Batteries going out of balance while charging.

[craig777]

So I recently upgraded my batteries to lithium from the old lead acids I had. I've got 2 12v 120ah batteries connected in series with an active balancer between them. according to the battery specs you can connect a maximum of 4 in series and 10 in parallel. Recommended charge voltage is 14.4 bulk, 14 float. I assumed you'd just double that to get to the 24v settings. When charging though the one stopped at 13.6 and the other one kept climbing. I eventually stopped it when the one battery reached 14.5, I didn't want to risk damaging the battery hoping the BMS would kick in. I've since changed the settings to charge at 27.6v which means the one reaches 13.6 and the other one then sits at around 14v. My concern is that the BMS supposedly balances the cells at 14.4v, so if the one doesn't reach that does that mean its cells are always going to be out of balance? About an hour after charging stops the batteries are back in balance with each other at around 13.2v.
Do I have a problem with one of my batteries?

 

[squowse]

most bms are set to start balancing the cells at 3.4V (adjustable). So that would be below 13.2V. many use much less than the maximum charge voltage of 14.4 to avoid cell voltage climbing so high. I use 13.9V and then 13.3V float. Batteries will never be perfectly matched and unfortunately you're going to have to use settings to suit the "worst" one.

 

[craig777]

most bms are set to start balancing the cells at 3.4V (adjustable). So that would be below 13.2V. many use much less than the maximum charge voltage of 14.4 to avoid cell voltage climbing so high. I use 13.9V and then 13.3V float. Batteries will never be perfectly matched and unfortunately you're going to have to use settings to suit the "worst" one.

Thanks for your response. Hopefully mine will then eventually balance themselves out to a closer range. I thought it was odd that the manufacturer would recommend such a high charging voltage. I'm okay with my batteries not being fully charged, my initial plan was to charge to around 80% to extend their lifespan but I was the worried if they didn't reach their balance voltage that that would harm them. I'm waiting on one of those battery monitors off amazon, I guess then I'll have a better sense of the charge they're holding and adjust the charging voltages accordingly

 

[rickst29]

Are the (presumably identical) BMS units within the "12v" battery packs programmable, or even viewable via cellphone/bluetooth?

The "going high, had to stop it by hand" battery pack BMS could properly be allowing 3.65 max per cell, which could "reach" 14.6V if the cells remained in good balance via the "active balancer". But the "stopping at 13.6V total" pack as an average cell voltage of only 3.4V, and the balancing "start voltage" might be at or above that voltage within the BMS, causing balancing to become active for only a very short time before the charging CMOS is disconnected.

Depending on the kind of BMS it has in it, it most likely contains one "weak cell" going to high voltage (the maximum per-cell voltage of the BMS) before the BMS has had any significant time to perform cell-by-cell balancing. Many BMS units, including nearly all Daly "Smart" BMS units, provide for only passive, low-current balancing between cells. On my two small home-built 12v packs with Daly "Smart BMS", It takes multiple days for the cells to fully balance after a charging cycle, and balancing can only take place at voltages where charging is still allowed.

13.4V total voltage on the "12v"battery pack is about 90% SOC (it/s more than 80%), but I "feel" that storing at that voltage should create no significant issues in total lifespan. Before days of balancing, the individual "weakest cell" is at higher voltage, and it's somewhat in trouble to begin with.
- - -
Are you always going to use the 2 battery packs in Series, as a "24V" battery bank?, In that case, I would remove the BMS from each "12v" pack and re-wire into a single 8-cell pack, electrically speaking. (You can keep 4 cells in each separate case, but instead of being managed by 2 "4S" BMS units it would be managed by one better "8S" unit. The interconnection between the cases would ALWAYS need to remain present, and they would always need to be adjancet to the BMS.)

Upon removal of the current BMS leads, they would be replaced by 9 leads attaching the end terminals and bus bars in sequence (at least 4 being "quite long" to reach into the second pack - presumably bundled within a single pack-to-pack "balancing leads cable"). The inter-pack cable is associated with a balancing lead on the "highest cell" of the first battery. If you want to do this, I strongly recommend this BMS (JK-B2A8S20P, in either the heated or unheated version) https://www.aliexpress.com/item/3256804034358347.html. (Disclosure: I received the heated version for free, in exchange for comprehensive testing which resulting in the test report here. I am not associated with the manufacturer or any selling vendor in any other way, and receive no benefit from sales of this unit by anyone.)

That BMS is cellphone/bluetooth viewable and configurable, it also has active balance current of up to 2A continuous. It would solve your issues, and turn you current "balancer" into a spare part.

However, your current single pack refusing to be charged above 13.6 total volts DOES indicate a likely problem with either one "weak" cell, or with the BMS. If the cells are not > 133Ah each, getting only 90% of "full" rated Voltage probably means that your pack can't truly have a working capacity of 120ah.

 

[mikefitz]

The balance at 14.4 volts is, I think, a mistake in the battery data. Most BMS start balance at 3.4 volts a cell, 13.6 volts per battery.
The fact that the BMS is shutting off the charge path on one of the batteries at a lower volts than recommended is not unusual with low cost batteries.
Reducing the charge voltage to a low value, fractionly less than the trip volts, and charge cycle both batteries as 12v batteries may improve the balance. The active balance in the series application will be causing more issues.

Mike

 

[craig777]

Are the (presumably identical) BMS units within the "12v" battery packs programmable, or even viewable via cellphone/bluetooth?

The "going high, had to stop it by hand" battery pack BMS could properly be allowing 3.65 max per cell, which could "reach" 14.6V if the cells remained in good balance via the "active balancer". But the "stopping at 13.6V total" pack as an average cell voltage of only 3.4V, and the balancing "start voltage" might be at or above that voltage within the BMS, causing balancing to become active for only a very short time before the charging CMOS is disconnected.

Depending on the kind of BMS it has in it, it most likely contains one "weak cell" going to high voltage (the maximum per-cell voltage of the BMS) before the BMS has had any significant time to perform cell-by-cell balancing. Many BMS units, including nearly all Daly "Smart" BMS units, provide for only passive, low-current balancing between cells. On my two small home-built 12v packs with Daly "Smart BMS", It takes multiple days for the cells to fully balance after a charging cycle, and balancing can only take place at voltages where charging is still allowed.

13.4V total voltage on the "12v"battery pack is about 90% SOC (it/s more than 80%), but I "feel" that storing at that voltage should create no significant issues in total lifespan. Before days of balancing, the individual "weakest cell" is at higher voltage, and it's somewhat in trouble to begin with.
- - -
Are you always going to use the 2 battery packs in Series, as a "24V" battery bank?, In that case, I would remove the BMS from each "12v" pack and re-wire into a single 8-cell pack, electrically speaking. (You can keep 4 cells in each separate case, but instead of being managed by 2 "4S" BMS units it would be managed by one better "8S" unit. The interconnection between the cases would ALWAYS need to remain present, and they would always need to be adjancet to the BMS.)

Upon removal of the current BMS leads, they would be replaced by 9 leads attaching the end terminals and bus bars in sequence (at least 4 being "quite long" to reach into the second pack - presumably bundled within a single pack-to-pack "balancing leads cable"). The inter-pack cable is associated with a balancing lead on the "highest cell" of the first battery. If you want to do this, I strongly recommend this BMS (JK-B2A8S20P, in either the heated or unheated version) https://www.aliexpress.com/item/3256804034358347.html. (Disclosure: I received the heated version for free, in exchange for comprehensive testing which resulting in the test report here. I am not associated with the manufacturer or any selling vendor in any other way, and receive no benefit from sales of this unit by anyone.)

That BMS is cellphone/bluetooth viewable and configurable, it also has active balance current of up to 2A continuous. It would solve your issues, and turn you current "balancer" into a spare part.

However, your current single pack refusing to be charged above 13.6 total volts DOES indicate a likely problem with either one "weak" cell, or with the BMS. If the cells are not > 133Ah each, getting only 90% of "full" rated Voltage probably means that your pack can't truly have a working capacity of 120ah.

Thanks for your detailed response.
No, the BMS isn't programmable or accessible via bluetooth or anything like that. They're under warranty for 24 months so I'm not keen on opening them up just yet and voiding that. After the 24 months I may play around with what you're recommending though.
I'll see how it goes for a while and maybe get it swapped out if it is a weak cell, problem is they're waiting on a shipment of stock so repairs are basically not happening at the moment.
Today though I did notice the weaker battery reached 13.8 so maybe the bms just uses an incredibly weak balancer that'll take some time

 

[rickst29]

Thanks for your detailed response.

.... Today though I did notice the weaker battery reached 13.8 so maybe the bms just uses an incredibly weak balancer that'll take some time

Yes, your understanding is EXACTLY correct. Low balancing current (and power transfer) is a common issue with many "cheaper" BMS units, and also with some otherwise pretty capable instances of Daly "smart" BMS I own two Daly units, in use on two smaller packs (home-built) on smaller battery packs which run in parallel with the larger pack (using the superior JK BMS). Following a charging cycle (or within a continuous cycle), my Daly managed packs require 2-3 days to reach a relativley blanaced state of cell voltages.
- - -
Te currently shown "13.8 total" limit (an average of only 3.45 per cell) probably indicates that the weakest cell risen has risen to the limit (likely 3.65 Volts). With the charge circuit "shut down"at that total voltage, the other 3 cells are only charge to an average of 3.40 volts each. There might be about 0.6 Amp-Hours of capacity left "unfilled" in those cells, but the overall difference is negligible and probably unimportant: 13.8V is already about 99.5% charged (on average), and most of us don't like to keep batteries charged that high under any circumstances.
- - -
The fact that the "weak" cell has reached the voltage limit early, at 100.0% SOC, will reduce it's lifespan if it is pushed that high on a regular basis. I would reduce the "float voltage of your charger to 26.9 Volts or less, corresponding to an average SOC of about 96.0% among all the cells.

The "14.0" float voltage recommendation (on each 12v pack) is IMO a crazy-high number, chosen with poor knowledge concerning LFP battery cells. My 26.9 recommendation corresponds to only 13.45 Volts on each pack, 96% average SOC. Squowse's setup, (post 2) stops at 80% SOC, increasing lifespan even more.

 

[craig777]

The fact that the "weak" cell has reached the voltage limit early, at 100.0% SOC, will reduce it's lifespan if it is pushed that high on a regular basis. I would reduce the "float voltage of your charger to 26.9 Volts or less, corresponding to an average SOC of about 96.0% among all the cells.

The "14.0" float voltage recommendation (on each 12v pack) is IMO a crazy-high number, chosen with poor knowledge concerning LFP battery cells. My 26.9 recommendation corresponds to only 13.45 Volts on each pack, 96% average SOC. Squowse's setup, (post 2) stops at 80% SOC, increasing lifespan even more.

Thanks so much, I'll change it as soon as I'm back home. I appreciate your advice.

 

[squowse]

Thanks so much, I'll change it as soon as I'm back home. I appreciate your advice.

I find 13.9 takes me to 100% or near as dammit. Obviously varies battery to battery. If all the cells are perfectly balanced with no "runners" you might be able to extend the capacity, but I am getting the rated capacity out of the cells or more.

 

[rickst29]

I find 13.9 takes me to 100% or near as dammit. Obviously varies battery to battery. If all the cells are perfectly balanced with no "runners" you might be able to extend the capacity, but I am getting the rated capacity out of the cells or more.

Yes, 13.9 is virtually 100% SOC. (A big "like" above for your phrase, "near as dammit" .) My post #7 presented 96% as the result of your charger stopping at "Float Voltage" on the basis of float voltage being reached. Some chargers, apparently including yours, stop "bulk charging" only on the basis of a time limit - rather than using either "float voltage reached" or "maximum bulk time reached" as alternative termination conditions.

 

[time2roll]

So I recently upgraded my batteries to lithium from the old lead acids I had. I've got 2 12v 120ah batteries connected in series with an active balancer between them. according to the battery specs you can connect a maximum of 4 in series and 10 in parallel. Recommended charge voltage is 14.4 bulk, 14 float. I assumed you'd just double that to get to the 24v settings. When charging though the one stopped at 13.6 and the other one kept climbing. I eventually stopped it when the one battery reached 14.5, I didn't want to risk damaging the battery hoping the BMS would kick in. I've since changed the settings to charge at 27.6v which means the one reaches 13.6 and the other one then sits at around 14v. My concern is that the BMS supposedly balances the cells at 14.4v, so if the one doesn't reach that does that mean its cells are always going to be out of balance? About an hour after charging stops the batteries are back in balance with each other at around 13.2v.
Do I have a problem with one of my batteries?

Yes 24v charging specs are exactly 2x 12v specs.

Hard to know what is going on unless the cell voltages are posted.

I do find it odd that while in series one battery stops at 13.6 as the other continues to climb. Would seem like they are in parallel.

 

[HRTKD]

I do find it odd that while in series one battery stops at 13.6 as the other continues to climb. Would seem like they are in parallel.

Exactly. If the BMS is working properly and shuts off the charge, then the other BMS/battery should see no charge. If that isn't what is happening then the BMS is a problem or the build is wrong.

 

[craig777]

Exactly. If the BMS is working properly and shuts off the charge, then the other BMS/battery should see no charge. If that isn't what is happening then the BMS is a problem or the build is wrong.

Yeah, I don't think the BMS is of the best quality. The company I bought them from is an established and trusted lithium battery supplier which is why I went with them. However, since I purchased these I have found out that they source these from a different manufacturer in china and they're already on their second "model" because of problems with the first one. I don't think they've worked out the kinks yet. Hopefully if I can get them to last a year or so then the warranty swap out will be with ones that have a better build quality, hopefully.

Having said that, whilst charging the voltages are now within 0.1v of each other. In standby they're exactly the same

 

[GSXR1000]

run them to line 50% charge then put them in parallel and charge to 14.2-14.4v.. I used a regular car charger

 

[Roswell Bob]

Can you use two chargers? Scrap the 24voltish charger for two 12voltish chargers.

 

[GSXR1000]

Can you use two chargers? Scrap the 24voltish charger for two 12voltish chargers.

yes while they are in parallel but no way in series

 

[rickst29]

What is the series "balancer" being used on the two packs? If you don't have one, this device might help: https://www.amazon.com/Mazava-Battery-Equalizer-Balancer-Balancer12V/dp/B09CYGCHZH

 

[GSXR1000]

What is the series "balancer" being used on the two packs? If you don't have one, this device might help: https://www.amazon.com/Mazava-Battery-Equalizer-Balancer-Balancer12V/dp/B09CYGCHZH

I would say no, this will balance at mid and lower state of charge also putting the batteries out of balance

 

[rickst29]

I would say no, this will balance at mid and lower state of charge also putting the batteries out of balance

I'm confused: Why is it NOT desirable to have the two packs in balanced with each other, while supposedly good BMS units (and one of them apparently isn't good) try to keep the 4 cells of each pack in balance with each other? AFAIK, that goes for lower SOC as well as high SOC.

 

[GSXR1000]

I'm confused: Why is it NOT desirable to have the two packs in balanced with each other, while supposedly good BMS units (and one of them apparently isn't good) try to keep the 4 cells of each pack in balance with each other? AFAIK, that goes for lower SOC as well as high SOC.

it is awesome to have the balanced at the top of the charge.. that's why it is called top balance.. this is for Lifepo battery only. but not good below that so BMS don't start balancing till 3.2-3.3v per cell or on this case 12.8v-13.2v before you balance