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Expanding from 4s to 7s... do I rebuild all packs?
#1
I have been using my 4s setup for awhile. I am ready to go to a 7s.

So my question, should I tear apart my existing packs, retest all the cells, add them in with my other cells and build 7 new?

Or would it be ok to add 3 more packs?

I have a bms so I am guessing adding 3 more isn’t a terrible idea, but I’m guessing not ideal.

Would like some feedback before I proceed either way lol.

Thanks!
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#2
If the 4s has been performing just fine, adding 3s should be fine. Just make sure the new packs have the same capacity as the existing packs, or at least within 100-200mAh of the others.
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#3
(07-08-2019, 12:22 AM)Korishan Wrote: If the 4s has been performing just fine, adding 3s should be fine. Just make sure the new packs have the same capacity as the existing packs, or at least within 100-200mAh of the others.

When i built the first packs I was shooting for around 160 amp hours each.  Was thinking of using that same baseline or close for the next packs.
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#4
it'd be good to double check each packs capacity to verify their integrity. No need to pull the packs apart, just connect to each Pos/Neg of each parallel segment and charge/discharge-test. Then you'll know what you need to aim for in the added packs
Proceed with caution. Knowledge is Power! Literally! Cool 
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#5
(07-08-2019, 12:43 AM)Korishan Wrote: it'd be good to double check each packs capacity to verify their integrity. No need to pull the packs apart, just connect to each Pos/Neg of each parallel segment and charge/discharge-test. Then you'll know what you need to aim for in the added packs

any recommendation on doing a big pack test?  i have testers for individual cells.  it would take DAYS to do that with a pack lol.
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#6
Create a resistive load, old toaster element, hair dryer element, you could even take some stripped 12AWG copper wire and coil it until you get 1-Ohm. Or you could go higher on the Ohms until you get something like 10A or such. Use a clamp meter or a shunt to measure the amps. Watch the voltage drop and keep track of the amps.

1A discharge over 1H = 1Ah (1000mAh)
If discharging at 10A
10A discharge over 1H = 10Ah (10000mAh)
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Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
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#7
i have some power resistors i could probably use in series to do that.  Ill give that a go!  Thanks!
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#8
I think i can use 3 x 1ohm 100watt resistors with a heatsink/fan.  

Then just do some voltage monitoring with the an arduino just like my other testers.  just need to compensate for that much higher amperage for cooling, wires and connectors.  

Ill post an update when I have something in place.
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#9
I had a similar issue going from 7s to 14s. I made the 7 new packs with same technique (similar ah) and then used Batrium BMS to discover packs a bit weak. For those weak packs, I added a few more cells and after a few rounds of this - had a balanced battery. Of course, this technique requires Batrium BMS and pack design that supports more or less cells by a few.
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#10
That could work too!  my packs are all 1s80p.  no reason they couldn't be adjusted to 1s85p if needed.  Ill keep that in mind.
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