Absolute lowest lipo voltage?

isaac_alaska · May 22, 2012

i'm wondering what the absolute lowest voltage is on a lipo where it might still be an option to recharge it. The snowmachine with the battery pack in it is up in greenland, and the mechanic said that the bms isn't working....he sent me pics and for some reason one of the bms slave modules isn't doing its job. either the voltage on the cell is too low (somewhere around 2 volts or less) for the module to work properly, or the module is damaged somehow. i told him to check the voltage with a multimeter and see what the cell voltage is actually at.

depending on the voltage...what do you think is a possibility to recharge from? i've read that if they get too low there is a really good chance they will catch on fire when charging.....
the charger for the pack operates at at a C/9 rate. (5 amps, into a 45 aH pack)

if the cell is at 2 volts is it worth trying to charge it? i realize there's always the possibility that it can catch fire...but i've never done the tests myself to know what the chances are. there are also temp snap switches inside the battery box that activate at 150F, which will open the main contactors and stop charging if they are tripped.

Thanks ES!

-Isaac

wesnewell · May 22, 2012

I've read 2.7v, but I've had a cell down to 2.59v and brought it back with no ill effects. I'd charge it back up and see if it puffs. If not, then monitor it closely for a while. If it starts puffing, discontinue charging and chunk it.

neptronix · May 22, 2012

If it's been at 2v for a very long period of time, it is very likely it is toast and maybe a potential fireball in the making. If it is not toast, it will have reduced capacity and be damaged.

If you JUST drained it down to 2v, you are going to want to start slow charging it at the lowest rate possible ( 1 amp or under ) ASAP.

2.7v is so far off the cliff that surely you have done some damage to the cell by then. Hobbyking says 3.0v for their RC Lipos.

( are we talking about RC Lipo here? because some other chemistries of lipo cells go down to 2.0v just fine. )

isaac_alaska · May 22, 2012

these are hobbyking RClipo, the 20-30C rated ones, in 6 cell packs.

from the 25 (out of 48) cells that still show voltages on the bms, all 25 are between 2.78 and 3.36 volts. 2.78 is the lowest, most are still above 3 volts.

try to make something idiot proof and the world will invent a better idiot...lmao. thanks for the information though. post the voltages here when he gets around to measuring them...(hopefully soon. but he's one mechanic trying to keep an entire base running haha)

captain387 · May 22, 2012

Does your bms draw current?

isaac_alaska · May 22, 2012

on the order of one mA, with 45 Ah cells.

nechaus · May 22, 2012

I personally never go below 3.5 volts However,
Some of the very first turnigy 5s 5ah 20 c lipo packs iv ever owned have been way over discharged a few times...
once i was riding my bike and all the sudden the power dropped and its like the throttle wasn't working, just a a huge power drop, the voltage cliff..
I continued to go up the hill and then down the botton of it was my house. It cut out almost at the top.

Went home in a rush and reversed polarity the charger by mistake, it blew it was a cheap turnigy, waited like 1 week for a new charger.
The battery Worked for another 50 or so cycles and then one of the cells become a big leaker, i then gave the pack to a friend and he removed the cell and used it at a 12 volt battery for a motor fan.

miuan · May 23, 2012

I've revived 20c lipo from 1-2V with zero puffing. Depends HOW you get the cell that low. If it's a slow discharge, chances are the cell wasn't damaged too much. Anyway check the cells and see if they receive charge without leaking or heating. Get them to 3.72V asap then do a fast discharge test to see if the cells hold voltage under load uniformly.

dogman dan · May 23, 2012

Sounds like they suffered a slow discharge, and could be brought back up.

2.7v is the lowest you can go and still call a cell "safe". But like others said, 3.5 v is where there is nothing left you can use.

isaac_alaska · May 23, 2012

sent instructions to the mechanic explaining how to turn the charger on without the bms, as well as instructions to charge it outside

the bms won't allow the charger to come on if it can't communicate with all the cells...and if one of the cells drops low enough then that cell can't communicate anymore.

looking forward to fixing all these issues next year

ZOMGVTEK · May 23, 2012

How long did it take for this to happen?

Sadly, the pack should have been fine if it had no BMS and was simply disconnected for storage.

The worst cell being at 2.78 isnt that bad. If most are above 3.0, it should be fine. Charging in an area that minimizes damage should a pack flame up is always a good idea.

Ypedal · May 23, 2012

snowmachine ?.. as in this pack is used in VERY COLD temps ?

if so, that's possibly a good thing, if the pack is outside right now in the cold, it needs to be warmed up to 15~20 celcius and voltages re-checked.

BEFORE charging..

if it's warm, then forget what, absolute minimum imo is 3.0v .. but check for swelling and if cells re at 0v.. remove BMS.. duh... then seperate the pack, taking a single brick at a time and using a low current slowly apply charge using a resistor or adjustable power supply ( 1 amp or less is a good start ) once you get all the cells in a pack to 3.0v + and nothing dramatic happens.. charge it up and watch carefully..

isaac_alaska · May 23, 2012

it happened while they were using it but it wasn't under heavy load, the current was limited to 3C at the worst so it wasn't a huge spike...and the other cells being pretty good tells me that if there's a low cell it's probably not at zero.

the mechanic says he was checking it when the cells should have been warm...it was supposedly inside overnight, which should have warmed it up.

haven't heard back from him yet but i'll let u know when i do.

dogman dan · May 24, 2012

FWIW, RC lipo is pretty unhappy at low temps. Even the relatively mild cold we get here in NM affects it a lot. No doubt you have insulated the pack, but perhaps a plug in heater should be added to the system, inside the insulated battery area. You can't charge lipo too cold without damaging it for starters. And the pack capacity plummets if too cold as well. So on a snow machine, you might need insulation, outside heat, and a monitor so you know if the pack is much below 50F.

EBJ · May 24, 2012

Does anyone know at what voltage the Hyperion (or most chargers) considers a LiPo to be at 0% charge ?

-I had the pack level LVC trip at 33V (for 10s) a few times... and if I remember correctly the Hyperion read out 4% total charge when I plugged it in.
But chances are since the 33V trip was under a load, the pack likely recovered to 34V. I just checked none of the cells were under 3.3V (because for some reason I thought that was the "end" zone)

dogman dan · May 24, 2012

It's pretty much the end of the having any wh left zone. They start over the cliff at 3.65v, go to 3.5v very fast, and from 3.5 on down it's nearly vertical on a graph. Free falling by 3.5v, at terminal velocity. It's about the last 10% of charge or less, and not worth trying to get it.

10s should stop at about 35v under load, to stop at 36v IMO. They stay balance so much better if you stop by 3.65v per cell. My own stuff usually stops by 3.7v most rides.

isaac_alaska · May 25, 2012

i'll keep that in mind and probably add it to the next version of the bms code...or at least change the cut offs a bit. i also need to add a hard cutoff (and a way to override it) so the contactors open if any cell drops below a certain voltage. that way no one can over discharge the pack unless they know how to override the cutoff...

ProDigit · May 25, 2012

It is possible to recover a Li-ion battery even if it is nearly completely depleted. I've done it before.
As long as there's still a remaining voltage over the cell, you can tricklecharge it for many hours (if not days) and restore it to a fraction of it's former capacity.
Not always, but sometimes, yes!

isaac_alaska · May 27, 2012

haven't heard back in about 4 days now. hoping this one doesn't have to get written off as "lesson learned for next time" and have to scrap it.

the least i can do is put the cells in my e-trike when they get back. i have it up to a sustained 45mph now, and it's scary fast at that speed. but it will also leave black rubber burn marks if you punch the throttle around 30 mph.

averaging about 8 miles per pack, and i can carry a stack of extra packs

isaac_alaska · May 31, 2012

Finally emailed me back, at least one cell is now at 0.0 volts... Th one that I made extremely easy to measure externally. Wonder if it will catch fire if it charged for an hour at 1/9th C. Guess we will find out. Maybe methods can build me a lvc circuit designed for one pack. To take up very little room. And put one on each of the 8 packs... There's a lot up e said about the department of redundancy department when it comes to lvc on lipo

miuan · Jun 1, 2012

isaac_alaska said:
Finally emailed me back, at least one cell is now at 0.0 volts... Th one that I made extremely easy to measure externally. Wonder if it will catch fire if it charged for an hour at 1/9th C. Guess we will find out. Maybe methods can build me a lvc circuit designed for one pack. To take up very little room. And put one on each of the 8 packs... There's a lot up e said about the department of redundancy department when it comes to lvc on lipo

Even if a cell gets shorted and no longer accepts charge, there is not enough heat to put it on fire unless charging at much higher C-rate. It may deteriorate if used as a resistor for too long, but with no stored charge it will barely smoke at worst.

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