Manual balance of new bank question FOLLOWUP Question

[kanelr]

Am in the middle of some upgrades which includes new batteries. I manually balanced eight new 12 V 100 amp hour lpo4 batteries to within .01, waited a day, re- checked, then put them on the inverter for their first bulk charge. At the end of the bulk the midpoint voltage, via victor bmv712, was 3% different. At one point during the charge it reached almost 6% I didn’t worry because we are far below the cut off. and I'm not putting a load on them till I get it right.
Do I manually re-balance them again? I’ll be using a high-quality power supply as the charger for this task, as opposed to the inverter/charger.
Used in p4s2 config. with xantrex sw4024. (24v)

 

[time2roll]

If the voltage is workable with little risk of over volt I would run them as is for a while. The new batteries are probably still balancing internally and may keep changing for some time.

As an alternative the low battery could be boosted to match the high battery when close to full charge. This can be done while connected in series.

The more permanent solution is to attach a balancer.

 

[rickst29]

Your concern is with the 'active' balancing and the BMS job to invoke it. Towards the end of a bulk charge with a fairly high voltage limit, the voltage of one cell will always rise in comparison to the others. This is a function of either slightly low internal resistance (creating a tendency to suck in power more quickly at the beginning and middle of charging) or a "smaller" cell, (the one capable of holdling the least amount of power before becoming full). It can be both factors.

I don't know what BMS you are using. Daly BMS, which I have on all 3 of my parallel 12v packs (because it was cheaper and I didn't know better), is pretty bad at performing "Active Balance", because the supplied current is too low. It takes forever. And, according to the BT App, my biggest BMS (200A, microcode version 2021-03) shuts down active balancing when charging is terminated on the pack as a whole. (In my opinion, it would be correct to CONTINUE pulling "balancing current" from the highest cell in order to feed the lowest, even while the main charging circuit has been disabled due to pack reaching "full voltage").

YMMV. What kind of BMS is involved, and what are your parameters for charge shut-off and balancing?

 

[kanelr]

Well some great thought out answers thanks guys! About bms- Before building and before posting this I spoke to a Victron rep with the intention of buying a battery balancer, he suggested with my particular system to start with a battery monitor, do a meticulous manual balance and then as I would be running the system with some regularity plan on doing a manual check/test once a month until I learn the behavior of this (24v) system. I am using eight 12 V batteries The new generation chinese internal bms 12v units placed 4p2s and based upon having to place the batteries in a single line, seeking short heavy equal length cables, and incorporating the factory suggested battery wiring diagram. See the before and after drawing. In the very fuzzy factory drawing The top is their suggestion - then we take that, bending it apart creating the unit that we used. (A heavy buss was used to compensate for the (orange) cross-linking but I don’t know how that affects balance.)

 

[time2roll]

Tear down and rebuilding that pack once a month is crazy. Get a balancer and check the voltage manually once a month.

Amazon $20. Shop around

 

[kanelr]

Tear down and rebuilding that pack once a month is crazy. Get a balancer and check the voltage manually once a month.

Amazon $20. Shop around

Sorry word choice may have been better I simply meant manually balance once a month and check everything being OK in the case of this one involves pulling off the terminal nuts along the buss but that’s fairly convenient

 

[kanelr]

Your concern is with the 'active' balancing and the BMS job to invoke it. Towards the end of a bulk charge with a fairly high voltage limit, the voltage of one cell will always rise in comparison to the others. This is a function of either slightly low internal resistance (creating a tendency to suck in power more quickly at the beginning and middle of charging) or a "smaller" cell, (the one capable of holdling the least amount of power before becoming full). It can be both factors.

I don't know what BMS you are using. Daly BMS, which I have on all 3 of my parallel 12v packs (because it was cheaper and I didn't know better), is pretty bad at performing "Active Balance", because the supplied current is too low. It takes forever. And, according to the BT App, my biggest BMS (200A, microcode version 2021-03) shuts down active balancing when charging is terminated on the pack as a whole. (In my opinion, it would be correct to CONTINUE pulling "balancing current" from the highest cell in order to feed the lowest, even while the main charging circuit has been disabled due to pack reaching "full voltage").

YMMV. What kind of BMS is involved, and what are your parameters for charge shut-off and balancing?

Bulk voltage 28.4 (brochure says charge at 14.2-14.6v so at 28.8- I was being cautious first charge)
absorption two hours
float voltage 27.6
equalize voltage 28.0
equalize time two hours
charging at 20 A
can charge as high as 35 brochure says 20 ideal.

 

[kanelr]

After setup snd first group bulk and a 12 hour float the mid point imbalance was 2.9%. though at one point it had reached 6% imbalance, but as we were still well below any high voltage cutoffs I just bit my nails and watched, praying desperately that the batteries which I purchased instead of a much needed implant were not self roasting into oblivion to the tune of "solly you boughtem" (ops). They them calmed down, then I did, as well. Next morning I loaded a general low impact load (lighting) for a few hours then after another eight hours mid point deviation was down to .1 and now 36 hours into first usage and live testing is reported as 0%. Was all set to have to manually balance again so happy to see it even out. Support from Power Queen the battery company has been outstanding as far as specifications, printed connection advice, followup etc. Now I have to load these puppies up pretty hard and see how they behave next couple months. Using LBX mode and the 16 watt search mode.

 

[kanelr]

WELL so there is an issue, maybe two. Today sunny so panels only to float , no load until evening lighting was the plan. then i decided to see how bad the standard AC sump pump would do just as a test. I think it did great, drew 780 watts for just about half a minute, brought voltag edown from 27.3 to 26.8 the bank stayed at 100%, Took pump back off solar until a later date. THEN as I was watchthe meters to see what it looked like through th epanels in good sun, the voltage AND the mid battery disparity shot up. volts webt above 30 a little and mid battery was 30%. i switch off the solar panel disconnect and in moments its down to ok27.7 with about a 3% mid voltage diff. .
1-The C40 charge controller is set to bulk charge around 28.4 so how is the 30v getting through?
2-does it sound like I may have a real bad single battery? I think one lagging behind to cause a huge differnce is more likely than one rinning ahead.....
3-My wiring setup has been seen by dozens of this forum now on another thread and no one has pointed out issues, could it be a mid point battery monitor connection issue?

I plan to wait until later today and individually check each unit for voltage. It may be better to do it immediately to find the scoundrel before he draws from the others if he is just lagging behind, but I can't. Ideas? I'm a lithium newbie who has read a lot this past few weeks.

 

[kanelr]

OK. Did a half day panel sort of float just to get the midpoint acting up again. it is 0% 0r just .01-.02% difference at midpoint, until about 27.25 v, then begins to move. By about 27.6 v, which is just below my charge controller float point it gets to about a difference of 2 or 3% difference (not .02 but 2%). Reminder, last time I bulked it went up to almost six percent. at above 28.2.
SO I just now separated the battery cabling to get the separate readings and the of the 8 batteries, the 4 closest to the positive were all 13.4v
and the four on the other end ere all 14.1. That's .7v difference when at the worst but in matched groups of four batteries. The serial numbers are random and not in order so its not a bad battery run. It has to be wiring. What am I missing in this? Attached is what the factory suggested, ten what we did to squeeze onto one shelf, then a photo of the actual setup. Any help is appreciated. I have lots of made up cabling so a rewire or adding doublers or jumpers isn't that bad to deal with.

 

[time2roll]

Changing the wire will not affect the the series differential. Need to manually adjust or get a balancer.

 

[kanelr]

Changing the wire will not affect the the series differential. Need to manually adjust or get a balancer.

There is no way that four are exactly the same and the other four exactly the some other voltage without some real electrical factor being involved I think. And it has begun each time with a careful balancing anyway. Intriguing

 

[time2roll]

The groups of four are same because they are parallel. Could mix two from each to reassemble.

 

[acdoctor]

I would have charged them individually to full charge ( bulk + absorb) first before paralleling and series. The problem you are fighting may be fixed.

 

[kanelr]

Agreed but why are the electrons piling up in the first 4 evenly it should not just jump to a higher voltage ocross the flip

 

[time2roll]

Agreed but why are the electrons piling up in the first 4 evenly it should not just jump to a higher voltage ocross the flip

By random chance I believe the parallel set that runs higher voltage is at a higher state of charge. So yes the electrons are filling closer to full where the low battery still has room. LFP does not allow energy to pour over the top and equalize the series battery like the old lead-acid does. This is what the balancer does.

 

[kanelr]

Well I will rotate/ alternate and re bulk. I dont have a 12 v charger to bulk
Individually. (I balance with just a power supply) should I bulk in 24 v pairs then re assemble?

 

[acdoctor]

Have you loaded the system and checked for balanced amp draw’s? Checked for voltage drops within the pack?

 

[acdoctor]

According to @kenelr you will still have the series problem but I would think it would be better that the 4 parallel.

 

[kanelr]

Hi what would be the tool and procedure for that? are there good search terms to use so i can go learn? thx