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Li-Po Cutoff @ 3.2 = 4%

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  • 1 Post By GerryH

Li-Po Cutoff @ 3.2 = 4%

Old 09-07-2019, 02:29 PM
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Default Li-Po Cutoff @ 3.2 = 4%

So, Got a new basher rig with batteries and was unhappy with the results. basically I got about 5-6 minutes of run time..
Its a DB8 nomad with an AXIAL yeti XL esc (AE-4) and motor(2200kv).
He gave me SMC 3S 4500mah, 90C li-pos, both are a bit puffy.
After running and charging and balancing and discharging and balancing etc.. one of them has a bad cell and the other I believe is just worn out.
I checked the ESC and changed the auto cutoff voltage from 3.7V to 3.4 V, which I believe was part of the short run time.
So I slapped my almost new 3S 5000mah 50C li-pos in there today.
Ran 15-20 minutes and it felt great. I get home throw my hitec li-po checker and it says 3.2V 4%.. 'Im like CRAP "Did I just kill my li-po's?"
I know the recommended rule is not to not pass 70-80% I drained these waaaayyyyy down...
Took 75 minutes for my charger to bring them to 100%

So, Did I hurt my batteries, shorten their lifespan? Should I change my cutoff voltage?
Thoughts? Opinions?
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Old 09-08-2019, 08:56 AM
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Technically no, you did not "harm" your batteries (beyond reasonable use), however you have reduced their lifespan ever so slightly. Basically, if you want the longest lifespan possible then you want to minimize the amount of time the voltage is away from 3.8V/cell, more info here:

How to Prolong Lithium-based Batteries

Everyone has their own personal preference when it comes to discharge, set your LVC to the max setting your ESC will allow to get the longest lifespan of your packs. Set the LVC to the lowest setting if you want to get the longest possible run time at the expense of nominally decreasing the lifespan of your pack.

The higher the C rating, typically the longer lifespan your packs will last as well.

Experiment with LVC and see how much longer run time you get between 3.7V and 3.4V, you might be surprised that you get less than an extra 30 seconds or so.
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Old 09-08-2019, 12:17 PM
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If your lipo checker was reporting one of the cells was at 3.2v at rest, it must have been much lower than that when the LVC kicked in. Here's the problem with a lower LVC with anything other than 1 cell battery. The ESC is measuring the voltage of the whole pack, not individual cells. Not all cells are equal in terms capacity. Lithium batteries have a steep voltage drop off as they approach "empty". The lower the LVC, the greater risk one cell will be pulled much lower than the others. The more cells in a battery, the bigger the chance of this.

Also you should immediately stop driving or at least pull off with the lightest of throttle when the LVC kicks in because the more you pull that one cell lower, the bigger the chance it's going to get damaged.

3.4v is too low in my opinion. During the winter, when there's not much else to do, I test all my batteries. I pull them down to 3.4v but I also have an automated test rig that shuts off the load when any cell gets to 3.4v. Most batteries, it can take about 2 mins to drop to 3.6 to 3.5v per cell, but only about 30 seconds to go from 3.5 to 3.4 and the voltages are dropping very rapidly at this point.
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Old 09-08-2019, 12:43 PM
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Originally Posted by NeoGP View Post
He gave me SMC 3S 4500mah, 90C li-pos, both are a bit puffy.
After running and charging and balancing and discharging and balancing etc.. one of them has a bad cell and the other I believe is just worn out.
I think you already gave the answer. A bit puffed and not good behavour on some cells says enough.

For those who think 3.2 or 3.4v as an LVC is too low, think again! With a load the voltage on the esc is always lower than inside the cell. Yeas, there is some resistance of wires, solder joints and connector but as with every battery there is also the cell its own resistance. So if a LVC at 3.2v grabs in the internal cell voltage can still be 3.6v
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