Maximum cell voltage difference for lipo?

[adamk]

ive searched Google and had no luck so I'll ask here...

What is the maximum difference in cell voltage for lipo batteries after being used? I have had 2 batteries now(dynamite reaction 2s 5000mAh) that have had a cell voltage difference that is extremely high to me. One battery had puffed and the cell difference was .4V. This was being run in an Axial scx10 with the AE2 esc at the factory lvc of 6V(3V per cell).

Now, the second battery had a .6V cell difference in the same rig with the same esc but, with the lvc set to 6.6V(3.3V per cell).

Dynamite says they batteries have a 1 year warranty and both were bought just under 2 months ago. They have been charged on a Venom PR2 charger at 6A and being balanced. Ending voltages are usually less than .03V per cell difference after being balanced.

I'm new to the electric RC world so any and all insight is welcome.

Thanks,
Adam

 

[GLwagon]

0.01 to 0.03 imbalance are some of my "old" packs after a run.
The cell Internal Resistance (IR) has a lot to do with this... my experience says packs puff for five reasons.
Over-volting them, an unbalanced charge where one cell goes to 4.3 and other to 4.1 (normally a balancer takes care of this)
Under-volting them, run to LiPo cut & the cells end up at 2.9v & say 3.4v (different IR or un-matched cells)
Over Loading them, running a low "C" pack in a uber high load application. (modern cells rarely do this unless you are dumb, example put a 25c 4000mAh 4s pack into a 2200kv 1/8th scale Race Truggy)
Improper storage, optimum storage voltage for a LiPo is 3.75v to 3.8v per cell. Period. Any higher & you take years off of a pack. Storing them at 4.2v per cell can cause even a decent pack to become unstable after a few weeks.
Junk Cells, unfortunately some times there are bad batches & cheap packs are sometimes prone to puffing. SMC had a bad batch about three years ago & I saw a lot of racers return a lot of packs.

 

[Johnnysplits]

Yeah I'd say .2v difference max. However, I.R. should be regularly monitored to really keep track of battery health.

 

[ednerin]

So what is the best method or tool to check IR?

 

[JokersWild]

So what is the best method or tool to check IR?

I use my digital fluke multimeter.

 

[hotwheels000]

I use my digital fluke multimeter.

You checking the ohms at the balance tap ?

 

[adamk]

Thank you for the replies.

The second pack with the voltage difference of .6 has not puffed. The lowest cell was 3.07V. Now, the puffed pack did read down to 2.9V after I ran it on a discharge cycle on the Venom PR2 charger. The settings for discharge are set at 6V for a 2s and I can't change that.

And could someone give more explanation as to how to read IR?

 

[GLwagon]

It isn't impossible, but I let my charger do the work & math.
BU-902: How to Measure Internal Resistance – Battery University
On the charger I have it is calculated over time & is part of the charge cycle.
Hyperion, Revolectrix, and others...
By the time you buy this you're $41 closer to getting a better charger.

If you see the charger sitting at a low amperage & always trying to balance it out or never balancing it out there is a suspect cell.
Learn your charger & scroll through the menus.
Charge in "BAL" or "Balance" mode only.
I personally use this, nothing better than needing to get a pack ready for the next race & charging upwards of a 5c rate if needed. A few of my packs are rated at a 5c rate.
6000mAh X 5c = 30amp rate. I rarely go over 2.5c rate.

 

[SilmarilS]

Normally you shouldn't discharge a battery below 3.5/3.6V...

3.0V is really too low, and you won't gain in runtime between 3.5 and 3.0V
I always set the LVC the higher the ESC can be set.. Normally around 3.5V.
You will have a little more headroom if you have unbalanced cells!

 

[Johnnysplits]

So what is the best method or tool to check IR?

My charger does it. Cellpro Powerlab 6.

 

[Johnnysplits]

Normally you shouldn't discharge a battery below 3.5/3.6V...

3.0V is really too low, and you won't gain in runtime between 3.5 and 3.0V
I always set the LVC the higher the ESC can be set.. Normally around 3.5V.
You will have a little more headroom if you have unbalanced cells!

3v/cell is absolutely fine to set the LVC at.

 

[Olle P]

... you won't gain in runtime between 3.5 and 3.0V

No more than 40-50% extra or so, which isn't exactly nothing...

The problem is that to actually reach 3.0V/cell the LVC must be set closer to 2.5V/cell.
With the LVC at 2.8V/cell (during load) it will cut the power somewhere above 3.4V/cell (at rest).

 

[SilmarilS]

No, you won't have 50% extra.... just check the discharge curve of lipo chemistry... below 3.5-3.4, you are nearly the end of the battery capacity..
But look, it's your choice to harm the battery just to have 10-15% more runtime, when you can just change the battery!

 

[JokersWild]

You checking the ohms at the balance tap ?

Yes sir.