Top Balancing with minimal gear - Flaws, please...

[KJU]

Here's what I have:
- 24 LiFePO4 nominal 80AH used cells to play with and learn from.
- a 12V 22A charger with adjustable constant voltage
- an inverter & loads, variable from 0A to 18A at the battery
- a JBD 4s BMS with Bluetooth and xiao xiang Android app (Overkill-similar)
- 4 50W 6 ohm resistors, in parallel, for ~2A draw on one cell; ~8A on 4s. I might get more.
- a very good digital multimeter
- time

(I don't want to buy a desktop power supply or other gear I won't use beyond this present compulsion.)

Here's my method so far, without fine details like millivolts and such.
- Assemble a 4s pack with BMS. Start charging.
- Hold back faster-rising cell(s) with resistors.
- Stop the charge or slow it with loading, using the resistors to even up.
- In the 3.5V/cell range and on up, go carefully to take each cell near 3.65, until all 4 have touched that. (BMS cutoff)
- Charge with cutoff set to 3.6V/cell and repeat, using resistors to aim for all cells hitting 3.6 simultaneously-ish.
- Set cutoff to 3.65 and go slowly up to it and if charge current is very small, I have called that Top Balanced.
(I've repeated those last two steps a few times.)
- To confirm, for a little faith in myself, I've discharged a good third or so and charged back to 3.6 or 3.65, all cells together-ish.
- Then I can test the battery capacity down to 2.75V/2.8/3.0, and watch the voltages dance apart and then waltz back together when they get to the top.

It's all very satisfying, but now please tell me the flaws.

It's tedious, yes, but this is a hobby. Bottom balancing seems way easier, but less wise if a BMS is to take over.
I have other LiFePO4 ready-made batteries that are serving serious purposes, but they're quite boring by comparison.

Thanks for any comments!
KJU

 

[Roland W]

The only flaw would be if you are doing your balancing at every cycle. Because the 3.65V means nothing to a LFP battery. The only thing a LFP cell wants, is reach its resting state. That would be somewhere around 3.33-3.4V depending on the age of the cell. Top balancing one time, and then run your pack between low and high resting voltage (3.1V-3.33V), and you won't touch that pack with any kind of balancer for a year if you are not asking more then the typical 0.2-0.4C given in datasheets! The range I told you here typically represents 90-95% of capacity. Do you need more of that?

 

[daromer]

You only need to set top voltage to 3.65v and then disconnect each pak when their current dropped down. I wouldnt call that complex or even hard to do. Dont over complicate it. Bottom balancing isnt better because guess whats happen when you charge.

Do you rather have them drift at the top and potentially disconnect or overcharge than in the bottom? Top balance is to 99% of the times better

 

[KJU]

The only flaw would be if you are doing your balancing at every cycle. Because the 3.65V means nothing to a LFP battery. The only thing a LFP cell wants, is reach its resting state. That would be somewhere around 3.33-3.4V depending on the age of the cell. Top balancing one time, and then run your pack between low and high resting voltage (3.1V-3.33V), and you won't touch that pack with any kind of balancer for a year if you are not asking more then the typical 0.2-0.4C given in datasheets! The range I told you here typically represents 90-95% of capacity. Do you need more of that?

Hi RolandW,
No, I certainly wouldn't charge that high every cycle - only to establish the pack. The reason I get them to hit 3.65/3.6V and very low current , is to coax them to a ceiling so I know they're full and even.
I set a pretty conservative charge limit of 3.55V/cell or lower for batteries in actual use, and cycle them in a range below that, turning off solar panels often.
Part of my present task is to select the worthwhile cells and combinations from the 24 used ones I have. The practical sets will get BMSs.
Thanks,
KJU

 

[KJU]

You only need to set top voltage to 3.65v and then disconnect each pak when their current dropped down. I wouldnt call that complex or even hard to do. Dont over complicate it. Bottom balancing isnt better because guess whats happen when you charge.

Do you rather have them drift at the top and potentially disconnect or overcharge than in the bottom? Top balance is to 99% of the times better

Hi daromer,
I agree about top balancing being better, generally. The recommended method with a regulated power supply is simpler and clearer, but more expensive and space-taking, so I'm choosing to top balance cells in 4s groups.
I can't just take a cell out, done. If I substitute a cell or two into the group, I'm back to square one in the game.
Anyway, I'm enjoying my only slightly overcomplicated hobby.
KJU

 

[Charly144]

Just put them in parallel and buy a 2 or 3 amp adjustable power supply for a few bugs, set it to 3,65 and let it go until amps are 0. alternatively go for a used one from craigs list or similar.
Or get a used rc charger...

 

[KJU]

Just put them in parallel and buy a 2 or 3 amp adjustable power supply for a few bugs, set it to 3,65 and let it go until amps are 0. alternatively go for a used one from craigs list or similar.
Or get a used rc charger...

Hi Charly,
Yes, I'm leaning towards a little more gear, and I've found a small CC-CV device for ~$30 delivered.
It would make it easier to rank and group my two dozen cells. Not that easy is mainly what I'm after.
I'd thought I would need to spend several times that, and store another breadbox-size item aboard.
KJU

Step Up Down DC-DC Converter 35w 0.5v~30v. Versatile Display CC & CV | Trade Me Marketplace

Very versatile EXTREMELY HIGH PRECISION Step Up Step down DC DC Converter Buck & Boost Complete with voltage display and constant current & voltage making set...

www.trademe.co.nz

 

[Charly144]

Go for an rc charger, eg imax knock off
€ 14,41 35％ Off | HTRC iMAX B6 80W Battery Charger Lipo NiMh Li-ion Ni-Cd Digital RC IMAX B6 Lipro Balance Charger Discharger + 15V 6A Adapter

17.9US $ 45% OFF|Htrc Imax B6 80w Battery Charger Lipo Nimh Li-ion Ni-cd Digital Rc Imax B6 Lipro Balance Charger Discharger + 15v 6a Adapter - Chargers - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

a.aliexpress.com

I personally do prefer icharger, a bit more pricey but top quality.

 

[KJU]

Go for an rc charger, eg imax knock off
€ 14,41 35％ Off | HTRC iMAX B6 80W Battery Charger Lipo NiMh Li-ion Ni-Cd Digital RC IMAX B6 Lipro Balance Charger Discharger + 15V 6A Adapter

17.9US $ 45% OFF|Htrc Imax B6 80w Battery Charger Lipo Nimh Li-ion Ni-cd Digital Rc Imax B6 Lipro Balance Charger Discharger + 15v 6a Adapter - Chargers - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

a.aliexpress.com

I personally do prefer icharger, a bit more pricey but top quality.

Hi Charly,
That is tempting. Something to compare with the little converter on TradeMe here in NZ.
Haven't bit yet.
Thanks for the idea!
KJU

 

[Charly144]

It is so universal, you will need it for lots of other things as well