Manual cell balancing

[Padzb]

I have 4 x 300Ah Catl cells on my full time liveaboard sailing boat. No balancing BMS, they've been constantly in use for over a year now. The centre 2 cells seem to be up to maybe 30mV less than the outside 2 cells but overall pretty closely matched. Has anyone tried using a separate battery & CV/ CC dc converter to do the job of an active cell balancer? Thinking maybe connect the separate power supply to one of the lower cells & push in a tiny bit of power then monitor again. And set the CV so it can't go much above the other cells. Obviously no common earth. Seems pretty much what an active balancer does anyway? The boat is never near mains power so it's quite difficult to top balance the cells again & carry on living with no electricity..

And what's the best way to test for balance anyway? Charged & let them rest then check the voltages?

 

[OffGridInTheCity]

Yes you can do this and I've done it a few times (even though I have a BMS) to quicken the balance of my Powerwall's 126 packs. I can explain but no need for extra verbiage unless requested.

One thing I've observed is that individually charging a cell/pack in a battery will raise the voltage to get charge current to flow - so it can take a little patience/experience to do this 'artfully', especially on an active solar battery that's charging/discharging at the same time. I use a hobby charger and record the ah(s) put in and then observe the results over several days and finally have it down to xxAh rather than rely on voltage when I do this kind of manual touch-up.

TMI: As an example of scale... My packs are 260ah each and I add ~18ah to push the voltage up ~30mv on 18650/INR type chemistry.

 

[Bob B]

I have 4 x 300Ah Catl cells on my full time liveaboard sailing boat. No balancing BMS, they've been constantly in use for over a year now. The centre 2 cells seem to be up to maybe 30mV less than the outside 2 cells but overall pretty closely matched. Has anyone tried using a separate battery & CV/ CC dc converter to do the job of an active cell balancer? Thinking maybe connect the separate power supply to one of the lower cells & push in a tiny bit of power then monitor again. And set the CV so it can't go much above the other cells. Obviously no common earth. Seems pretty much what an active balancer does anyway? The boat is never near mains power so it's quite difficult to top balance the cells again & carry on living with no electricity..

And what's the best way to test for balance anyway? Charged & let them rest then check the voltages?

I have done similar to what OffGridInTheCity describes ..... but with only a 30 mv difference ...... I don't think I would bother.

 

[Padzb]

Great, thanks for the quick replies
Usually I log the voltage of every cell onto a Raspberry Pi but blew the ADS1115 sensor recently so next monday a new one should arrive, when that's back in action I'll have a gentle play & see how it goes.
Thnx

 

[gotbeans]

Are they only 30mv difference when all cells are charged to 3.5v or more per cell? (I assume 300ah Catl is lifepo4)
If so I wouldn't bother like bob said lol

And what's the best way to test for balance anyway? Charged & let them rest then check the voltages?

Not really, all kinds of factors make them rest at different voltages from what I've seen with my 64.

I have all the data on charging all 64 of my 280 amp hour cells and I initially thought the voltage drop at 24 hours was related to the overall capacity of the cells (270-285 amp hours for me) however it isn't directly related to that either. I also have internal resistance and temperature mapped out and then started to climate control the 10x10 are within 1 degree for the last 20 cells and those are still all over the place with where they rest at voltage.
So I think it's some internal stability on the molecular level that we couldn't possibly measure.
So I'd just say get all of them charged to 3.5-3.6 volts and then see if they are unbalanced there..
If you want to get them balanced just connect them in parallel for a few days but I bet the resting voltage would change on 1 of them after a week anyway (mine at least.. yours seem identical)
For yours I wouldn't bother(assuming the voltage diff was at the charged voltage). I've seen them drift more in 1 charge than yours have in a year lmao

I assume all of my internal resistance is identical. I have noticed that where I hold the prongs and where I push them and what angle of approach etc has influenced the readings of the resistance since it's such a tiny reading everything throws off the accuracy

Put my discharge/charge table headers in a separate pic forgot to copy the voltage range I used and amps

 

[Padzb]

Today when I checked the solar reg had gone as high as 13.8v then switched to 13.3v, when I measured so the solar was pretty much just keeping up with house loads.

This is what they looked like over a day a while ago, the blips in voltage is the fridge cycling. Don't think any difference gets more than 40mV. Can't remember if cell 1 is most negative or most positive. Not really worried but fancy a play now
Data is such fun!

 

[mikefitz]

Pull down the high cell/cells by connecting a suitable DC load across the cell. Apply for several seconds whilst monitoring cell volts.
Repeat as necessary.

 

[Padzb]

might be replying in the up in smoke channel if I start down that road

 

[exmarkretired]

Great, thanks for the quick replies
Usually I log the voltage of every cell onto a Raspberry Pi but blew the ADS1115 sensor recently so next monday a new one should arrive, when that's back in action I'll have a gentle play & see how it goes.
Thnx

Thanks for asking this. I am in the planning stage of a 48v DIY bank of cells but thought why not "add 4 more" and build a 12v portable bank for the toy hauler. Was wondering if I could do the same No BMS . Thanks again and to all those who answered this question.

 

[OffGridInTheCity]

Thanks for asking this. I am in the planning stage of a 48v DIY bank of cells but thought why not "add 4 more" and build a 12v portable bank for the toy hauler. Was wondering if I could do the same No BMS . Thanks again and to all those who answered this question.

I wouldn't suggest 'No BMS' as the takeaway here for a daily operational battery bank. A lot can happen in a few hours during charge and discharge of an active, daily system, and it's impossible to stay on top of it.

Not here to 'shame', just sharing that when I started, I too skipped the BMS for my initial 7s60p solar battery thinking I'll just measure it every other day. After a month or 2 I finally realized it was discharging unevenly and some packs were way too low - very hard on the cells. Since then, I've become a huge fan of BMS / monitoring.

Now, if you have a limited use battery......
For example, I have a 4s12p headway battery I use for my winch a couple of times a year and it just sits the rest of the time (no charge/discharge). I don't have an active BMS but I do have a BattGo - https://www.amazon.com/ISDT-Battery-Meter，LCD-Capacity-Balancer/dp/B07797N9BG - to monitor (and balance) for the few hours I use it and recharge it after use so I can see what's happening.