LIPO CELLS BALANCING DOUBTS

[topgun78]

Good afternoon,
I have been a happy owner of a DIY e-bike since 2015.
My batteries are 12X 24v5000mah Turnigy and they are configured in 3sX4P (72v - 20 Ah ). Also Balance leads are connected accordingly.
They have always been balance charged with relatively low currents ( 5 to 10A) to 4.1V and never discharged below 3.6V seldom below 3.7. When I recharge them I put all 12 packs in parallel and recharge at 24V.
Now after 3 years I noticed that one of the cells was repeatedly going out of balance falling quite low compared to others.
So I took the packs apart and I noticed that 2 packs were puffed and several cells in these 2 packs had higher internal resistance compared to the other packs.
Now here's the questions that I would like to submit to the experts:

1- If in a group of packs ( 4 in my case ) one pack goes bad (again as in my case ) is it a possibility that the bad pack will deteriorate also the good ones? I ask you this because when also balance leads are paralleled there is little way for me to tell if one single pack is bad .Measuring from the balance lead of the group of four I will only be able to read the average voltage of the group of 4 but no of the single one. And this leads me to the next question

2- Ideally when using the bike the cells do not need to be in parallel but it would be much more beneficial to have them separate so that I could monitor each one of them and cut the power when the lowest one falls at a predeterminate voltage (like 3.6 x es.) Does anyone know if there is a way to monitor balance lead of each single pack ?. I was also thinking that inexpensive arduinos properly configured could be used to have this job done.

3 - What do you guys suggest to do with puffed lipos? Toss them? Probably I could reassemble them and use one of them for lighting system of the bike.

4- I recently bought some new LIPO packs same capacity different brand ( Multistar) with also different discharge rate ( 10c instead of 20c) . Do you think is ok to mix different packs? My idea would be to configure it 3sX7P to reach 72v 35Ah. What is the best strategy to put the battery packs toghether?
Cheers

Alberto.

 

[flippy]

1: yes.
2: such a thing already exists, it's called a BMS.
3: toss them immediately.
4: dont mix different cells.

 

[Chalo]

You can put a parallel string of different cells alongside your existing battery, as long as the nominal voltage is the same and the actual voltage is the same at the time you join them together. Whichever batteries are stronger under load will serve proportionally more of the current demand. As long as they're kept in parallel, they'll support each other's voltage.

 

[topgun78]

Thanks to both of you for the reply. For quest.number 4 I'll go for Chalo answer + explanation. It makes more sense for me than just a generic "Do not do that". My idea is that even if Multistar are not the best lipo to release power ( average IR is around 6-7 micro ohms per cell ) even a mild 2C in a 35A pack can deliver 70A flawlessy without even thinking of getting hot. Of course I will make sure that all voltages are at the same level and I have already started to park the packs to storage level that for my Turnigy reaktor means 3.75V.
For queston 3 I understand that swelling lipos are dangerous . So what I'll do is separate the bad and keep the good ones and repack the usable elements for a new pack or keep them there for further failures.