Ok you win... Cell imbalance

[RCinFLA]

I suppose the other option would be to lower my charge voltage from 13.8 to 13.6... The cells seem to track to a 0.00x Delta until just above 3.4 volts per cell

That would make the imbalance grow over time as most BMS's do not balance a cell until its cell voltage rises above 3.4v.

Give the BMS some balancing time by holding the absorb voltage to 14.2 to 14.4v for several hours.

 

[ericfx1984]

That would make the imbalance grow over time as most BMS's do not balance a cell until its cell voltage rises above 3.4v.

Give the BMS some balancing time by holding the absorb voltage to 14.2 to 14.4v for several hours.

Yeah there really is no reason to balance the cells until you hit 3.4 volts per cell... The problem is that even if you are wildly different in state of charge you could show virtually identical voltages... And it's not really until about 3.4 volts that the cell voltages will be even remotely different

 

[fafrd]

That would make the imbalance grow over time as most BMS's do not balance a cell until its cell voltage rises above 3.4v.

Give the BMS some balancing time by holding the absorb voltage to 14.2 to 14.4v for several hours.

Do the math on how many hours it will take the passive balancing of a BMS to balance out 1% of excess charge on one 1216Ah cellblock (or even a single 304Ah cell)…

 

[fafrd]

Yeah there really is no reason to balance the cells until you hit 3.4 volts per cell... The problem is that even if you are wildly different in state of charge you could show virtually identical voltages... And it's not really until about 3.4 volts that the cell voltages will be even remotely different

When your battery spends much more time in the lower knee near full depletion than in the upper knee near full charge, that is when a bottom-balanced battery and an active balancer make much more sense…

Top-balancing a battery that never gets close to fully charged makes no sense at all.

 

[ericfx1984]

Do the math on how many hours it will take the passive balancing of a BMS to balance out 1% of excess charge on one 1216Ah cellblock (or even a single 304Ah cell)…

I taught balanced these a year and a half ago before I put them in this configuration

And then about a week ago I brought them up to around a little over 3.4 volts per cell... Then I fed them with power supply to get them to 3.45 volts per cell and ran the active balancer (5a)

Eventually I was able to get them to 3.65 volts per cell and I held them there with the active balance are going for well over 8 hours...

At that point I took the active balancer off and everything showed a balance of around 0.002v. That's pretty darn close... But after a cycle or two the problem was still there

 

[MisterSandals]

At that point I took the active balancer off

Is there a reason you removed the active balancer?

 

[fafrd]

I taught balanced these a year and a half ago before I put them in this configuration

And then about a week ago I brought them up to around a little over 3.4 volts per cell... Then I fed them with power supply to get them to 3.45 volts per cell and ran the active balancer (5a)

Eventually I was able to get them to 3.65 volts per cell and I held them there with the active balance are going for well over 8 hours...

At that point I took the active balancer off and everything showed a balance of around 0.002v. That's pretty darn close... But after a cycle or two the problem was still there

3.363V versus 3.365V is ‘pretty darn close’.

3.373V versus 3.375V is not even in the ballpark of being ‘pretty darn close (0.5% SOC delta).

Also, these voltages we are talking about need to be measured when the cells are not being charged or discharged and have been allowed to settle for at least 15 minutes..,

 

[ericfx1984]

Is there a reason you removed the active balancer?

Yeah a lot of people say that these cheap active balancers should not be used unless you're babysitting them

They also suggest they should only be used for maintenance... Not for permanent installation

 

[fafrd]

Is there a reason you removed the active balancer?

If you leave an active balancer attached to a battery that spends much more time fully depleted than fully charged, the result will be a bottom-balanced battery.

The lowest-capacity cell will still be a ‘runner’ but now you can set boost voltage based on a sensible rationale.

For example: 3 cells at 3.35V and one cell at 3.60V = 13.65V.

 

[MisterSandals]

Yeah a lot of people say that these cheap active balancers should not be used unless you're babysitting them

Well, when people use cheap electronics of any kind they should rethink what they are trying to accomplish vs the risks.

I think there are a lot of folks who swear by decent balancers. I have a Heltec running 24/7/365 in my RV that is there just in case. I have no idea how much it does. Things ain't broke so i ain't fixin it.

If you leave an active balancer attached to a battery that spends much more time fully depleted than fully charged, the result will be a bottom-balanced battery.

Weren't you just recommending this? (asking because thats how i read this

When your battery spends much more time in the lower knee near full depletion than in the upper knee near full charge, that is when a bottom-balanced battery and an active balancer make much more sense…

 

[fafrd]

Yeah a lot of people say that these cheap active balancers should not be used unless you're babysitting them

They also suggest they should only be used for maintenance... Not for permanent installation

To maintain a top-balanced battery, that’s probably true (at least you need to rig up something to control the active balancer to only be on near the upper knee).

But for a bottom-balanced battery, the opposite is trie.

I’d been connecting my active balancer before bed and disconnecting it in the morning but have left it connected and running 24/7 for 3 weeks now.

My highest and lowest cell had a 69mV delta when I started (3.208V versus 3.277V) and has improved nightly since then.

Last night, ~20 days later the delta has dropped to 15mV (3.232V versus 3.247V).

 

[fafrd]

Well, when people use cheap electronics of any kind they should rethink what they are trying to accomplish vs the risks.

I think there are a lot of folks who swear by decent balancers.
I have a Heltec running 24/7/365 in my RV that is there just in case. I have no idea how much it does. Things ain't broke so i ain't fixin it.

Same here (8S).

Weren't you just recommending this? (asking because thats how i read this

I am recommending to leave an active balancer attached 24/7 to any bottom balanced battery.

I’m also recommending any battery that is fully-discharged nightly but rarely fully charged daily will be more effective when it is bottom-balanced.

 

[Skypower]

I’m thinking that leaving a balancer on 24/7 is not for every circumstance. Why it works for some and horribly for others probably has to do with the pattern of use.

 

[fafrd]

I’m thinking that leaving a balancer on 24/7 is not for every circumstance. Why works for some and horribly for others probably has to do with the pattern of use.

Precisely. Leaving a top balancer on always with a top-balanced battery is a bad idea (it will rebalance the battery to be bottom-balanced if I spends more time in the lower knee than the upper knee).

As a minimum, controls should be added to only turn on the active balancer when the battery nears full charge (such as when the SCC switches from CC to CV).

 

[ericfx1984]

As a minimum, controls should be added to only turn on the active balancer when the battery nears full charge (such as when the SCC switches from CC to CV).

I need to research how to do that with the JBD BMS

 

[ericfx1984]

I’m thinking that leaving a balancer on 24/7 is not for every circumstance. Why it works for some and horribly for others probably has to do with the pattern of use.

My pattern of use varies with the seasons of course... But generally speaking I almost always reach a full charge during the day... But I'm oftentimes under load so meaning even once I hit a full charge the battery management system will shut off charge turn on charge shut off charge turn on charge and so on

So I don't spend a whole lot of time at a high state of charge... Then typically I'll have a good deal of usage in the evenings obviously way more in the summer then compared to the other months

Then the sun comes up and I do it all over again

 

[fafrd]

I need to research how to do that with the JBD BMS

Is your active balancer standalone or integrated into a JBD BMS?

If stand-alone it’s pretty easy - get a battery monitor with relay output and program the relay to turn on when battery voltage rises above the threshold you want.

If your active balancer is integrated into your JBD BMS, I don’t know whether the turn-on voltage is programmable or not.

 

[ericfx1984]

Is your active balancer standalone or integrated into a JBD BMS?

If stand-alone it’s pretty easy - get a battery monitor with relay output and program the relay to turn on when battery voltage rises above the threshold you want.

If your active balancer is integrated into your JBD BMS, I don’t know whether the turn-on voltage is programmable or not.

Okay if that's actually a really easy solution... The JBD is highly programmable... I currently have it set to start balancing at 3.4 volts per cell...

 

[fafrd]

Okay if that's actually a really easy solution... The JBD is highly programmable... I currently have it set to start balancing at 3.4 volts per cell...

OK, so with that BMS, you can only top balance - no way to bottom balance.

And the problem with that is that if you rarely charge above 3.4V / 13.6V, your balancer will never kick in and your balance will stray.

The other problem is that your balancer has a precision of 6mV.

So even if you program the balancer to start balancing at 3.375V and your battery regularly spends several hours charged above 13.5V, one cell can be at 3.381V while the others can be at 3.375V which is an 0.67% difference is SOC.

That’s about the best you can hope for. If you rarely charge above 13.6V and try to go lower, activating the balancer at 3.363V / 13.45V will only get you to a precision of 3% SOC (and it gets even worse at lower voltages).

I’ll say it one more time, trying to maintain a top-balanced battery in a system that rarely charges into the upper knee is a fool’s errand.

Leave your BMSs active balancer as is (where it will never get used), pick up a Heltec 4S Active BMS, and leave it running 24/7. Your battery will slowly become bottom-balanced and you’ll be much happier.

Top-balanced batteries are better for all those battery-based systems intended to provide backup power when the grid goes down. Those batteries rarely get discharged into the lower knee and are generally charged into the upper knee daily and held there as long as the sun is shining in CV mode.

But for systems like yours (and mine ) where the primary purpose of the battery is to capture excess solar energy generated during the day and to use that stored energy to offset loads overnight until the battery is depleted, a bottom-balanced battery makes much more sense…

[P.S. if the JBD BMS has an option you could check for ‘Bottom Balance’ which changed the meaning of ‘Balanced Turn-on Voltage’ from ‘Balance Turn-on Voltage Above’ to ‘Balance Turn On Voltage Below’ you wolf not need to purchase a stand-alone active balancer…]

 

[Aussieguy]

15amp balancer...... will get it done ?

Andy also had videos for:
you can leave balancer attached and externally switch it on/off

use a voltage sensing relay
at x volts turn on a balancer
at y volts turn off

don't know if the 15amp is a candidate for leaving it permanently attached?