extreme team 72v 150 lipo battery pack low voltage

[gforce15]

so coming out of winter i have left my cart plugged in all winter i have a 72v 150 lith pack from extreme team. went to take the cart out and nothing put a voltage meter to it and got 16v. the charger is flashing low voltage output short-circuit. any thoughts? when i unhook the charger from the battery pack it gets 87 volts at the leads. does this mean the bms is asleep or whatever and needs to be woke up somehow and if so is it possible.

[Pat911]

Sounds like something has failed and either your BMS is faulty or has drained your battery. Connecting a charger should wake a sleeping BMS but it seems like it’s not doing that.

ET is no longer in business so you’re on your own. You need to open the battery and check the cell voltages to see if the battery is able to be saved. If the 16v terminal voltage translates back to cell voltages then your battery is shot. Hopefully the cells are ok and it’s the BMS that’s faulty. That’s the cheaper option but I don’t like your chances.

[augiedoggy]

Is there anything on the cart that stays on with the key off? like a voltage reducer? Most chargers will charge the pack up and then shut off as an energy star certification thing. They wont just turn back on later and top the battery off automatically. So if you have something drawing off the battery pack it will drain down and the charger wont help. this is why some people connect thier chargers to a timer to cycle the power every now and agin to turn it on and recharge the batteries.

Not saying this is what happened but it is a concern. some chargers (or bms) will not allow the pack to charge if it has fallen below the safe minimum voltage. the bms honestly should disconnected the pack from the load but its possible the bluetooth and bms itself drained the battery further down over the winter although that seems excessive. Ive been told the best way to recover the battery from this situation is to charge each cell to 3.6v individually. if it is your charger thats not providing the current as a safety measure you could try connecting the battery to another 48v source in parallel to "jump start" the charge but dont take my word for it. I did have luck doing this with one of my ryobi 40v packs for my trimmer when it fell below the minimum voltage.

I did not watch this video but I believe this is the same situation that you might be experiencing (even though this is lipo and you have lifepo4)

https://www.youtube.com/watch?v=KmHoiOfVhRs

[Pat911]

The BMS should have protected the battery from parasitic loads. The BMS/Bluetooth shouldn't have brought the pack down to 16v even if the battery was shut off and at minimum SOC due to parasitic loads. There's a problem with the battery, or more specifically, the BMS. Best case scenario, new BMS, worst case, whole new battery.

There are risks involved in trying to revive cells that have gone below the minimum voltage, even with LiFEPO4. At 16 volts, and 24S configuration (72v battery), that's an average of 0.67v per cell. If the cells are really at that voltage then they are done for. No bringing them back with any sort of safety or reliability from that point.

I have also revived bad cells before but I know the risks and how to mitigate them, and I've never got anything near original performance out of them. There is usually a very high self discharge associated with revived cells, as well as increased heating during charging. I also would never suggest to an amateur to try it.

[augiedoggy]

Originally Posted by Pat911

The BMS should have protected the battery from parasitic loads. The BMS/Bluetooth shouldn't have brought the pack down to 16v even if the battery was shut off and at minimum SOC due to parasitic loads. There's a problem with the battery, or more specifically, the BMS. Best case scenario, new BMS, worst case, whole new battery.

There are risks involved in trying to revive cells that have gone below the minimum voltage, even with LiFEPO4. At 16 volts, and 24S configuration (72v battery), that's an average of 0.67v per cell. If the cells are really at that voltage then they are done for. No bringing them back with any sort of safety or reliability from that point.

I have also revived bad cells before but I know the risks and how to mitigate them, and I've never got anything near original performance out of them. There is usually a very high self discharge associated with revived cells, as well as increased heating during charging. I also would never suggest to an amateur to try it.

What percentage of state of charge is this? To be clear I'm not suggesting anyone do anything. I do not know this person or his skill level. I only stated what I've done and what I've seen others do. My limited knowledge of lithium batteries comes from others on the Internet. I have no professional training on the subject. I do know that it is dangerous to revive certain forms of lithium chemistry. I also see many videos and statements of people doing it. I will say I am confused when I'm told one day that draining a battery down to 0% charge is ok and then told the opposite the next? I can only assume zero percent charge means something different than the amount of electrical charge stored in them here.

[gforce15]

thanks for the advice. i kinda figured id be screwed i dont feel like blowing something up so ill just bight the bullet and deal with it

[Pat911]

Hi Augiedoggy,

0% SOC is 2.5v.

These cells might be at or below 0.67v. That is well below 0% SOC and in the area where permanent irreversible damage HAS been done to the cells.

Yes, I know you’re going to question how a cell can be below 0% SOC so I’ll try to explain how capacity is measured and calculated for lithium cells. I’ll keep the discussion to LiFePO4 but applies equally to other forms of lithium and even FLA cells, albeit with different top and bottom voltages

1. Cell is fully charged to 3.65v
2. Cell is discharged to 2.5v and discharging stopped
3. Amount of energy drawn from the cell during this process is it’s rated useable capacity. This is the capacity that is advertised for the cell. So an advertised 100Ah cell will have 100Ah available from 3.65v down to 2.5v.
4. There is minimal energy (maybe 1%-3%) still remaining in the cell at 2.5v but this is not useable energy and is not counted toward the cells capacity because trying to use it will irreversibly damage the cell.

Referring back to the other thread where this came up, a 100Ah cell has a full 100Ah useable capacity and using all 100% of this useable capacity will not damage it. What will damage it is if you try to use (say) 103Ah of a 100Ah cell, bringing the capacity to -3% (yes I know, not mathematically possible but hopefully you get my point). Doing so will bring the voltage of the cell below 2.5v, permanently damaging it.

The point of the BMS is to stop any possible discharge once any one cell in the battery pack gets down to 2.5v, hence preventing damage to that, and any other cell.

Cheers
Pat.

[Pat911]

Originally Posted by gforce15

thanks for the advice. i kinda figured id be screwed i dont feel like blowing something up so ill just bight the bullet and deal with it

Measure cell voltage before throwing the towel in. You might just be lucky.

[jasonojordan]

BMS seems like a common point of failure on these Extreme Team packs.

[Pat911]

Yes it does unfortunately. I don’t know why as they are using either Ant or Daly units which are usually quite reliable. My only thought is that the configured parameters may be the culprit.