pack with the same voltage (doesn't have to be the same capacity), and parallel them. It has been done many times here, mostly feasable if you don't use regen.

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careca   10 mW

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Re: Chinese pack + new cells

Post by careca » Aug 18 2019 2:06am

So...packs in parallel are ok, even if with different capacities?
Wouldn't one charge the other in reverse?
About the regen, what is the issue with it?

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MadRhino   100 GW

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Re: Chinese pack + new cells

Post by MadRhino » Aug 18 2019 2:36am

This answer was about your original post. Adding your new, better cells to your Chinese pack is meaningless. Better building another pack with new cells and use the 2 packs on parallel. Then, when your Chinese cells start to fail, you will still have the good cells together and working.
Make it fool-proof, and I will make a better fool.
Current bikes
Street: https://s20.postimg.org/ewrvugywt/Session_04_2015.jpg
Dirt: https://s20.postimg.org/lbqwr55ml/IMG_0157.jpg

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careca   10 mW

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Re: Chinese pack + new cells

Post by careca » Aug 18 2019 2:43am

What do i have to make sure, when connecting two packs, each with it's own bms?
Im thinking about charging too, do i charge them isolated or in parallel too?
Also, i assume the discharge current adds up as it does with any parallel setup, right?

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MadRhino   100 GW

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Re: Chinese pack + new cells

Post by MadRhino » Aug 18 2019 7:10am

You got it right. Charge them together, at a rate that suits your cheap battery. Discharge rate too, should not exceed the C rate of your cheap battery.

You see where I am going? Best is to build one battery, using only high C rate cells of the same capacity. I would ditch the cheap Chinese battery, to use on another bike with lower performance/range expectations.
Make it fool-proof, and I will make a better fool.
Current bikes
Street: https://s20.postimg.org/ewrvugywt/Session_04_2015.jpg
Dirt: https://s20.postimg.org/lbqwr55ml/IMG_0157.jpg

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careca   10 mW

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Re: Chinese pack + new cells

Post by careca » Aug 18 2019 8:38am

I see, there's just no way to increase C rate besides a totally new battery.
Gonna put 30 of those sanyo's and 130 25R's.
Thanks.

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