Top balancing should i use a bms

[mr-k]

hi, just bought a seplos mason kit with the 16 grade A ev 280 cells, i started off top balancing by setting them in banks of 4s, then charged them with a victron 12v charger, now ive put them all in paralell and charging them with a desktop charger at 3.6

Should i get a bms while charging them in paralell, i work from home so i can monitor the voltages, also should i charge them under compression, i turn the charger off at night, i guess this will increase the charge time considerably

 

[Supervstech]

ANY TIME THE CELLS ARE IN SERIES THEY MUST BE PROTECTED BY A BMS.

 

[Supervstech]

To top balance you need a power supply that outputs steady voltage AT 3.65V no load.
The cells get placed in parallel, and you charge them until amps drop to zero.

Anything else is just playing with the cells.
Or damaging them.

 

[mr-k]

ANY TIME THE CELLS ARE IN SERIES THEY MUST BE PROTECTED BY A BMS.

Should i get a bms while charging them in paralell

 

[mr-k]

To top balance you need a power supply that outputs steady voltage AT 3.65V no load.
The cells get placed in parallel, and you charge them until amps drop to zero.

Anything else is just playing with the cells.
Or damaging them.

hi. but should i buy a bms when top balancing 16 ev cells in parallel

 

[#40Fan]

 

[mikefitz]

started off top balancing by setting them in banks of 4s, then charged them with a victron 12v charger, now ive put them all in paralell and charging them with a desktop charger at 3.6

How did you top balance In banks of 4? Did you use a 4s BMS?

Should i get a bms while charging them in paralell

BMS are designed for cells in series.

The 'conventional' technique of putting all the cells In parallel and charging to 3.65 may end badly.

 

[Bob B]

hi. but should i buy a bms when top balancing 16 ev cells in parallel

If they are in parallel, what you need to do is carefully control the power supply voltage being used to top balance.

Adjust the voltage from the power supply BEFORE connecting the leads to the battery ..... verify the voltage setting with a DVM .... Connect to the parallel cells with a good ring terminal connection, not alligator clips .... don't touch the power supply setting again.

The 4S charging you described is what is concerning people.

 

[Supervstech]

hi. but should i buy a bms when top balancing 16 ev cells in parallel

It depends on what you mean by in parallel...

Are the individual cells ALL in parallel?

Or do you mean, sets of 4 in parallel?

Please in detail, explain what you are asking so our answers line up with what you are wanting to know

 

[houseofancients]

Should i get a bms while charging them in paralell

You have a BMS..
The seplos one...

If top balancing you should connect them in parallel, not in series..
The option I use nowadays is not too balancing them, rather charge them untill 3.43 , let the seplos BMS control the amps
When the BMS slows charging down to 10a , change the charging voltage of the inverter to that voltage and keep absorbing .
my neey active balancer will have plenty of time to balance them out perfectly, when they are within about 10 mv, step up the charging voltage to 56.8 in increments of .5 v.

Meet is permantly connected, but doesn't start untill 3.45 and stops at 3.40, but has barely run after above balancing

Big advantage, cost less time, I can do a proper build ( and compress) before giving the cells the chance to swell, and my BMS keep a eye out on my cells..

Also, not unimportant..
I can build the case in my rack , then add the cells, so I don't have to lift 110kg ( 220 pounds)

 

[toms]

It is never a good idea to connect cells at differing SOC in parallel.

No manufacturer has or ever will recommend this.

If you are charging a single cell I’d recommend using a disconnect that prevents the cell from being charged over 3.65V, or being charged at lower than 0.05C

The most practical way is (and always has been) to completely build your pack including BMS, then charge to your designated “full” voltage. If the BMS disconnects the charger due to a cell going over voltage, connect a power resistor across that cell for a fixed time to reduce its SOC.

The easiest (less economical) way is to use a high current active balancer.

 

[SilverbackMP]

The most practical way is (and always has been) to completely build your pack including BMS, then charge to your designated “full” voltage. If the BMS disconnects the charger due to a cell going over voltage, connect a power resistor across that cell for a fixed time to reduce its SOC.

I have seen Andy (Offgrid garage) use that method with six or eight incandescent bulbs wired onto a board with leads.

I just went with the standard parallel method.

 

[toms]

I just went with the standard parallel method.

That is not the “standard” method. Parallel top balancing became popular after some YouTubers with very limited knowledge of LiFePO4 operation that couldn’t work out how to stop their e-bike derived BMS’ from disconnecting due to imbalance found that parallel top balancing “solved” this issue.

This coincided with the introduction of the relatively cheap aluminium cased prismatics which allowed a new group of people into the DIY LiFePO4 scene (also around the time this forum was initiated), and several high profile YouTubers were gaining popularity.

These people had one thing in common - they thought they had discovered something new without realising it had been around for a decade and all the mistakes they were making had been made before.

Those that got it wrong in 2010 are on to their second (or third if they are slow learners) set of cells.

 

[Supervstech]

That is not the “standard” method. Parallel top balancing became popular after some YouTubers with very limited knowledge of LiFePO4 operation that couldn’t work out how to stop their e-bike derived BMS’ from disconnecting due to imbalance found that parallel top balancing “solved” this issue.

This coincided with the introduction of the relatively cheap aluminium cased prismatics which allowed a new group of people into the DIY LiFePO4 scene (also around the time this forum was initiated), and several high profile YouTubers were gaining popularity.

These people had one thing in common - they thought they had discovered something new without realising it had been around for a decade and all the mistakes they were making had been made before.

Those that got it wrong in 2010 are on to their second (or third if they are slow learners) set of cells.

Can you explain what the standard is then?

 

[Quattrohead]

BMS battery management system, does what it says on the box. Why reinvent the wheel.
Toasters are automatic too, they can get your toast done just perfectly. Now of course you could try and make toast on a regular grill but if you don't keep an eye on it you're going to burn your toast. A burnt lithium battery is far more spectacular and interesting but doesn't taste as good as toast.

 

[toms]

Can you explain what the standard is then?

Sure. Same as what every manufacturer in the world uses.

Charge each cell to 3.65V at 0.05C, immediately disconnect charger when the voltage is reached.

The only time it is OK to connect cells in parallel is if they are at the same SOC.

Ask anyone that parallel top balanced their cells around 2010 when the internet forum experts were saying it was a good idea how it worked out for them.

 

[a619]

Ask anyone that parallel top balanced their cells around 2010 when the internet forum experts were saying it was a good idea how it worked out for them.

It would be more productive discussion instead of just saying "ask anyone" or "do your research", you could explain the physics/ electrical reasoning of why it's not a good idea to top-balance cells in parallel. I don't see any reason why paralleling LiFePO4 cells would be an issue.

 

[sunshine_eggo]

Really? No one?

Top Balancing LiFePo4 Cells using a low cost benchtop power supply.

To get the paper, click on the orange at the top of this page. This is a tutorial with detailed steps on how I top-balance my LiFePO4 Cells using a low-cost benchtop power supply. To get the tutorial, click on the orange "Download" button...

diysolarforum.com

Download the doc and read it beginning to end. Time saving tips (use BMS to charge in series first, fabricate better leads for better parallel charging) and a full process start to finish.

FOllow the directions and have success.

Note. Not worth bothering with unless you're going to be ready to deploy the battery and start using it shortly after balancing.

 

[toms]

It would be more productive discussion instead of just saying "ask anyone" or "do your research", you could explain the physics/ electrical reasoning of why it's not a good idea to top-balance cells in parallel. I don't see any reason why paralleling LiFePO4 cells would be an issue.

If you parallel LiFePO4 cells of different SOC and then charge them to full, due to the flat voltage curve you will end up with one cell being held at full voltage while the others are still charging. This will cause lithium plating.

Nobody disputes this, the only argument i’ve had is that the level of plating is going to be insignificant. Hence my relaying my experience of a trend of people that i have followed where parallel top balanced cells have failed prematurely.

Surely if you were going to follow a completely unnecessary process (alternatives listed earlier in this thread), that was unsupported by any manufacturer, you would be wise to research any side effects. I can only relay (and have done for many years), my own experience.

I have zero skin in this game - i don’t even install customer power systems any more, too old and grumpy now.

It’s pretty much got to the point where i only help people that have had early system failures due to following poor advice.

 

[sunshine_eggo]

Sure. Same as what every manufacturer in the world uses.

Charge each cell to 3.65V at 0.05C, immediately disconnect charger when the voltage is reached.

I think you meant that as tail current, not an actual charge rate?

I measured it, and my 280Ah cells took on another 400mAh between 14A and 0A... Tapered FAST.

The only time it is OK to connect cells in parallel is if they are at the same SOC.

BUT... this is impossible to determine based on voltage. I had 9 cells with identical voltages to 3.29X, and they varied in SoC by 22%. What is the harm in putting two cells at identical voltage when they won't transfer any current between them?

Back in 2010, weren't "they" still charging to 4.2 and discharging to 2?