Top Balanced Needed?

toms

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Jul 24, 2020
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@Short_Shot
give it up mate, you are making yourself look silly..

When you connect SOC imbalanced cells with differing voltages they will then have the same voltage. If you disconnect them, they will have different voltages again.

That doesn’t mean the voltages were different when they were connected.

If you connect a fully charged and fully discharged LiFePO4 in parallel and connect to a 3.65V charger, the fully charged cell will not go over 3.65V

If that pair is then placed in a series pack that has BMS cell level overvolt protection - the same applies.

That is basic electrical knowledge, backed up by even the article you linked to.
 

samwise

New Member
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Nov 18, 2021
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I bought 8 grade 8 cells from Amy. They all tested at the same voltage when they arrived. Since they are matched and at the same state of charge, do I need to do a top balance charge each cell?
Hi, I am new to this forum and this is exactly the question I was going to ask myself.
Do I really need to top balance?
The background is that I need the battery for a sailboat. I can assemble a battery pack and top balance the cells at home - but I cannot carry the battery with me to the boat (via airplane). I can assemble the battery on the boat - but top balance the cells, having all the bits and pieces underfoot for a week? No way.
So what am I going to lose, if I buy a pack of 4 cells from Amy and just put them together, then charge them with the BMS?
A bit of capacity? What? 10%?
Buying a ready-to-use battery pack from e.g. Basen is almost twice the price. And... have they top balanced the cells?
 

Short_Shot

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Jul 13, 2021
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It sound be a good idea for you to make a new thread here @samwise

You may lose total capacity until the BMS can catch the cells up. If you're fine with that then life is good.

However if the balance is significantly off, you may find it going into high voltage cell disconnect even if the total pack voltage is low and then you'll have many issues until a reasonable balance is achieved.

An active cell balancer may be a workable solution to your problem as long as you take it off the pack after a good top balance is achieved in your boat during charging, unless that balancer is designed for top balancing only and will stop balancing under a certain cell voltage, which may or may not be a thing.

I'm far from an expert on active balancers and I suspect @Steve_S can help you here since he seems to know virtually everything.

Doing this on very imbalanced cells without repeatedly triggering high voltage disconnect can become somewhat complex/annoying and may involve setting a slightly lower pack charge voltage to start - if you can configure your charger that is.
 

time2roll

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Mar 20, 2021
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3,449
Hi, I am new to this forum and this is exactly the question I was going to ask myself.
Do I really need to top balance?
The background is that I need the battery for a sailboat. I can assemble a battery pack and top balance the cells at home - but I cannot carry the battery with me to the boat (via airplane). I can assemble the battery on the boat - but top balance the cells, having all the bits and pieces underfoot for a week? No way.
So what am I going to lose, if I buy a pack of 4 cells from Amy and just put them together, then charge them with the BMS?
A bit of capacity? What? 10%?
Buying a ready-to-use battery pack from e.g. Basen is almost twice the price. And... have they top balanced the cells?
I have built several batteries and skipped the parallel top balance. If you charge in series and the voltages come up together then the balance is fine to put in service. May need to limit voltage to 13.6 to 14.0 volts to stay in the working zone. Going to 14.6 will be trouble. Once in service, most people only charge to around 13.8 anyway. Worst case you can balance manually or add an active balancer to speed up the process.
 
Last edited:

samwise

New Member
Joined
Nov 18, 2021
Messages
2
Thank you @Short_Shot!
It sound be a good idea for you to make a new thread here @samwise
I did not want to clutter up the forum with threads with the exact same topic. I am sure there will be others who ask themselves "Top Balance needed?" and it would be great if they find the answers in just one thread.
So from your explanations and from time2roll 's experience I'd say I have my answer:
With decent cells (matched and balanced) the battery should work just fine without top balancing. I would only have to break it in like a horse. ;)
And worst case, if the cells really turn out to be out of sync, I would have to re-open the pack.









 

toms

Photon Sorcerer
Joined
Jul 24, 2020
Messages
972
Just to clarify - don’t disassemble the pack to top balance once you have done a series charge. Simply disconnect the load from the pack, and individually charge the cells. (You can charge to 3.65V, but it is better to charge to your BMS high voltage disconnect voltage at the current you normally charge at).

A lot of people have trouble with this concept, and misunderstand what a top balance is trying to achieve - these people should stick to parallel top balancing.
 
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