diy solar

diy solar

Top / Botton Balance question

skiffmaster

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Joined
Sep 27, 2022
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Hello all, I am brand new here and to Lifepo4. The thing that got me into this emerging technology is that my son is an avid Bow fisherman. This sport requires many LED lights that shine into the water at night illuminating the bottom and the fish. Also utilized is a 24 v trolling motor that runs constantly. Both lights and electric motor use a lot of power and run for 4 to 6 hours per outing. Lead acid just wasnt cutting it, so I recently purchased (16) 3.2 v 100 AH CALB Lifepo4 cells. I used 8 of the cells to make 2 - 4s 12v 100 ah batteries each with a Daly 100ah (nonbluetooth) BMS, These two batteries I sent to the same feed to give me 200 ah of 12v. With the other 8 cells I put together an 8s 24v 100 ah battery for the trolling motor. For this battery I purchased a really nice bluetooth 100ah BMS from Overkill Solar. All 3 BMS came pre programmed for the voltages and applications and I have made no changes to them. I have an onboard charger wired in for each battery so that on plug in gets them going. I have read enough about this technology to confidently say that I am a bonified NOVICE and I'm still trying to get my head around some things. I tried to use best practices building these but due to some time constraints I did not top or bottom balance the cells. I read in a few places that if the cells you purchase were quality same batch cells that tested close at the factory (which is what I have) that the BMS would balance the cells for you over time. With the time constraints I decided to let this part go and went ahead with the build. That was a week ago and my son has used the batteries 3 times with great success. So much so that I havent been able to charge them up fully until today. The Dalys are (non bluetooth) so I have no idea whats going on in those cells. The Overkill however gives me more information in more ways than I understand or know how to interpret (That will come later for me i hope). My question to the community refers to the screenshot here showing each of the 24v batterys cells individual charge. What is this screenshot telling me? Should I go back and top balance or bottom balance the cells or both? Will the BMS balance these cells? As far as the (nonbluetooth) Dalys go I guess we are just supposed to trust that they are doing their job? How can I know what they are doing without a monitoring system? Are there any good SOC meters for Lifepo4 that I can dash mount? Any good advice is very much ss.pngappreciated.
 
The BMS will only balance cells that are slightly out of balance, not gross deviations like yours.

See the top-balance threads here for more information on how to do it. There are a few different ways. You need to find the time to do this.

Bottom balancing is a different story, where your existing BMS will do no good, designed for top balance. It is a different technique, and your existing bms would have to be removed, since it is designed for top balance, and a new monitoring circuit to cut off the charge would have to be used. Not common to find off the shelf for the consumer.

Consult the sticky threads and more here on how it is done. Typically you will use a programmable bench charger (and KNOW how to use it so you don't damage the cells), or a simpler method would be to purchase a "single cell" LiFePo4 charger, and charge each one up individually before reassembling the cells into a battery.

A single-cell LFP charger is a nominal 3.2v type. One example would be to consult the chart here on Batteryspace for 3.2v, and pick your desired charge rate:


Welcome to the fun!
 
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Your BMS shows two cells are quite a bit over the recomended 3.65 volts per cell. I would attempt to discharge the pack until they get below 3.6 volts. They then should settle around 3.33 volts per cell. I do not know how you got there but you should check the settings on the Overkill BMS. Perhaps it was set up for a different chemistry than LFP (LiFePO). Welcome to the forum As mentioned earlier it may take a while for the balancing function to get them top balanced.
 
Someone was charging battery pack before they hooked up BMS. ?

Cells 1 & 5 are overcharged.

Better bring them down soon.
 
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OK... so two cells on the 24 v are especially out of whack and the other 8 on the two 12 v I have no way of knowing. So I will discharge all 16 cells to around 3.33. Then I'll probably charge a cell at a time to top balance because IMO I'm simply too inexperienced to do it any other way. There was no charger attached to the cells before BMS. The cells did sit in storage for 1 year after purchase however. Maybe that is the culprit
 
So I will discharge all 16 cells to around 3.33.
3.33 is the resting voltage so it is impractical to think you can discharge them to 3.33 volts. They will slowly decline to 3.2 volts and so on as your discharge them, but the curve is very flat so voltage will not tell you much at that part of the curve. If you discharged them to under 3.0 volts you may see the cell with the least capacity declining faster.
 
That the cells are well matched or have the same voltage has little to do with whether or not they need to be balanced. They _always_ need to be balanced. For our uses, always top, and never bottom.

You have an overkill solar BMS. The overkill manual has one of the better explanations of this. Section 2.3 explains how, and Appendix C explains why. I suggest against doing it one cell at a time. Follow the directions in the Overkill manual, and buy the required equipment if you don't have it. There are countless examples of people taking shortcuts and ruining their cells.
 
Someone was charging battery pack before they hooked up BMS. ?

Cells 1 & 5 are overcharged.

Better bring them down soon.
Or they didn't read the directions and the BMS isn't setup properly. The overvoltage might be set wrong.
 
Bring up the parameter settings page, and screenshot that here.
The BMS should have disconnected charging a LONG time before those two cells hit 3.8V…
My guess is you have LiNMC BMS settings and need to fix the parameters…
But, yes, hook up a load and get the cells below 3.6V asap.
 
Thank you all for helping me here. I am discharging as we speak. This was their first full charge after manufacture and its been less than 24 hours so maybe getting them down now I have avoided any harm to the cells. Can anyone suggest a way for me to discharge them to a common SOC? Right now I'm just running the motor and lights to get things out of the danger zone but what should I do next ( to all 16 cells)?
 
Bring up the parameter settings page, and screenshot that here.
The BMS should have disconnected charging a LONG time before those two cells hit 3.8V…
My guess is you have LiNMC BMS settings and need to fix the parameters…
But, yes, hook up a load and get the cells below 3.6V asap.
ok, hold on
 
My apologies but I have to turn in and pick this back up tomorrow. I will try and come back with more info from the BMS and get myself a little bit more educated on top balancing in the mean time. Thanks again.
 
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