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Balanced Charging

pacard17

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Nov 18, 2021
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hello! need some help from the brain trust!

built a 100ahr lifepo w a jbd bms. using xiaoxiang bms v3.1 1026. thoroughly top balanced the cells to 3.65v. when i put it in use, i noticed that cell 3 was charging faster and discharging faster. meanwhile cell 1 wasnt fully charging. i think this was due to cell 3 hitting charge limits and disabling charging.

so i disassembled it and top balanced the battery again. then i swapped the position of cell 1 & 3. now, cell 1 is overcharging and cell 3 isnt getting to full charge. is this symptomatic of a bad bms? anyway to fix it? or should i just buy another?
 
BMS is just measuring and monitoring the cells. BMS does not control the charge of individual cells.
Most likely this is a cell issue.
 
Yes the BMS does do balancing. However the balancing is very slow and done over an extended period of many charge cycles.
With everything brought up to 3.65 in parallel (twice) the cells should not need balancing. They are balanced and would seem to be of variable condition and capacity.

How close are the cells? Within 30mV at the top is fine. Just limit charging voltage. Going to 3.65vpc at each charge cycle is unrealistic. 3.45vpc is fine. Then run them down to 2.9vpc and post the delta.
 
hmm. thx for the responses. im watching the bms balance rather quickly. so for instance, one cell was at 3.55v and the other was at 3.65v. within an hour, theyre both within .015v. this is actually better than it was.

have also noticed it wont balance while charging. once i disable charging, balancing kicks in.
 
Charging and balancing are really 2 different issues. One balanced, healthy cells will charge (and stay balanced) without active balancing and only slowly drift over time.

For example, I have a large powerwall with 112 different packs. Once balanced, the packs don't need balancing day by day... and in fact I only do touch-up balancing every 6-9 months because that's all that's needed, my BMS let's me choose, and it reduces wear on the Batrium longmons.

I'm not saying you shouldn't use BMS balancing if you're BMS has it built-in, I'm just saying that every day balancing should not be critical for healthy cells once they are balanced - and you should be able to charge without balancing.
 
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have also noticed it wont balance while charging. once i disable charging, balancing kicks in.
That may not be the actual balancing as much as just settling to resting voltage in which they will naturally be closer in voltage.
 
the balancing icon turns on so thats why i assumed it was balancing thx
 
jbd bms. not sure of the model number. connecting to it w xiaoxiang v3.1 1026 build.
 
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hello! need some help from the brain trust!

built a 100ahr lifepo w a jbd bms. using xiaoxiang bms v3.1 1026. thoroughly top balanced the cells to 3.65v. when i put it in use, i noticed that cell 3 was charging faster and discharging faster. meanwhile cell 1 wasnt fully charging. i think this was due to cell 3 hitting charge limits and disabling charging.

so i disassembled it and top balanced the battery again. then i swapped the position of cell 1 & 3. now, cell 1 is overcharging and cell 3 isnt getting to full charge. is this symptomatic of a bad bms? anyway to fix it? or should i just buy another?

Sometimes running a battery cells through a few full charge discharge cycles will improve the cells balance. Suggest trying this first.

Typically once the cells are top balanced at 3.65 volts don't charge the cells past 3.5 to 3.525 volts and some charge up to just 3.45 volts or less.

Take a look at the Overkill Solar BMS manual. The Overkill Solar is a JBD BMS and the manual is well written and should help.

Also check out the web site for BMS apps etc.


The Overkill Solar BMS (JBD) is often set to balance when charging while cell voltage is above 3.4 volts. The problem with the JBD BMS the balancing current is small and my not be enough to balance some grade B cells. If that is the case an active balancer like the Heltec 5A will usually balance the cells when enabled while cells are 3.4 volts and above.

Also be sure to check all the wiring for a bad connection(s) as that is often the problem, although swapping the batteries probably ruled that out.

Finally maybe this will help. I have used it to blead down runner cells.

A 2 ohm 100 watt resister works good for a single runner cell dissipating close to 2 amps at 3.65 volts.

First ensure all connections are good on each cell of the battery.

Wire an alligator clip to each end of the 2 ohm resistor. Charge entire battery at a low current and clip the alligator clips on the 2 studs of the runner cell observed on the BMS. Watch the runner cell bleed down on the BMS. Repeat as necessary on each runner cell while slowly increasing voltage up to around to 3.60 or so volts for each cell as read on the BMS until all cells are balanced at a vey low current.

As long as the BMS is connected and set for 3.65 volt per cell high voltage disconnect this method should safely top balance a battery with the cells wired in series.

2 ohm 100 watt resister link.

https://www.amazon.com/dp/B07CVRH6DF
 

Attachments

  • Overkill_Solar_BMS_Instruction_Manual.pdf
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Sometimes running a battery cells through a few full charge discharge cycles will improve the cells balance. Suggest trying this first.

Typically once the cells are top balanced at 3.65 volts don't charge the cells past 3.5 to 3.525 volts and some charge up to just 3.45 volts or less.

Take a look at the Overkill Solar BMS manual. The Overkill Solar is a JBD BMS and the manual is well written and should help.

Also check out the web site for BMS apps etc.


The Overkill Solar BMS (JBD) is often set to balance when charging while cell voltage is above 3.4 volts. The problem with the JBD BMS the balancing current is small and my not be enough to balance some grade B cells. If that is the case an active balancer like the Heltec 5A will usually balance the cells when enabled while cells are 3.4 volts and above.

Also be sure to check all the wiring for a bad connection(s) as that is often the problem, although swapping the batteries probably ruled that out.

Finally maybe this will help. I have used it to blead down runner cells.

A 2 ohm 100 watt resister works good for a single runner cell dissipating close to 2 amps at 3.65 volts.

First ensure all connections are good on each cell of the battery.

Wire an alligator clip to each end of the 2 ohm resistor. Charge entire battery at a low current and clip the alligator clips on the 2 studs of the runner cell observed on the BMS. Watch the runner cell bleed down on the BMS. Repeat as necessary on each runner cell while slowly increasing voltage up to around to 3.60 or so volts for each cell as read on the BMS until all cells are balanced at a vey low current.

As long as the BMS is connected and set for 3.65 volt per cell high voltage disconnect this method should safely top balance a battery with the cells wired in series.

2 ohm 100 watt resister link.

https://www.amazon.com/dp/B07CVRH6DF
All good advice but his runner cell is moving? Sounds like something is up with position 3, not cell number 3.
 
Sometimes running a battery cells through a few full charge discharge cycles will improve the cells balance. Suggest trying this first.

Typically once the cells are top balanced at 3.65 volts don't charge the cells past 3.5 to 3.525 volts and some charge up to just 3.45 volts or less.

Take a look at the Overkill Solar BMS manual. The Overkill Solar is a JBD BMS and the manual is well written and should help.

Also check out the web site for BMS apps etc.


The Overkill Solar BMS (JBD) is often set to balance when charging while cell voltage is above 3.4 volts. The problem with the JBD BMS the balancing current is small and my not be enough to balance some grade B cells. If that is the case an active balancer like the Heltec 5A will usually balance the cells when enabled while cells are 3.4 volts and above.

Also be sure to check all the wiring for a bad connection(s) as that is often the problem, although swapping the batteries probably ruled that out.

Finally maybe this will help. I have used it to blead down runner cells.

A 2 ohm 100 watt resister works good for a single runner cell dissipating close to 2 amps at 3.65 volts.

First ensure all connections are good on each cell of the battery.

Wire an alligator clip to each end of the 2 ohm resistor. Charge entire battery at a low current and clip the alligator clips on the 2 studs of the runner cell observed on the BMS. Watch the runner cell bleed down on the BMS. Repeat as necessary on each runner cell while slowly increasing voltage up to around to 3.60 or so volts for each cell as read on the BMS until all cells are balanced at a vey low current.

As long as the BMS is connected and set for 3.65 volt per cell high voltage disconnect this method should safely top balance a battery with the cells wired in series.

2 ohm 100 watt resister link.

https://www.amazon.com/dp/B07CVRH6DF
thank you for the manual! so glad there's one out there. when i disassembled the battery to top balance again, i tested the bms connections and they were solid. ea connection is dual crimped from both directions and heat shrinked w an adhesive lined connection. will definitely try a few cycles before moving to the next step.

what's interesting to me is that the runner cell is in the same location in the pack even after i switched the position of the cells. that's why i was suspecting something was up w the bms.

unfortunately, i didnt take a pic of the connections and they are now covered w electrical tape. my batteries came w wired leads. so they are bolted together along w the bms leads.

appreciate everyone's input. much appreciated.
 
740GLE said:
All good advice but his runner cell is moving? Sounds like something is up with position 3, not cell number 3.

my thought too.

I missed that part thanks for the correction.

It would seem there are a few options and probably more.

The first is to check the wires on the cell with problems. Measure the voltage directly on the cell in question with a good quality voltmeter.
Then (being careful not to short) measure the voltage on the connector like Will does in this video starting at 1420 seconds. If there is much difference between the voltage at the cell and the connector suspect a bad wire. If there is a difference check the rest of the voltages on the cells and connector for comparison. Also check for a bent pin on the connector where it plugs into the BMS. Nice video Will did on the BMS.


If the wire checks out there is a section in the overkill manual on how to do a voltage calibration on each channel of the BMS. Again a good quality (and accurate) voltmeter is needed.

Also cleaning connections with 99.9% isopropyl alcohol is something I always do before making the connection.
 
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