houseofancients
Solar Wizard
True, however I simply do not to stress out the BMS if it is very easily preventable
Do I have to top balance if cells are all the same voltage?
- Thread starter Rob99
- Start date
I kinda see what you’re getting at, however if you’re doing it right, the balance circuit should never see anything over 3.65 and the balancer may be attempting to adjust the other cells that need help as well. Unless you’re trying to somehow use the same balance leads to boost the cell like a terminal strip ( that would be a very bad idea). I’m thinking we’re all talking about direct temporary adjustment via the terminal bolts and alligator clips. In many cases it’s probably more productive and less stressful for a bms’s passive balancer to be disabled when there’s an active balancer added to it. I’ve seen an improvement in reduction of balance time primarily when the delta is smaller if a Heltec is used. When the delta is high, both balancers are on the same page.
houseofancients
Solar Wizard
No it is not
littleharbor2
Solar Addict
Absolutely helps. Thanks. I had never thought of using a BMS in the balancing phase. Plus, my "smart" battery charger maxes out at 10 amps so it's really no faster than the bench power supply. I will rig up a 12 volt BMS just for this purpose. Lots of people in the South Campos of San Felipe are starting to make the switch from lead acid and I find myself building batteries for some of them.
Right, so I've got both of my Seplos packs built and they power up nicely and I can connect using the Seplos battery monitor. Had a bit of a mare getting them to work together as a parallel pair.
Seplos build instructions say you have to set the DIP switches to configure master and slave but that didn't work at all!! Trawling through the battery parameters in the monitor I spotted a function switch for Autocoding and another for Parallel, both right at the bottom of the list so I guess recent additions.
Seemed likely candidates, so through a process of trial and error worked out that master has to have Autocoding ON / Parallel OFF and the slave both set to ON. This was the only combination which seemed to work and allowed me to see both as Pack00 and Pack01 in the battery monitor.
Clearly the DIP switches are now redundant.
Next stage is waiting for my installer to return later this week to connect up the batteries and commission my Sunsynk inverter.
Seplos build instructions say you have to set the DIP switches to configure master and slave but that didn't work at all!! Trawling through the battery parameters in the monitor I spotted a function switch for Autocoding and another for Parallel, both right at the bottom of the list so I guess recent additions.
Seemed likely candidates, so through a process of trial and error worked out that master has to have Autocoding ON / Parallel OFF and the slave both set to ON. This was the only combination which seemed to work and allowed me to see both as Pack00 and Pack01 in the battery monitor.
Clearly the DIP switches are now redundant.
Next stage is waiting for my installer to return later this week to connect up the batteries and commission my Sunsynk inverter.
Demille
New Member
You are lucky, you do not need to first put them all in parallel. The first time using a new setup battery You do need top balancing and absorption. Having the same voltage does not mean they have the same capacity and saturation. At 3.3V they are supposed to have+- 15% VOC, ready to go in flat charging curve. Charging a 16s at 58.4V the voltage will slowly increase following the flat curve till reaching +- 55;2V;In reality the BMS will show 100%VOC most of the time not knowing the saturation. For first charging do not set the inverter batt on Lithium( %VOC) but use the inverter voltage settings, otherwise the batt set will never reach max capacity. After the long flat curve the voltage will go up and the cells will unbalance as they have not the same saturation and capacity, some cells the volt will increase fast , others not; if nothing is done charging will stop when the first cell reaches max voltage. The end of the flat curve is the moment a good active balancercer can do its job, reducing the fast cells, increasing the slow ones, making it possible to reach 100% VOC with balanced cells. Then the set is kept at absorbtion voltage for at least a few hours. After top balancing and absorption the BMS can be set to 100% VOC and the inverter on Lithium. Now your battery system is ready for use.
littleharbor2
Solar Addict
Am I the only one who is confused here?
Me too.
Seems to be a lot of conflicting advice, although I'm less swayed by a new member than those who've been here for some time and clearly have lots of valuable experience.
Doesn't make things any easier though.
You just spent 4000 Pounds on new batteries and want to take a shortcut?
Top balance them in batches of whatever size battery you intend to build. 16 for a 48 volt battery, 8 for a 24 volt battery, etc. You have no idea of the state of charge on each cell and voltage is NOT a good indicator. Search the forum for sob stories from people who took the easy way out and destroyed their cells.
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So what gives you the impression that I want to take a shortcut? That's not what I've said. I've had lots of good advice on this thread.
Unfortunately Demille chose to reply to my original post from nearly 2 weeks ago and maybe not have considered the subsequent 40odd replies which has given me more things to think about and a couple of options to consider.
Your original post:
Just had delivery of 32 Eve 304Ah cells to build into two Seplos 280L kits. Have unpacked the cells and checked voltage on each to identify any differences and find that every one is 3.3v Because they all read 3.3 volts doesn't indicate they are anywhere near the same SOC.
Am I right in thinking that they don't need top balancing as they are all the same voltage and the inverter and BMS will just charge them when connected? That's a shortcut. A proper top balance will take close to 2 weeks with a 10 amp power supply.
I have been through loads of threads on here and through the top balancing tutorials and videos but they all seem to focus on having cells out of balance." If you haven't top balanced, they ARE out of balance.
Well unfortunately you've made the same mistake that Demille did by referring to my original post. If you'd bothered to read all the responses then you'd see that further discussions and advice from others will have given me a different view from the one I originally expressed. Isn't that the point of a forum.........to ask a question and then take on board the responses from other experienced members.
And in post 49, you said you’re still confused. I tried to help you avoid an expensive mistake. Whatever you decide to do, best of luck.
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Demille
New Member
First use, after top balancing, add absorption time. let the BMS know what is 100% VOC. I know this from experience and it took me a year to find out why my batteries, I have 2, would never charge to full capacity although showing 100%VOC. For over a year I probably used +- 50-75% of its full capacity. I started with putting the individual cels in parallel and started to use them after being balanced at +- 3.2V, using the inverter set on lithium and DalyBMS with canbus ( pylon protocol). Reaching 54V the BMS showed 100% VOC for BAT1 and at 53.8V for BATT 2. Knowing something was not correct it took me a lot of time to find out what was wrong.
Using LFP the first time the BMS is not able to know what is 100% VOC unless you bring the batt to 100% VOC. During the Flat curve off charging 15% to 90% VOC voltage diff is only + 0.1 to 0,15V. and BMS may show 100% VOC. Even if cells are charged individually to 3.6v this does not mean they have the same capacity and saturation. A cell charged to 3.6V and 100% VOC may show +- 3.55 V when discharging starts. This means this cell has 100% VOC at 3.55V and I used this voltage for the absorbtion voltage. A weak cell may drop to 3.5V or much less the moment of decharging. To bring the BATT to its max capacity, 100% VOC, all the cells, absorption is needed to bring all cells to 100%VOC. In total batt capacity it will not make a big difference, mayby +- 2-5% VOC, but it will make a big difference next cycle as weak cells will stay balanced during the flatt curve of the battery cycle. Top Balancing the battery cells (voltage) will not bring the battery to 100% VOC but it will prevent a weak cell will bring the total capacity down ( still showing 100% VOC) or forces the BMS to stop charging due to max cell voltage of one cell. Charging cells individualy to 3.6V should give a batt set at 57.8V, 100%VOC. What will be 100% VOC off this batt the moment of decharging? My advise is let the BMS know what is 100% VOC for your battery.
littleharbor2
Solar Addict
I'm still confused. What is VOC ? I know what SOC is when talking about Batteries. Never seen VOC used in this context. Batteries in a system won't be in an open circuit condition.
Demille
New Member
Sorry, I meant % SOC. State of charge, I used Value or Volume of charge. I understand VOC is used for solar panels, for max Voltage open circuit.
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Rednecktek
Solar Wizard
Aaahhh, yeah terminology can really throw a wrench in the works, especially when asking for help. Just don't say the word "ground" and you'll probably be fine. ?
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