How to connect big lithium packs for the first time with different SOC

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bashers

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Oct 1, 2021
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16
I've got 6 lithium packs from a BMW i3 that i wish to integrate in parallel.
The thing is they have been used individually before i purchased them so they are all of different SOC.
How should i safely get them balanced so they are all the same SOC before connecting them to the same bus bar for parallel use?
 

Redpacket

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Feb 28, 2018
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Because they are larger capacity packs, more care is needed to prevent high current flow.
You should have a DC clamp meter to check what's happening here.
Charge lower voltage packs up (or discharge down higher cells) so they are within about 100mV (ie 0.1V) of each other.
Initially connect using only medium thickness cables (eg maybe 14AWG/2sqmm) not thick ones to help limit possible higher flows.
Use DC clamp meter to check current flow, should only be a few amps
Leave connected for a while, eg approx 3-4 hours to let packs further equalize charge states.
Then fit final heavy duty links/wiring, etc for parallel use.

If you're planning to use in series later, you should capacity check each one separately & check they are reasonably close.
 

bashers

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Oct 1, 2021
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They are from two sources, 5 are at almost 100%, the others at 65%
Potentially that's 5kw that will want to flow from the full ones to the partially charged ones!

This is the most worrying part of the whole install so far
I need to do some calculation and see what cable i need to limit the flow to 25amps or so.
I think i may try to find a 48v immersion heater and use that to balance these batteries
 

floydR

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Aug 23, 2017
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Being BMW i3 packs I assume they are all the same pack layout and you have access to the balance leads if packs are 12s get a icharger x12 and do a regenerative discharge of the high packs into the low packs. this way you only lose the power that is needed by the X12 at least theoretically.
Later floyd
 

Oberfail

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Jun 22, 2021
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You could connect them parallel, with a resistor / low voltage light bulb in between them.
That would slowly balance them out, while some electricity gets turned into heat / light.
 

bashers

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Oct 1, 2021
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Thanks guys. That X12 looks great, but this build is already get expensive in parts and tools alone and i want to spend as little as possible to balnce these one time, let alone £170 for the X12
I've opted to buy a 48v immersion for £20 so that will do the job, albeit slowly and gently at 6ah
 

Korishan

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Jan 7, 2017
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As Oberfail mentions, just using some light bulbs would work, and they cost a lot less. It wouldn't matter if they were DC bulbs, or incandescent AC house bulbs (not led or fluorescent). Not only would you be able to tell that they are balancing, but you could easily tell at a glance when they are done as the bulbs will go out.
Could even go with an automotive headlight bulb too, since they are designed for high current application.
 

Charly144

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Nov 22, 2017
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287
Why not fully charge each individual pack first and then connect them…
 

AngeleToR

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Nov 28, 2021
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What I did when I connected my batteries was (supposing you have individual protection/fuses for each battery pack as I do):
  1. Connected the most discharged batteries to the solar charger, all of them with less than 0,1V difference
  2. I attached labels to each battery, with its measured voltage
  3. Using the SAME voltmeter, connected it to the main DC bus to check the batteries voltage slowly raise
  4. As the voltage raises and reaches the one written in the labels, I only had to connect the fuse for every battery until all of them were connected
I used fuses as I still had no BMS setup for every battery. If you already have your BMS setup, you can program it to take care of the task and limit the current flowing in and out of the batteries.

That worked pretty fine for me, with no energy lost in any point. When all the batteries where connected and balanced, than I activated the inverter and started to discharge them. That was like 6 months ago and everything's working smooth like silk yet ;-)
 
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