Lithium battery looses power like NIMH pack

[Irishchamp]

I have a 4000mah 5s lipo pack that work fine for a few minutes, plenty of punch, smooth power etc. then falls off super fast like an old NIMH pack. Back to the work bench, check the voltage at the bullet connectors and balance plug, 4.1v per cell, 20.XX v at plug. Pack is barely warm, no puff. voltage with wheels off the ground is good, has full wheel speed, voltage stays at about 19v. This is not a good load test I know. Drove again, pack functions properly.
Packs are 100 amps continuous, rig is pretty big and heavy.
16lbs fully rtr. 2x Dewalt 18v motors. RC4WD 40 series rock crushers.
Packs don't get hot.

[losikid]

Thermal shutdown on the esc?

[Irishchamp]

ESC is cool. Nothing seem too be getting overworked. Motors, ESC, and battery are all cool.
The truck ran great on 6s, temps on everything were within normal ranges.

I'm 99% sure it's the battery.

Originally Posted by losikid

Thermal shutdown on the esc?

[Irishchamp]

Ran it again for 5-10 minutes. Everything was fine then the external voltage alarm I hooked up started beeping. cell voltages, 3.7, 3.6, 3.3, I missed the last two. Definitely the battery! Not sure what exactly it's doing. Internal Resistance issues? Non-matched cells combined with not enough amperage headroom?

Obviously this battery is not up to the task but I'm still perplexed. I've overworked batteries and had them get toasty but this is weird to me. No heat.

[JohnRobHolmes]

Your battery is big enough. When the cells reach about 3.55v under light load typical of crawlers, they start to crash in voltage in order of capacity. Sounds like you may have a weak cell though, you shouldn't have one up at 3.7v and another at 3.3.

[Irishchamp]

When I put the pack on the charger a few minutes after my last post all cells showed 4v per cell, only having temporarily dropped to the low and uneven voltages. The pack charged up fine only taking 775mah.

Originally Posted by JohnRobHolmes

Your battery is big enough. When the cells reach about 3.55v under light load typical of crawlers, they start to crash in voltage in order of capacity. Sounds like you may have a weak cell though, you shouldn't have one up at 3.7v and another at 3.3.

[JohnRobHolmes]

yep, got some poopy cells in that pack for sure. Even a 450mah pack wouldn't have major issues dealing with your setup.

[Irishchamp]

So I have another identical pack, used it with the same result, fine for a few then power gone, almost none at all, crawling along at 1-2mph. I didn't see any one cell abnormally low this time, all were at 3.7-3.8 but no power at all.

These packs prior to this were used in a 10s setup, worked flawlessly. Probably 1 hour of runtime total, stored correctly but they have been stored for quite a while. They charge and balance fine...

No doubt the packs are bad, just strange. Two 5s packs down the drain.

Thanks to all for our input!

Originally Posted by JohnRobHolmes

yep, got some poopy cells in that pack for sure. Even a 450mah pack wouldn't have major issues dealing with your setup.

[fyrstormer]

Voltmeters measure voltage indirectly, by measuring the difference in amp draw before and after a resistor with millions of ohms of resistance. (this prevents burning out the voltmeter circuit.) That much resistance in the circuit will allow even a very weak battery to appear to have proper voltage, while the battery's own internal resistance can be so high that it can barely push more amperage than the voltmeter circuit needs -- so when you hook it up to something that can handle substantial amperage, like an ESC and motor, the voltage will drop like a rock.

TL;DR: Unbalanced cells aren't the only indicator of a worn-out LiPo. High internal resistance will also cause the behavior you're seeing, and it's irreversible.

You might be able to disassemble the packs and build new packs from the individual cells that still work, but don't do that unless you're prepared to deal with a possible battery fire. It's easier to just buy multiple lower-voltage packs and connect them in-series to get higher voltage; you can replace a single pack if it goes bad.

[JohnRobHolmes]

Your lipo cutoff set to 6 cells?

[Irishchamp]

Lipo Cutoff set to Auto, 3.5 or 3.6v per cell.

Originally Posted by JohnRobHolmes

Your lipo cutoff set to 6 cells?

[Irishchamp]

On a slightly unrelated note, what batteries are people having success with? I'll be looking for some 4, 6 and maybe 5 cell packs. 4000-6000mah.

[Olle P]

Originally Posted by fyrstormer

... will allow even a very weak battery to appear to have proper voltage, while the battery's own internal resistance can be so high...

While the internal resistance is a concern when a cell has gone bad it's not that important.
It's more relevant to know that the cell voltage recovers quite fast when a (nearly) drained cell no longer needs to deliver any current. So when feeding a moderate current it might be down to 2.7V and then as the current is cut the voltage slides to 3.4V or more.

[JohnRobHolmes]

Originally Posted by Irishchamp

Lipo Cutoff set to Auto, 3.5 or 3.6v per cell.

You might want to lower this to 3.4. 3.6 will trip up as soon as the fresh charge gets knocked off the pack. Might be the issue.


I would suggest you Force a specific voltage or remove the cutoff for testing purposes before you start cutting into your batteries. The more I think about it, the more it sounds like the ESC is just giving you a premature soft cutoff.

[Irishchamp]

I'll try that. There is no need to be concerned with the larger than normal voltage fluctuation? Ripple voltage, is that the correct term?

Originally Posted by JohnRobHolmes

You might want to lower this to 3.4. 3.6 will trip up as soon as the fresh charge gets knocked off the pack. Might be the issue.


I would suggest you Force a specific voltage or remove the cutoff for testing purposes before you start cutting into your batteries. The more I think about it, the more it sounds like the ESC is just giving you a premature soft cutoff.

[JohnRobHolmes]

No worries, just pay attention and stop when it gets slow.

[Irishchamp]

Got it! I'll report back on the results.

Originally Posted by JohnRobHolmes

No worries, just pay attention and stop when it gets slow.

[fyrstormer]

Originally Posted by Olle P

While the internal resistance is a concern when a cell has gone bad it's not that important.
It's more relevant to know that the cell voltage recovers quite fast when a (nearly) drained cell no longer needs to deliver any current. So when feeding a moderate current it might be down to 2.7V and then as the current is cut the voltage slides to 3.4V or more.

You are describing the exact same phenomenon from a slightly different perspective. There was no need to imply that I was wrong.

[fyrstormer]

Originally Posted by Irishchamp

Lipo Cutoff set to Auto, 3.5 or 3.6v per cell.

Wow, that's high. I have mine set to 3.2V normally, and 3.1V on some of my higher-powered vehicles that experience more voltage sag at full throttle. I suppose for a crawler application, the cutoff voltage can be set higher because the voltage doesn't sag as much at full throttle, but still, LiPo batteries generally say the minimum voltage is 3.2V/cell and I take them at their word. I have batteries a couple years old that still hold more than 90% of their rated charge.

[Irishchamp]

So I lowered the voltage cutoff to 3.4, definitely better. However, after repeated full throttle bursts the power still cuts, if I stop for 30 seconds it's back to normal. I lowered the low voltage cutoff to 3.2, I'll try that in a bit. The battery actually gets warm now, symptoms are less convoluted. Warm pack and power loss makes sense to me. :-) these packs are great for trailing in low gear, lighter weight and plenty of power for occasional bursts, they're not the right pack for high gear wheelie popping though.

Funny, I've never, ever ran a pack to low voltage cutoff or had it trigger prematurely, ever...
This was totally foreign to me......