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Battery pack question (18650 cell related)


alcatraz
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Hey fellow euc riders...

So I got a 2y old pack with 40pcs LG MH1 cells. It's starting to show signs of degradation.

I will be transferring the bms to a new pack. My problem is to decide what cells to use.

I'm stuck between choosing the same kind which is very cheap or a newer kind that has maybe 5-10% more capacity and invest 2-3 times the price.

Because I ride a lot, I do anticipate I will have to do this pack rebuild again in 2y. 

For this reason I'm thinking that instead of having a bunch of old useless cells why not reuse the best of them. Lets say 30-50% of them. I mean the ones with lowest internal impedance.

Does this make any sense or would you just recommend to discard them all and get a whole set of new cells each time?

(What I didn't mention is that I will rebuild the new pack into a 60 cell battery so I'm not that worried about range. I just don't want to discard 60 cells every 2 years.)

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I would not mix old and new cells. You risk overly draining the weak cells to dangerous levels as the newer ones keep the overall voltage high enough that the wheel keeps rolling. Cells in a pack should be as closely matched as possible. 

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22 minutes ago, WARPed1701D said:

I would not mix old and new cells. You risk overly draining the weak cells to dangerous levels as the newer ones keep the overall voltage high enough that the wheel keeps rolling. Cells in a pack should be as closely matched as possible. 

Some further reading for @alcatraz http://batteryuniversity.com/learn/article/bu_803a_cell_mismatch_balancing

You could also decide to make wires to all cells available to charge them directly (everytime 100% balanced!) And just rely on the bms for overcurrent and undervoltage protection...

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Thank you for your replies.

Because MH1 cells are not made anymore I have the opportunity to get some used cells at a good deal. They have under 35mOhm impedance and guaranteed over 3000mAh capacity.

My idea is to match the cells as closely as possible. Both from internal resistance and capacity in each serial group of cells. There are affordable 18650 chargers that can measure resistance and capacity. With this info it should be possible to produce 20 similar groups of 3 cells.

I'd like you to help me confirm my assumption here that if all these 20 serial groups of 3 cells in parallel have the same group resistance and group capacity the pack would be balanced. 

Then when the chargers 84V is put on the chain of cells their equal resistance should make them charge at a similar rate.

What would happen if a 2800mAh battery is connected in parallel with a 3000mAh. Wouldn't the equal voltage automatically take care of their mismatch?

Group resistance and capacity is then the key. Lets say two sample groups are:

2800+2800+3000 with resistance of 0.01Ohm

and

2900+2900+2900 with resistance of 0.01Ohm.

Each group has same total resistance and capacity.

/a

Edited by alcatraz
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2 hours ago, Chriull said:

Some further reading for @alcatraz http://batteryuniversity.com/learn/article/bu_803a_cell_mismatch_balancing

You could also decide to make wires to all cells available to charge them directly (everytime 100% balanced!) And just rely on the bms for overcurrent and undervoltage protection...

That is a cool idea to use two 10s hobby chargers.

Bit noisy and messy though.

Because it's an inmotion bms and that the board is quite huge, I have a feeling of confidence that it can supply a decent balancing current.

I like the idea of charging the cells with the original charger so that the BMS balancing can keep up with the charge rate. They are most likely designed to work together. Besides I don't need quick charging.

Also by using the same cell type and going 3p instead of 2p I effectively reduce the discharge and charge rate with 50% which increases the cells capacity. LG makes the best cells for high current. Just don't wanna pay premium for the latest new cells.

Edited by alcatraz
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