Parallel battery question - charging and styles

Stucco

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So I just ordered 100 lbs of 18650 cells from Battery Hookup. Looking to create a 21s~p battery to match my 21s flat pack battery I build for my electric cycle.

I was thinking of wiring these in parallel since the voltage would be the same. But if they have different AH should I instead look at some type of switch mechanism. Where I drain one out, then switch to the other one.

If it is ok to have them in live parallel, can I still charge them in parallel? Or should I do independent charging for each pack.

They will have separate dedicated JBD BMS if that helps.
 

sunshine_eggo

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I was thinking of wiring these in parallel since the voltage would be the same. But if they have different AH should I instead look at some type of switch mechanism. Where I drain one out, then switch to the other one.

The concern with different Ah is if they support their "fair share" of the current. If you can wire them such that they are passing an appropriate current, then fine. If not, then the next question is the disparity still within the cell specs? If so, then it should also be fine, but the cells supporting the disproportionately high current will degrade faster.

If it is ok to have them in live parallel, can I still charge them in parallel? Or should I do independent charging for each pack.

Since charging is likely at a much lower current than discharging, parallel should be fine.

They will have separate dedicated JBD BMS if that helps.

(y)
 

Stucco

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The concern with different Ah is if they support their "fair share" of the current. If you can wire them such that they are passing an appropriate current, then fine. If not, then the next question is the disparity still within the cell specs? If so, then it should also be fine, but the cells supporting the disproportionately high current will degrade faster.



Since charging is likely at a much lower current than discharging, parallel should be fine.



(y)
Any good method you can think of for tracking AH for cells like this? Or is it just, hook up a load, run it to "empty" then math?
 

Stucco

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Oh, yeah, I definitely will be testing each cell. This will be a several month build I feel. ;) Again, looking at each pack as a whole.
 

Diysolar123

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I was thinking of wiring these in parallel since the voltage would be the same. But if they have different AH should I instead look at some type of switch mechanism. Where I drain one out, then switch to the other one.
when it comes to batteries of the same chemistry, when you put them in PARALLEL you will get the sum of their AH ratings NOT the sum of the output current.
for example:
2Ah battery in parallel with a 1AH battery will provide 3AH of "power" as they will both be at the same voltage (they are in parallel after all)

keep in mind that the max current you can draw is NOT the sum of the two!

what will happen is that as one pack can no longer provide a share of the current, all the outgoing current is coming from the battery that still has capacity; you basically have the current capacity of just one pack. You cannot predict exactly when that will happen so you should not use this technique to try and pull double outgoing current, just to increase the "capacity" of your system.

also keep in mind you do not want to let such as unbalanced system ever get close to causing a BMS to "disconnect" due to an internal cell voltage hitting a tripping point. You could get into the situation where:
Pack A is the "good" 2AH pack
Pack B is the "weak" 1Ah pack
each pack is rated to provide 1C worth of discharge current.
these are each 21S packs so the low voltage discharge danger point is around 2.8V * 21s = 58.8V

you have both packs fully charged and with the same BMS settings (keep in mind BMS settings are not calibrated to 5 decimal places ;-) )
You have set the low voltage CELL disconnect at 2.8V/cell which is well into the steep voltage discharge curve so voltages change a lot and quickly.

you begin draining at 1C; each pack is sharing some of the load for some unknown amount of time (it will NOT be equal but it will be shared).
now the fun part...
as you start to drop below 3V/cell you will suddenly see a rapid voltage change in all the cells in the packs as their individual voltages start to vary more and more. The pack voltages will be the same as they are in parallel, but the individual cell voltages are watched by the BMS and will be driven by the single FIRST cell to hit the disconnect voltage which will result in the BMS suddenly disconnecting.

so now you have a single battery pack providing all the outgoing current, and the other battery pack has disconnected itself.
The tricky part comes into play when you start charging the system backup.
depending on the BMS, you start to charge the system back up, but one of the BMS's could still be disconnected and when it reconnects you could have a significant voltage difference suddenly between the two packs...and that means the higher pack is suddenly going to move a LOT of current to the lower voltage pack when the lower pack BMS decides to "reconnect"..this can cause a BMS short fault depending on how much current suddenly moves.

You can somewhat avoid this by setting your minimum discharge voltages well above the BMS disconnect voltage so you will not get anywhere near this situation. Depending on the BMS you may be able to set it up so that in the event of a disconnect event it will NOT reconnect automatically so you can manually intervene and make sure that the pack voltages are the same before re-enabling the charge fets.
 
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