Easy Bottom Balancing

[Marnus101]

Hi Everyone
I have a question regarding bottom balancing.
Here is my question/hypothesis:
Lets say I buy 16 LiFePo4 100Ah brand new cells from a supplier. After purchasing I hook them up to a DIY discharger that uses a 100W resistor and an arduino to measure when voltage reach 2.5V, when it does reach 2.5V the arduino will trigger the relay to open and discharging will stop. After this process I will rest cells for 24hr and allow to them to recover to their different voltages. Thereafter I will hook them up in series and connect my Smart BMS, I will then enable manual balance and the bms will start balancing each cell. Due to the low state of charge (Cells are a about 2.85-2.95V after recovering) the balancing wont take that long. After couple of hours I have all cells at lets say 2.88V. I will then charge up entire pack and I am done.
Is this a good approach?

Any reply would be appreciated!

 

[John Frum]

Does your application require bottom balancing?
Typically for solar top balancing is indicated.

 

[Marnus101]

All the research I done suggested bottom balancing for solar. Cell supplier also recommended it.

 

[John Frum]

All the research I done suggested bottom balancing for solar. Cell supplier also recommended it.

Dang near consensus here to top balance.
The bms's that we use typically top balance.
Are you planning on using a bms?
What chemistry are you using?

 

[Sojourner1]

"All the research I done suggested bottom balancing for solar. Cell supplier also recommended it."

Think about it for a second.

If charging with solar that's always hooked up to the batteries and can charge while the sun is out and keeps the batteries at full more than not, that's top balancing.

If you have an EV and want to drive as far as possible putting the batteries at the bottom end then plug in to charge that's bottom balancing.

 

[fafrd]

If the cells are matched in terms of capacity it doesn’t matter whether you top-balanced or bottom-balanced - either will provide you the capacity of your lowest-capacity cell (boosted by however much your BMS can balance while charging).

And if your cells are not capacity-matched, your going to have less capacity than you paid for (whether bottom-balanced or top-balanced).

From what I have understood, the main advantage of top-balancing over bottom-balancing is that many suggest that the impact as far as reduced lifetime of charging to 100% is less than the impact of discharging to 0%.

If this is so, charging all of your cells to 100% (or whatever % you elect to back off from 100%) will maximize the lifetime of your cells.

For a solar install, the ideal situation is that all cells reach 100% / 3.65V at the same time (or whatever %/V below that level you have elected to program your charger for).

In the case the battery was not completely depleted overnight, no cell hit 0% SOC (or whatever lower limit you have set) and all cells should recover to 100% the next day.

In the case one cell discharged to 0% and BMS overdischarge protection kicks-in, all cells will have had the same amount of charge pulled out and so all cells should again recharge to 100% together the next day.

And finally, in the case you have used an SOC setting to halt discharge before any cell reaches 0%, it’s exactly like the first case and all cells should recharge to 100% together the next day...

 

[Marnus101]

"All the research I done suggested bottom balancing for solar. Cell supplier also recommended it."

Think about it for a second.

If charging with solar that's always hooked up to the batteries and can charge while the sun is out and keeps the batteries at full more than not, that's top balancing.

If you have an EV and want to drive as far as possible putting the batteries at the bottom end then plug in to charge that's bottom balancing.

Thank you! Make sense. Sorry for the stupid question. Okay but last, what is the fastest and most effective way to top balance cells? Im using ANT bms

 

[Marnus101]

If the cells are matched in terms of capacity it doesn’t matter whether you top-balanced or bottom-balanced - either will provide you the capacity of your lowest-capacity cell (boosted by however much your BMS can balance while charging).

And if your cells are not capacity-matched, your going to have less capacity than you paid for (whether bottom-balanced or top-balanced).

From what I have understood, the main advantage of top-balancing over bottom-balancing is that many suggest that the impact as far as reduced lifetime of charging to 100% is less than the impact of discharging to 0%.

If this is so, charging all of your cells to 100% (or whatever % you elect to back off from 100%) will maximize the lifetime of your cells.

For a solar install, the ideal situation is that all cells reach 100% / 3.65V at the same time (or whatever %/V below that level you have elected to program your charger for).

In the case the battery was not completely depleted overnight, no cell hit 0% SOC (or whatever lower limit you have set) and all cells should recover to 100% the next day.

In the case one cell discharged to 0% and BMS overdischarge protection kicks-in, all cells will have had the same amount of charge pulled out and so all cells should again recharge to 100% together the next day.

And finally, in the case you have used an SOC setting to halt discharge before any cell reaches 0%, it’s exactly like the first case and all cells should recharge to 100% together the next day...

Thank you so much for the thorough explanation! Any suggestions if i need to top balance a lot of cells? Should I discharge them all to 2.5 and then just connect them in a 16cell pack and let the BMS top balance them for me? Im using ANT bms

 

[Ampster]

Any suggestions if i need to top balance a lot of cells? Should I discharge them all to 2.5 and then just connect them in a 16cell pack and let the BMS top balance them for me?

No the process for parallel top balancing is to connect them in parallel like you were going to do for bottom balancing but charge them to 3.65 or close. You will need a power supply that you can set at 3.65.

 

[fafrd]

Thank you so much for the thorough explanation! Any suggestions if i need to top balance a lot of cells? Should I discharge them all to 2.5 and then just connect them in a 16cell pack and let the BMS top balance them for me? Im using ANT bms

I’m just learning about this now, so take any advice from me with a grain of salt.

I’m in the middle of charging up my 90Ah 8S battery for the first time.

If I believe what the BattGO tells me, I got some bad cells.

But if ignore the BattGO and read the cell voltages with the same multimeter, all of my cells are at exactly 3.310V, meaning ~80% SOC.

So now I’m going to add ~5% / 4.5Ah at a time and see whether the voltages start to stray away from each other as I start getting into the ‘hockey stick’ above 3.315V...

Obviously, getting a 3.65V charger and charging all of your cells in parallel with large common busbars is the ‘best’ / recommended way to top balance. I just plan to see how close I can get using my 20A 28.6V charger, my BMS, my BattGO, and my multimeter before making that investment.

Will report back as my experiment unfolds...

 

[Ampster]

I top balanced my cells and then assembled them in a 2P16S pack. As I began exercising them some started moving toward 3.65 per cell so now I am manually top balancing the runners with a one Ohm 50 Watt resistor. The issue with a 2P pack is you don't know which of the two cells is the weakest. I have already removed one pair and replaced them with a spare pair that had to be adjusted to the rest of the pack. Now the task of individually capacity testing each of those two at the rate of 40 Amps, which for a 280 Amp cell is going to take 14 hours round trip.
My point is with new cells you never know. It may take several weeks of exercise before a weakness reveals itself. Fortunately I ordered spares so I cans substitute and not have to wait for my vendor to send me replacements.

 

[fafrd]

I’m just learning about this now, so take any advice from me with a grain of salt.

I’m in the middle of charging up my 90Ah 8S battery for the first time.

If I believe what the BattGO tells me, I got some bad cells.

But if ignore the BattGO and read the cell voltages with the same multimeter, all of my cells are at exactly 3.310V, meaning ~80% SOC.

So now I’m going to add ~5% / 4.5Ah at a time and see whether the voltages start to stray away from each other as I start getting into the ‘hockey stick’ above 3.315V...

Obviously, getting a 3.65V charger and charging all of your cells in parallel with large common busbars is the ‘best’ / recommended way to top balance. I just plan to see how close I can get using my 20A 28.6V charger, my BMS, my BattGO, and my multimeter before making that investment.

Will report back as my experiment unfolds...

Well, after 4.25 hours of charging at 10A, all 8 on my cells are 3.337V with the exception of 2 which are at 3.338V.

I’m going to call it quits for the day, but so far, pretty happy with how closely-matched the cells are charging.

3.337V is ~90.6% SOC or ~81.54Ah based on the 90Ah rating of these cells.

I started at an average of 3.243V or ~26.35% SOC so 42.5Ah went into the battery and changed the SOC by ~64.25% or 58Ah based on the 90Ah rating (meaning the capacity of the cells may come out closer to 66Ah than the rated 90Ah.

Very happy with how well the cells are matching but a bit concerned about how quickly they are charging up.

Will try to charge up to 3.575V (~98% SOC) tomorrow and will check how well the matching holds.

There is so much uncertainty on the starting SOC of the battery, I’ll need to run a discharge cycle to determine the actual capacity of these cells after they are fully-charged...

 

[Marnus101]

Can I leave the power supply unattended at set voltage 3.65V overnight?

 

[Ampster]

Can I leave the power supply unattended at set voltage 3.65V overnight?

If you are certain that 3.65 is where your power supply will stay at. Most of the mistakes I have seen are that the power supply is not set properly. What do your cells read now? What are the Amps? How big are the cells you are charging? It is morning where I am, so you have some time to gather data.

 

[Marnus101]

I will make sure power supply is at 3.65 using my fluke. Most cells are at 3.337V and two cells are at 3.5V

 

[Marnus101]

And my bench power supply can supply 5A max

 

[Ampster]

I will make sure power supply is at 3.65 using my fluke. Most cells are at 3.337V and two cells are at 3.5V

Then you must not be in parallel and you already have two that are already at the knee. I would watch them carefully at that point.

 

[Marnus101]

Thanks
No cells are connected to each other at the moment. I wil discharge those two cells a bit and then connect all of them in parrallel?

 

[Ampster]

I doubt your BMS can pull down those two before the BMS cuts out when they reach 3.65. Do you know the settings on the BMS?

 

[Marnus101]

I mean ill discharge them manually and them connect them in parallel and connect the power supply set at 3.65V. No BMS connected tight now