Problem with 72v LiFePo4

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Oazer
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Problem with 72v LiFePo4

Hi from Germany again!
(relocating my question from another forum topic as suggested),

I am new here, but i am reading in this forum for quite a while.

I purchased 2 36v 20Ah Lifepo4 batteries and connected them in series for 72v in order to use it for a x5304. The controller should work until 48amps.

I have the following problems:
- I was only able to get at most 2000 watt out of the packs, then probably one of the BMS cut off
- Out of the 20 Ah on the first ride I got only 6 Ah out with constant cut offs towards the end at decreasing watts (cut offs at around 400 watts in the end was when I stopped running the vehicle)
- Charging them did not work afterwards

I got the hint that may be some of the cells are out of balance, so i should run them empty on very low power. I did that with one of the packs seperately at 36v and it worked until a total of 14.3 Ah (which seems okay for the first charge)and charging this pack works now fine. BUT the second pack did not do anything, even the braindrain control unit showed nothing and charging is not possible.

Now: Today I let somebody check if there is still voltage with the problem pack. Result: 36.5 volts overall and all 12 cells show voltage. The whole Pack looks fine, the BMS as well and no smell whatsoever...STILL I cannot charge it or run the motor even at the lowest current.

Has anyone an idea what I can do?

Regards,

Thomas

dogman
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Re: Problem with 72v LiFePo4

Sorry, but I'm truly stumped now. Glad to hear you didn't toast the battery though.

Be the pack leader.
36 volt sla schwinn beach cruiser
36 volt lifepo4 mongoose mtb
24 volt sla + nicad EV Global

Oazer
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Re: Problem with 72v LiFePo4

Its getting even "better"... I put some 3 12v bulb in series and connected it to the pack...a big nothing! I called shops, people who might know...They all dont know...

My theory: One cell is completely down, therefore BMS cuts off and charging is not possible because at least one cell is full so the charger recons it as charged...I am screwed, am I?!

dogman
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Re: Problem with 72v LiFePo4

My 36 volt pack,from ping reads up around 45 volts when fully charged. wierd that they call it a 36 volt. So I think your pack is not fully charged, and it might be the low voltage cut off preventing the current from flowing. I'm not sure what the spec for that is, but ping can tell you. If each cell had the same voltage you should have all good cells. Are you sure the charger works? Over at endless sphere forum there are tons and tons of posts on lithium batteries, some 65 pages on the first duct tape battery thread, plus others. One of the things discussed is how to bypass the bms to charge or discharge. I'm still not the guy to tell you how to diagnose the bms though. good luck!

Be the pack leader.
36 volt sla schwinn beach cruiser
36 volt lifepo4 mongoose mtb
24 volt sla + nicad EV Global

Oazer
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Re: Problem with 72v LiFePo4

I made a post there as well and i also tried to find similar topics over there...but i didnt find anyone having the same problem, hopefully someone who knows will help me. I wrote also to ping, but no answer so far.

jbird
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Re: Problem with 72v LiFePo4

Did you make sure that it was okay to put both 36V battery packs in series to get 72V? Some BMS modules cannot handle this which hopefully the seller told you about. Also did you fully charge and discharge each pack separately at least a couple of times before trying to go to 72V? This would have conditioned the cells and allowed the BMS to do some initial balancing. If you do not have a "real" warranty or all else fails, you will need to very carefully disassemble each pack (avoiding short-circuits, wiring/tab damage, BMS damage, etc.), check each 3.2V cell with an accurate digital voltmeter (should read about 3.7V when full), and attempt to manually charge each cell with a single cell lifepo4 charger like this one //s.p10.hostingprod.com/ [at] www.voltphreaks.com/ssl/catalog/product_info.php?products_id=28&osCsid=6db2bef97c0432ad6f8194228aa0ee18">https://s.p10.hostingprod.com/ [at] www.voltphreaks.com/ssl/catalog/product_info.php?products_id=28&osCsid=6db2bef97c0432ad6f8194228a...

You could check the battery chargers with the digital multimeter for proper voltage and current output.

Oazer
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Re: Problem with 72v LiFePo4

The seller recommended to not put them in series (so, if anything is broken it is my fault for sure). I now know that i should have charged and discharged them a couple of times before using them...but I charged them just once, connected them and rode them...

Is there no possibility to do something without disassembling them? Instead of charging each cell...wouldnt it be enough to discharge the full ones?

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