LiFePO4 Bottom Balancing Tricks

[atatistcheff]

While I was sitting staring at a volt meter today I was thinking of ways to make bottom balancing easier. Since this seems to be a critical step when dealing with DIY LiFePO4 packs it would be really nice to make it easier. I'm sure there are some tricks out there and my purpose here is to brainstorm a little and see what others may want to contribute. This discussion assumes we are using LiFePO4 packs with a small number of cells. What I mean is we're not building using 18650 cells where you would have dozens in a pack but probably using something like 8Ah Headway to 200Ah Sinopoly cells.

One technique seems to follow steps similar to this:

1. Verify your cell voltages are close. Yes "close" is vague but follow best practices you can find in Will's videos.
2. Assemble your pack, let's say 4s for example.
3. Connect the pack to a load and drain until the lowest cell is at 2.5V
4. Connect resistors to the other cells one at a time to bring each cell to 2.5V
5. Charge the pack and proceed with load testing.

These are high level steps and your idea of the bottom voltage may vary somewhat but I believe these are fairly close to reality.

Now the monotony comes in step 4. You have to be careful when draining your cells that you don't go below 2.5V. Somewhere around 2.0V you will begin to permanently damage your LiFePO4 cells. I generally hook up resistors, set a timer, check back, rinse and repeat until I'm at 2.5V. This is a time consuming process and requires babysitting the cells to make sure you don't hurt them.

What would be nice is some way to automatically discharge the cell down to 2.5V and then disconnect the load.

One thing that occured to me is that my BMS automatically cuts off anytime a cell goes below 2.5V. A simple solution would be to use a 12V N.O. relay to disconnect my resistor load when my cell hits 2.5V. That's great if you have a BMS that cuts out at 2.5V. Are there other solutions? Here's where I'd like to solicit some input and see what others might be doing to automate this process a bit.

 

[Jejochen]

Use an icharger for example and run lifepo4: discharge to any current you set it to.

 

[Power4life]

I use this particular BMS to bottom balance my cells and while it has a slow balance current it makes for super easy balancing. Initially, I tried to top balance my cells but realized I had to charge, run balance, charge and run balance and so on and so on until all cells matched at the top end. After realizing it would take a considerable amount of time to do this I decided to just bottom balance. So I took my pack down below 12 volts where the cells begin to separate. Once one of the cells reached 2.7 volts I stopped the discharge and activated the static balance (no electricity flow) option on my BMS where it would burn off the energy of the higher cells to match the lower cell. Took about 4 days but I didnt have to do anything during the process. Super easy.

 

[Power4life]

Here it is all balanced with little effort to do so

 

[Supervstech]

I like to tie them to a regulated load with a single cell disconnect.

I use a cheapo single battery usb bank, hooked to a usb fan.
I wire it to the cell, and let it run till it is at disconnect volt...

 

[Maast]

Well you can use these 2.5v to 3.8v adjustable low voltage alarms along with a standard 4S balance connector and set them to trigger at 2.7 volts or so and then hook up your discharge load. They'll trigger when the lowest cell hits your set point.

These things are wake-the-dead oh-my-god-make-it-stop loud so they'll get your attention if you're anywhere in earshot. Would save you having to baby sit it.

You can also desolder the buzzers and hook the output to a cheap relay control board to energize a normally closed relay and disconnect a load, you'll need a 100uf smoothing capacitor for the alarm output to reliability trigger the relay.

On those particular lipo low voltage alarms the output signal is the top left and bottom right contacts of the buzzers, or you could just solder leads to the backside of the circuit board to connect to the relay control board so you'd get both the alarm and the automatic disconnect.

 

[TCgreg]

Now the monotony comes in step 4. You have to be careful when draining your cells that you don't go below 2.5V. Somewhere around 2.0V you will begin to permanently damage your LiFePO4 cells. I generally hook up resistors, set a timer, check back, rinse and repeat until I'm at 2.5V. This is a time consuming process and requires babysitting the cells to make sure you don't hurt them.


I used a Cellpro Powerlab 8 which has a discharge feature and a precise voltage disconnect. My battery consists of (24) 3.2v 100Ah CALB cells configured into (2) 12v/300Ah batteries. The cells all arrived at the same voltage within a tenth aside from one which was markedly lower. CALB replaced that cell. Where buying raw cells I would compare delivered voltages. They should be very close if the factory runs the same capacity tests on all of them. If they vary I would wonder if the cells are even tested for capacity.

Each cell is discharged to 2.5v at 10 amp rate. After that the cells are allowed to recover overnight and settle at around 2.8v. A second round of discharging follows at 2.65v at 3amp rate. Following that all cells are allowed to sit overnight and compared after resting. In my case they all recovered to 2.70 within a hundredth of each other.

This probably isn't the easiest way to bottom balance cells but I think it is probably the most accurate and eliminates the above noted concern. No babysitting required and highly accurate voltages.

Youtuber RV Exodus has a nice multi series tutorial including using the Cellpro Powerlab.

 

[Maast]

I used a Cellpro Powerlab 8

I just took a look and thats a nice bit of kit! Very feature rich, only limiting factor is 40A @ 24V/20A @ 12V but if you're willing to accept less than a .5C charging and discharging will handle any size cell. Especially like the software.