Top Balancing an older pack. Eve 3.2v 280, 32 cells.

[Sunnytheskoolie]

Hello,

I have been running my battery for the last 4 years and it has started to have some issues. When reaching the upper limits it starts to have cells that charge faster and others that charge slower. There are only 2 or 3 of the cells that drop down, most of them stay together during the charging.

I am running the REC Q1 BMS, I do not have an active balancer on it.

Everything was running fine for a couple years with the right BMS settings.

I just pulled the battery apart, it's a 2p16s battery. I top balanced it to 3.65v last night and am going to let it sit for a few hours/day to see how it settles out.

Should I put the cells back together in the exact same arrangement or should I mix up the parallel cells?

Should I also bottom balance the batteries as well?

I used a small charger to get the batteries to the same voltage when the pack was together. 80% of the pack would be at the upper limits and then the system would go into an absorption state and the BMS would start trying to balance it all out. I would take the small charger and put a charge into the cells that were lower so that they would climb up to meet the other cells. I would repeat this process until all the cells were at the top 3.5 limit I had them set at. Then the pack would stay together. It was just from 3.37-3.5 that they would split.

If they still separate like this I will go ahead and add an active balancer since it seems like it would keep the cells together while charging. I think the cells are all still in great shape after pulling them all apart.


Any advice or if you have done the same thing please let me know the results.

Thank you

 

[HRTKD]

Maybe split the cells into two separate batteries using a BMS that has better cell balancing, like the JK BMS.

 

[time2roll]

Has the charging and balancing profile been reviewed to determine why the balance was being lost?
That is where I would start.

 

[Brett V]

You don’t want to bottom balance the cells. I would pull all the bus bars and make sure you haven’t developed any corrosion on the mating surfaces. Be very critical of your BMS connections too. It doesn’t take much when you’re looking at millivolts. I picked up a battery internal resistance meter, it’s useful to evaluate the cells health and measure resistance on connections.

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[Skypower]

It took a while for it to get out of whack so you might have a while before it gets bad again since you top balanced , that is if everything stays status quo. Besides being sure all the connections are good, the other thing that can keep it in balance is more frequent trips to full charge and increasing the bulk/absorb time. More time at full state of charge to ensure balancing, unfortunately has a downside to the battery chemistry. An active balancer will definitely speed up the process to the point of minutes not hours or days (regular maintenance). As for pairing the cells differently, yes that works quite well IF connections weren’t the cause of the imbalance. Also the voltage would have to be recorded before balancing or it’s just not worth it and best to just put it back as it was in that case. You generally pair a low cell with a high cell. Highest with the lowest first and the following pairs with diminishing differences. Generally once a pack is assembled I strongly resist disassembling. My first build was a 2P 16S , but bms’s being so cheap I like the idea of redundancy.

Two options are to use a JK bms 2amp balance version or keep the setup as it is and add a cheaper and more powerful option, an EnerKey 4 amp balancer. If you go the latter route, be sure that the balancer has its own leads to each cell, NOT spliced in and sharing the BMS leads.

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[Sunnytheskoolie]

I set the pack up again and am now taking it down to the bottom 5% and then will charge it back up to the max and see if the cells stay together. I don't really want to add a balancer if I don't have to. And I would prefer to keep the pack as one and not use a second BMS. If it comes to that I will but hopefully it just needed to be balanced out. I'll post back once I have drained and charged it a few times.

 

[nc73]

Well there's your problem, no active balancer. I have mine setup with jk bms and only ever top balanced. Been great for couple years now. You do not need to bottom balance. I made that mistake and stuck with top only.

 

[Brett V]

I have five batteries, all using an Overkill BMS without active balancing. They have passive balance only. Above 3.5 volts, my delta is always below 10 mv and usually around 3-5 mv.

 

[Sunnytheskoolie]

Well there's your problem, no active balancer. I have mine setup with jk bms and only ever top balanced. Been great for couple years now. You do not need to bottom balance. I made that mistake and stuck with top only.

I didn't end up bottom balancing it. I top balanced it and now I have drained it to less than 5% and charged it back up and am doing that again a couple more times over the next few days to see how it runs. It takes 30 hours to drain it with a space heater so it takes some time.

I haven't used an active balancer since I built the battery it ran well for over two years of continuous use. What active balancer would you recommend with a REC Q1 BMS and 2p16s set up?

 

[adobopr]

I didn't end up bottom balancing it. I top balanced it and now I have drained it to less than 5% and charged it back up and am doing that again a couple more times over the next few days to see how it runs. It takes 30 hours to drain it with a space heater so it takes some time.

I haven't used an active balancer since I built the battery it ran well for over two years of continuous use. What active balancer would you recommend with a REC Q1 BMS and 2p16s set up?

For most cases you shouldn't need an active balancer, but you have to make sure your pack gets to the passive balance threshold for enough time each day or it will cause problems.

I made this mistake and I'm bow considering trying an acrive balancer temporarily to top balance, but will then go back to passive balancing.

 

[Sunnytheskoolie]

Coming back to this thread.

I top balanced the battery and edited the settings in the REC Q1 BMS so that the balance start was a bit higher and ended at the same time as the battery stopped charging and now the battery is working like a charm.

It was out of balance and I didn't balance it initially because I had purchased what I though were batched and matched cells. I did put them in parallel because I didn't have any way to charge them up as I was already off-grid.

So after a couple years of great usage they went out of balance and a top balance fixed the issue.

 

[time2roll]

Yes and the battery should have slowly balanced over that time. Keep an eye on it for if the cells start to drift again.