How to keep Lifepo4 cells balanced without charging to 100%

[meetyg]

Hi.
I'm building a 24v 40Ah 8S Lifepo4 pack, with an 80A JBD BMS.
This will be charged with a CC/CV AC-DC Lifepo4 charger (voltage is adjustable).

I have done all the top-balancing procedures, at 3.65v, and did a capacity test on each cell separately. Each one gave slightly above 40Ah.

Anyways, now I have connected them in series, and hooked up the BMS. Cells were only at 4mv deviation from one another.

My question is: How do you keep the cells in balance after a discharge-charge cycle, if you only want to charge up to say 3.4v ?
I know some say that the cells should keep balance if they were properly top balanced, but in reality they will drift.
What parameters should I use in the JBD to allow effective balance?
Having that the charge curve is mostly flat, will it be possible to (true SOC) balance at 3.4v at all?

I have seen many of Andy's (Off grid garage) videos about balancing and active balancers etc... but I'm confused and not sure what the bottom line is? Do I have to go higher (say 3.5v) in order to balance ?
Should I keep my SOC in between the high/low "knees" of the charge curve (say 3.0v to 3.4v) ? Will that minimize the chances of getting the cell imbalanced?

Thanks.

 

[newbostonconst]

Mine balance all the time. I am at 500 ah though. Max deviation I see is .020 volts cell to cell when I change the SOC from like 20% to 100%, but I only charge to 3.45 volts dew to my set up. I am running almost a 70 volts setup.

 

[upnorthandpersonal]

Balancing at 3.4V makes no sense, so keep the balance voltage at 3.45V, or 3.5V or so. You'll charge to 27.2V or whatever as usual, and any cells that start running will get balanced.

 

[John Frum]

In order to maintain top balance you need to charge into the high knee.
How often you have to do so depends on how much your cells drift.
If your cells are decent quality and you don't discharge into the low knee or otherwise stress your cells you may get away with only charging into the high knee once a month or so.

 

[meetyg]

Balancing at 3.4V makes no sense, so keep the balance voltage at 3.45V, or 3.5V or so. You'll charge to 27.2V or whatever as usual, and any cells that start running will get balanced.

Thanks. So what you are saying is that the balance voltage should be a bit above the actual charge voltage? (but still under overvoltage of course).
I always thought that my balance voltage should have been below my charge voltage.

I will your suggestion, it seems to make sense now.

 

[Horsefly]

3.4V is still in the flat part of the charge curve, or at least just on the edge of it. There isn't much point in trying to maintain a top balance there.

If I recall, Andy's videos also showed that if you only charge to 3.4V it takes a very long time in absorption to get close to full batteries. He showed that if you just bump it up to 3.45V you can fully charge, and just that small change gives you an opportunity to have the JBD BMS do some passive balancing. Set the balance voltage to 3.4V (I think that is the default).

 

[upnorthandpersonal]

Thanks. So what you are saying is that the balance voltage should be a bit above the actual charge voltage? (but still under overvoltage of course).
I always thought that my balance voltage should have been below my charge voltage.

I will your suggestion, it seems to make sense now.

You need to put it high enough so you're in the past the knee on the higher end of the charge curve. 3.5V is a good start, sometimes 3.45 is possible, 3.55 and up is better.

By putting it above your individual cell charge voltage, you'll be able to catch the runners (the cells that are at higher state of charge compared to the others) and balance those back in line with the rest.

 

[meetyg]

Thank you all for the informative replies!
I guess I should choose the "always balance" mode on the JBD (not "balance on charge only"). Otherwise, when I charge, the BMS balance rate might not be able to compensate for the charge rate, possibly causing a cell overvoltage, and stopping charge prematurely. Does that sound correct?

 

[Horsefly]

Thank you all for the informative replies!
I guess I should choose the "always balance" mode on the JBD (not "balance on charge only"). Otherwise, when I charge, the BMS balance rate might not be able to compensate for the charge rate, possibly causing a cell overvoltage, and stopping charge prematurely. Does that sound correct?

It depends on how you use your battery. If you discharge regularly to a pretty low SoC, "always balance" will be trying to balance down at the bottom end, which can actually undo your top balance. I think "balance on charge only" is the way to go. Even though it isn't doing much on any one cycle, over time it will help.

 

[meetyg]

It depends on how you use your battery. If you discharge regularly to a pretty low SoC, "always balance" will be trying to balance down at the bottom end, which can actually undo your top balance. I think "balance on charge only" is the way to go. Even though it isn't doing much on any one cycle, over time it will help.

Yeah, but if I set starting balance voltage high enough, I would think that it won't try to balance while at low SOC.
This is different from a simple active balancer that always tries to balance, no matter what the voltage is.

 

[Horsefly]

Ok. I thought you were talking about trying to balance all the time, meaning setting the start voltage lower. So yeah, you could go ahead and disable the charge balance, and balance above 3.4V even if not charging.

The thing is, if you are not charging you won't be above 3.4V much anyway. When you stop charging, the voltage will settle to around 3.4V anyway, maybe even a bit below. But there is no harm in doing it.

 

[meetyg]

Ok. I thought you were talking about trying to balance all the time, meaning setting the start voltage lower. So yeah, you could go ahead and disable the charge balance, and balance above 3.4V even if not charging.

The thing is, if you are not charging you won't be above 3.4V much anyway. When you stop charging, the voltage will settle to around 3.4V anyway, maybe even a bit below. But there is no harm in doing it.

Charge only balance has been problematic so far while testing, but again, I was putting balance voltage lower than charge voltage, and the BMS balancer didn't keep up while charging. I will try the opposite way, and see if charge only balance works better this way.

 

[MisterSandals]

and the BMS balancer didn't keep up while charging.

Most BMSs balance in the milliamp range and most chargers charge in the amp (or 10's of amps) range. That's 1000x difference so its not surprising (to me anyway) that they often cannot keep in balance.

 

[Diesel Pro]

Even a 2 amp active balance is really only 1 amp equivalent. It's about a 50% duty cycle charging capacitors from the high cell(s) and then discharging to the low. Takes time to catch up once the cells fall out of balance.

 

[87 Dream]

Hi guys my first post here and glad to meet some very talented like minded ppl. I have built a fair few 16s LFP packs and I have incorporated active balancers in all my build with great results. Diesel Pros` explanation is 100% on. So many say the active balancer is working all the time. Actually not. It applies its Max 5A balance current at high delta, so at low delta has low amps. High delta is usually at high SoC if you bulk and float at close to 3.5V per cell or above. Any other times in the middle of the LFP curve the cells might be drifting in SoC but the Voltages are all pretty similar. I bulk and float at 55V (3.43V) and once a month do the top up of BUlk and float to 56V (3.5V).

Many ways to skin the cat. Many guys will tell you many things. I can only share my successes. More than happy to share my failures. I also use the JBD 300A for all of my installs and also have the odd Daly.

I have found and echo what work pioneers like Andy has done. I stay away from the bottom and top ends of the LFP curve. This is where the stress is encountered.

87

 

[Horsefly]

I bulk and float at 55V (3.43V) and once a month do the top up of BUlk and float to 56V (3.5V).

That's really interesting. I've never heard of anyone doing bulk and float at the same voltage. For that matter, your bulk is lower and your float is higher than most anyone I've seen post here.

So many say the active balancer is working all the time. Actually not. It applies its Max 5A balance current at high delta, so at low delta has low amps. High delta is usually at high SoC if you bulk and float at close to 3.5V per cell or above. Any other times in the middle of the LFP curve the cells might be drifting in SoC but the Voltages are all pretty similar.

I would completely agree, except some folks here like to run their packs down to below 3.1V, where the curve starts to get steeper again. If you run an active balancer on cells that get down to 3.0V or below you may have some wider imbalances, and the active balancer running all the time will attempt to correct that imbalance. Doing so at the bottom can can undo a top balance, at least over time.

I also use the JBD 300A for all of my installs and also have the odd Daly.

I don't know that I've seen a 300A JBD. I assume you are still talking about the 16S packs? Do you have a link for that BMS?

 

[87 Dream]

That's really interesting. I've never heard of anyone doing bulk and float at the same voltage. For that matter, your bulk is lower and your float is higher than most anyone I've seen post here

With the help of a Victron Smart shunt I have nailed and tracked down ah values and to be honest with a start 95% SoC (55V) and discharging all the way down to +-20% SoC (51V) I get quite a good shift out of these cells.

The stress at the bottom and top ends accounts for the last & top +-10%. It's at these values where the cells difference in resistance and drift becomes apparent. No two cells are made 100% the same no matter what. Even so the difference in the connections and busbars will attribute to make the drift possible even if the cells were direct carbon copies.

Over many many cycles the delta or drift becomes apparent.

ould completely agree, except some folks here like to run their packs down to below 3.1V, where the curve starts to get steeper again. If you run an active balancer on cells that get down to 3.0V or below you may have some wider imbalances, and the active balancer running all the time will attempt to correct that imbalance. Doing so at the bottom can can undo a top balance, at least over time

Hence I don't like to go below these values. I find zero value in doing so. Except squeezing the last 10 to 15% SoC and creating stress for little to no gain.

I will try to dig out the link, and will post a pic.

I say all that I say with a heavy hand of salt. I cycle my various packs on plants daily with a maintenance Top balance to fix the imperfections every 30days.

That's why I love the LFP DIY battery building. One never stops learning and seeing how others do things. It's very abstract.

Will dig out some pics but they are JBD Single Contactor BMS.

87

 

[John Frum]

I bulk and float at 55V (3.43V)<snip>

I take that to mean you bulk and float while the sun is up, confirm?

 

[87 Dream]

149.37US $ |Jbd Smart Bms Continuous Current 300a Automatic Identifica 7-20s With Built-in Bluetooth Free Ntc Lifepo4 Bms Board Peak 1000a - Battery Accessories & Charger Accessories - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

a.aliexpress.com

 

[Horsefly]

Hence I don't like to go below these values.

I didn't mean to imply you were. I agree with you. It's just that I fear some people here want to leave their active balancers on all the time and drive their batteries a bit harder than you do.