premature lvc by overdischarge?

Enpro

Enpro

10 W
Joined
Dec 27, 2011
Messages
97
Location
The Netherlands
I got two 36v/15ah goldenmotor lifepo4 packs (rated for 30amps continue,60 amps peak according to goldenmotors that is..) in series and already after a couple of miles the lcv kicks in.
I got them hooked up to a 72v 45a crytalyte controller which is a little too much i know but even if i run only at half throttle the same thing happens.
At full charge they both measure 41v and after the lvc one pack reads 41v and the other 39v even with diodes. (without diodes same thing happens)
Faulty pack or am i just pulling too many amps?
 
It's cold and you are pulling too many amps, yes.
I am sure those packs are overrated.

Put some kind of voltage measuring device on them and you will see the voltage sag.
This will answer your question, enpro.
 
Okay i'm almost convinced but why is one pack reading a full charge?
And is it still drawing that many amps when half throttling?
I will put a multimeter on my bike tomorrow and take it for a test run :wink:
 
Because it's sagging and hitting low voltage cutoff, then shutting down.
OR a cell is sagging extremely, hitting the per cell low voltage, then shutting down.

All it takes is for one of these batteries to go into LVC cutoff to disable both of them.

Sorta like a light in a string of christmas lights, all wired up in series.

You will see the problem by watching the voltage.
 
neptronix said:
Because it's sagging and hitting low voltage cutoff, then shutting down.
OR a cell is sagging extremely, hitting the per cell low voltage, then shutting down.

All it takes is for one of these batteries to go into LVC cutoff to disable both of them.

Sorta like a light in a string of christmas lights, all wired up in series.

You will see the problem by watching the voltage.
Click to expand...

But if a cell is sagging like you say wouldn't that cell be "balanced back"after a while by the remaining higher charged cells?
Which would make the pack "stabilize" again after a couple of hours?
And putting diodes between the packs should provide a even output right?
 
Enpro said:
But if a cell is sagging like you say wouldn't that cell be "balanced back"after a while by the remaining higher charged cells?
Which would make the pack "stabilize" again after a couple of hours?
Click to expand...

Series strings of batteries cannot equalize their voltage, there is no way for that to happen.
That is why we use balancing chargers and BMSes to make that happen.

The only exceptions are NiMH, NiCD, certain Li-Mn, and i believe SLA. - but this 'series balancing' effect only occurs while they are on a charger.


If you have a very weak series string, it will smack the per-cell level LVC on a load.
Poorly designed BMSes can drain 1 or 2 cells very low and damage them over time. This could be the case with your batteries.

We don't know until you collect some data. Go have a ride with a multimeter hooked up to each battery!
 
More likely, one weak cell in a cell group sags, which sags the whole group, which trips lvc.

And yeah even if it was warm, you could still be crossing the line for amps. Even at half throttle.

Bear in mind the old rule, whatever they claim, half of that is reasonable. So yeah, 45 amp controller on a 15 amp battery is pushing it a tad.

Getting a 20 amp controller would be a good idea.
 
Okay i got it about half empty and at full throttle the voltage drops to 70v
I'm guessing that means both packs drop about 5v ?
Is that too much?
its raining now so i try to ride it empty later today.
 
As several of us said, It only has to be too much for one cell group, then the bms cuts you off. 5v sag sounds normal, but if one cell group is getting too much sag, then the bms cuts out.
 
dogman said:
As several of us said, It only has to be too much for one cell group, then the bms cuts you off. 5v sag sounds normal, but if one cell group is getting too much sag, then the bms cuts out.
Click to expand...
Well its 10v in total... so 10v sag is okay?
 
you do not need diodes in series with the batteries.

the LVC is a cell level shut off. you have to know the individual cell voltages at the point when it cuts out. that will tell you which cell is not getting fully charged. the diodes make it worse.

you will need to do a bench test of the packs to find the one which is not balanced.

5V or 10V none of that matters, you need to know what each cell is dropping to.
 
if not using ( or disconect ) the E-brake switches.. you can use your bike's brakes to load up the battery and cause cutouts while bench testing... ( yes, it wears brake pads, but no big deal for a quick test. )

http://www.youtube.com/watch?v=58_jLXw-KWk&list=UUJCgYcCXBAuxEgVT1bpMnyQ&index=40&feature=plcp

http://www.youtube.com/watch?v=nJKsMbYf4Ys&feature=autoplay&list=UUJCgYcCXBAuxEgVT1bpMnyQ&lf=plcp&playnext=1

edit, also see :
http://ypedal.com/Lbd.htm
 
okay this is whats happening.
Full charged packs (81v) only drop to 70v max at fullthrottle.
After 4 km of easy low throttle riding combined voltage starts to drop to 79v (Pack 1= 41v , pack 2= 39/38.6)
Then voltage drops to 68v when full throttling only for a few seconds.
Then aftr couple minutes more riding lvc from pack 2 kicks in and pack still measures 38.6v
Trying to restart, lvc kicks in at only 5v sag and like 30% throttle.
 
I feel kind of lost here...
Why are those idiot people that sell these packs state that they can handle 30amps continuously.
They sell a 50amp controller that is made for a 48v 10ah pack and the sales man told me 45 amps would be no real problem for my setup... :evil:
 
There is a difference between " Will work with no real problem " .. and " Work Properly " ..

What's happening now is that you are stressing those cells beyond their specs, they will die within a short period, the BMS is cutting out ( as it should ).. etc etc etc....

you want a high power setup.. get high power batteries to go with it ! :wink:
 
he already has essentially dead cells but he refuses to bench test it to find them. bot a golden motor battery, so i guess i should not expect he would wanna test it. i have never seen the golden motor BMS either so there is no way to know if it does even stop discharge below 2.1V anyway.
 
I am going to bench test it tomorrow and disassemble it to measure each individual cell.
I will also post some pics of the bms and cells :wink:
And you guys got me convinced to get some better battery's all along but i just had to try it.
I'm always pushing things like computers,cars and bikes and other household items to the limit and most of the time you can go far beyond the specs but with battery's and e bike controllers its the opposite.
 
Okay i measured the balance leads and they all read about 4v accept for one.
I got 4v,8v,12.1v, 16.2v, (19.1v), 23.2v, 27.2v, 31.3v, 35.3v, 39.4 thats without load.
So do i got a dud cell now?
Their are 20 cells in the pack so i guess they are 8000mah each.

Also i noticed a very strong chemical smelling odor coming out of pack? might be a burning smell but its not the familiar electrical component burning smell. Lifepo smell?

I post the pics and do some more testing with a load tomorrow.
 
i don't think this is lifepo4 if you have those voltages. the smell is the electrolyte from the ruptured pouches.

don't disassemble it.

you bench test by discharging the pack into a load, like an electric space heater, which has no fan, and measure the voltages as the battery discharges. the bad cell will drop so fast you will know right away. if you want to evaluate capacity, discharge through an ammeter at the same time, or a wattmeter if you have one. then you will have an idea of how bad off the pack is now, and what to fix.

you can measure the voltages on the pouches if you can reach them or on the sense wires if it has them. which it may not if this is limno2
 
Well i have seen the limno2 packs and they have smaller and more cells in them. These cells are about 12 cm in length and 1.5cm diameter so they sure look like lifepo4 cells to me.
I measures those voltages at the balancing wires on the bms so the voltages are from 2 cells.
 
This whole thread reminds me of a friend of mine..
Years ago... He was saving up his money to buy a car.. Never really drove (took the bus/bike a lot) and wanted a really nice (re:fast) car.. So he saved up until he could afford his Camaro Z28 (this is back in 1987)..
Within 6 weeks, he wrecked it...
Moral of the story is..
Newbies shouldn't try to go "all out" on their first vehicle..
Start small, learn a lot, then when you're capable to do certain things, THEN go crazy!

Running 72v on a bike when you've never even ridden an ebike or have any experience with ebike maintenance or electronics, usually turns out bad..
(This is the second person in 2 months, I've heard that went right to 72v (with seried batteries), high end controllers, etc, etc before they really had experience with ebikes before.. The FIRST tool, to have to start, should be some form of watt meter/ammeter.)

No offense meant, Enpro..

Btw.. Can you link us to the battery(s) you bought?
 
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