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Top balance estimated time?

Swamplizard

Ready to unplug and wander the USA
Joined
Oct 21, 2020
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TGIF All.

So my 24v 280AH battery bank has been working flawlessly for the last year however I noticed 2 cells seem to charge faster/more than the rest and it shuts down BMS.

Wanting them to be more balanced, I decided to dissassemble it, put them in parallel, and top balance them with a 10amp bench charger ... goal was to get them to 3.6 so as not to "push it" and then re-assemble the bank. BTW when in the RV I use conservative min/max charge with my LV2424 but the 12/24 1oAmp DC to DC charger seems to want to go 3.65/cell.

After 6 days of 4-6 amps - battery bank is only at 3.4v

I have a trip planned - how long from 3.4 to 3.6? Not sure of the math as the charger will slowly push less juice as battery tops off.

Thanks!
 
TGIF All.

So my 24v 280AH battery bank has been working flawlessly for the last year however I noticed 2 cells seem to charge faster/more than the rest and it shuts down BMS.

Wanting them to be more balanced, I decided to dissassemble it, put them in parallel, and top balance them with a 10amp bench charger ... goal was to get them to 3.6 so as not to "push it" and then re-assemble the bank. BTW when in the RV I use conservative min/max charge with my LV2424 but the 12/24 1oAmp DC to DC charger seems to want to go 3.65/cell.

After 6 days of 4-6 amps - battery bank is only at 3.4v

I have a trip planned - how long from 3.4 to 3.6? Not sure of the math as the charger will slowly push less juice as battery tops off.

Thanks!
Right now i am balancing 6 cells and take me 1 week.
 
are they 280's?
I was thinking 5 Amps/hour times 20% of 280AH was 56 hours but .... not sure that is the math
At least the bench charger is going down form 6 to5 t 4 and now it shows 2.33
 
LF280K

The amp is going down with the time, my charger disconnect automatically 0.1 amp.
 
are they 280's?
I was thinking 5 Amps/hour times 20% of 280AH was 56 hours but .... not sure that is the math
At least the bench charger is going down form 6 to5 t 4 and now it shows 2.33

Yes 56 hours @ 5amp.

if you're down to 2.33a its real close.
 
If you're balancing all of them in parallell, then it's 280A times however many cells. If there are eight, that's 2240A from empty. At 5A charging that can take a very long time.

If you charge them to bms cutoff in series, then break down, you'll be much much closer.

But there's really no need to do any of that, make up a headlight bulb with some leads and bleed off the high cells 'till they match the others. This is best done while charge and the pack is near full - or 'high knee'. If your bms does cell balancing this will greatly assist it and speed up the process.

My 230A cells will bleed off a few millivolts in a few minutes. I spent some time doing this even though they were fully top-balance before assembly. They still go out of balance at the very top - one or two will get full and spike. So I cut charging at 5% tail current max volts 14.1 or 14.2 (12V system).

My bleeder:
 
Love the lightbulb bleeder! Will go that route in the future for sure as this battery is normally under a bench in a box mounted to the vans floor and is heavy…. Big pita to remove and disassemble.
 
It's not my idea - comes from Steve at Overkill Solar (JDM BMS) installation manual for DIY lifepo4 batteries.
It does work killer though, but you want to be at high-knee, where one or more cells are peaking. You can bleed them off while the other ones catch up, getting them all the same.

Unless you get perfectly matched grade-A cells, there's going to be some imbalance. Relying on the bms to keep them aligned just isn't realistic - would take many charge cycles over months of time. You can rebalance at any time.
 
I agree with bleeding the two cells down. Although after a year into this I suspect the cells may have issues larger than the BMS can self balance.
If I tore the battery apart I would seriously consider replacing the pair while it is apart next time.
 
Thanks all - topped off and ready to rebuild but it took nearly two weeks - wasnt expecting that but onward and upward.
 
Unless you get perfectly matched grade-A cells, there's going to be some imbalance. Relying on the bms to keep them aligned just isn't realistic - would take many charge cycles over months of time.
I wonder if the balance on an Overkill with factory settings is a gimmick.

The factory settings require a voltage to be above and charging for the balance to happen. I have mine set to 3.4 and the cells charge to 3.475, and the daily time spent at that state is less than an hour. How many amps those tiny 20 gauge wires balance the cells is a mystery, but my guess is less than an amp.

With that, I wonder if that could ever effectively balance a battery pack that was not top balanced prior to installation. You could readjust the settings and change it so it is always balancing, but I still wonder how much current flows through a 20 gauge wire.
 
I wonder if the balance on an Overkill with factory settings is a gimmick.

The factory settings require a voltage to be above and charging for the balance to happen. I have mine set to 3.4 and the cells charge to 3.475, and the daily time spent at that state is less than an hour. How many amps those tiny 20 gauge wires balance the cells is a mystery, but my guess is less than an amp.

With that, I wonder if that could ever effectively balance a battery pack that was not top balanced prior to installation. You could readjust the settings and change it so it is always balancing, but I still wonder how much current flows through a 20 gauge wire.

Exactly. My pack doesn't spend much time at high-knee, so balancing is extremely limited via bms. I don't charge to 3.6, but max it about 3.55. Any higher than that and one or two of them are full and their voltage starts to climb much quicker than the others. This is where you can do pretty effective balance by bleeding them off, allowing the low ones to catch up. Current has slowed to a trickle here, maybe an amp or two.
 
LF280K

The amp is going down with the time, my charger disconnect automatically 0.1 amp.
Is 100mA considered top balanced? I have 16 cells charging at 3.65V running at 40mA right now. It just won't drop to 0.
 
Yes. You can always do some fine tuning after they’re put in service. Some don’t even advocate for extensive top balancing, employing an active balancer instead.
 
But there's really no need to do any of that, make up a headlight bulb with some leads and bleed off the high cells 'till they match the others. This is best done while charge and the pack is near full - or 'high knee'. If your bms does cell balancing this will greatly assist it and speed up the process.
Thank you! While the pack is still intact, I assume?

Every time I think I'm ready to build my first pack I bump into posts with people struggling with issues like this, so I'll keep just studying a bit longer.
 
Yes. You can always do some fine tuning after they’re put in service. Some don’t even advocate for extensive top balancing, employing an active balancer instead.
That's good to know. I've been agonizing over not being able to get the current down to absolute 0 and cell voltages not actually reaching 3.65. However, 3.646 is close enough for me.
 
I ended up going for 3.55 and it eventually stopped charging. Good enough for me as I set my Charger/MPPT using the 90% rule anyway to keep them alive longer. I still get more than 270AH out of my 280s which is good enough for my needs.
 
Thank you! While the pack is still intact, I assume?

Every time I think I'm ready to build my first pack I bump into posts with people struggling with issues like this, so I'll keep just studying a bit longer.

Yes, and best done while it's charging, at high knee. If your bms is balancing you can help it along. On my 230A pack the bulb will lower cell voltage about 5mAh a minute. Doesn't take long to bring them all into perfect balance. So you should really be doing this above 3.4Vpc if that's where your bms is set to start balancing. And that tends to be where one or two start to run ahead - they're reaching full charge before the others and starting to spike up. The idea is to limit current to the high one(s) so the others can catch up.

Go ahead and build it, ask questions along the way. (y)
 
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