cell resistance

[kevinpratt823]

At what resistance would you consider a cell "bad"?

[jcald2000]

Below 3 for racing, below 5 for sport running and 5 and above only low amp draw project. at 10 in the trash.

[785boats]

Kevin.
Are you talking about just one cell in a pack having a higher resistance than the rest or all cells in the pack having a high resistance?
I've got some old packs with all cells equal but over 20 m/ohms & it runs fine as a sport pack.

BUT!! I recently had a 6s pack that came off the charger with 5 cells between 1.7 & 1.9. the sixth cell was 13.8.
I said to a fellow club member as I launched the boat that I didn't think it would last the 5 laps. And guess what.
Yep. after two laps the boat slowed dramatically so I shut it down. When it was recovered & the hatch removed, that cell was like a balloon & hot. The rest of the cells were fine.
It was a practice day, That's why I gave it a go. Even tho' I pretty much knew what the outcome was likely to be. It's now been converted to a 4s pack.

Do you have a suspect pack? If so, what are the numbers?

Paul.

[kevinpratt823]

Thanks for the feedback. I always monitor my packs closely and just wanted to know what people consider acceptable. I currently have a 6s TP pack that took forever to balance the last couple times, all cells are under 3 except one, which was over 5-6 if I remember. I may give it another run then decide if it goes back for warranty.....

[Brushless55]

This IR stuff I am really trying to wrap my head around..............

[bikergreen]

I am wondering what would cause lipos to go bad and cause high internal resistance. I have just had my first experience with some used packs which all had very high internal resistance, all cells above 30!

What I ran into:

I charged up one set and ran them and right off the gun LVC kicked in instantly, didn't know what to think and didn't have my programer with me so I had to wait to try again. I dropped the LVC to the min on the run the next day and still the same result. Turned it off and did a couple passes and brought it back in and the batteries were hot and swollen. I cycled the batteries a couple times to see if it would help and it didn't, at that point I checked the internal resistance using my i-Charger and the results explained all the problems. Other things happened also including a set which was fully charged dropping from 4.2v to 4.0v in a matter of 24-36 hours.

I am posting this simply to learn more - thanks

[jcald2000]

The low voltage cut off in all ESC's is unreliable. Do not run them below 3.7 volts resting which means you have to time your runs, it only take one time running them too low to damage them as you found out.

[bikergreen]

The low voltage cut off in all ESC's is unreliable. Do not run them below 3.7 volts resting which means you have to time your runs, it only take one time running them too low to damage them as you found out.

I have been running the same setup on the same ESC's in multiple boats for a couple years and have not damaged a battery - also as it said in my post the boat did not move 2 feet before LVC engaged on a set of fully charged batteries. so my run time was about 5 seconds total once it returned to shore at cut throttle.

If you have any input on what leads to increases in internal resistance in lipo's as my question asked that would be great. But I am not new to all of this.

[jcald2000]

The fact that you bought USED batteries leaves us without any accurate info to start from which is why you are getting basic usage info.
Resistance increases from age, the amount of amps draw and how low you run them down. There is no magic bullet to restore them. You have one USED pack left, follow the directions left in all the previous post and good luck.

[bikergreen]

Ok thanks - was just looking to know more about the factors that contribute to increased internal resistance, and was pretty sure that there is no reversing it. Thanks for the info.

[785boats]

bikergreen.

There are several factors that will increase the IR of a pack.

One of the main things is excessive heat.

Over discharging. That is less than 20% remaining in the pack.
Taking too much out of the pack at a high current load. This creates heat.

Over amping the pack.
That is, for example, pulling 300A out of a pack that is only rated for 250A This creates heat.

Over charging the pack.
Charging higher than 4.2V/cell will shorten the life of the pack by increasing the IR.

Putting full load on a very cold pack.
Nailing the throttle with a cold pack will increase the IR.
Packs should be warmed to around 100F before really loading them up.

A dead short across the main power leads will obviously do a bit of damage too. And up goes the IR.

Physical damage like being dropped on to the ground can cause a local crystallisation in the damaged area which increases IR.

Leaving a pack fully charged for long periods will increase the IR.

It's a cumulative thing.
Each time a pack is abused in any or all of the above. The IR goes up a bit more. Then one day it's just too high.
Of course even the best kept packs have an increasing IR. Just to a much much lesser extent.
If you manage to avoid all of the above, your packs should last for years.

Over the years, I've been guilty of every one of the above, at one time or another, except over charging.
I suspect that I'm not the only one.

Cheers.
Paul.

[bikergreen]

Paul - Thank you very much for taking the time to lay that out for me, lots of good info there and exactly what I was looking for!

Thanks
Matt

[785boats]

No worries Matt.

[Brushless55]

Yes, good stuff Paul
thank you