# Li-ion: Reasonable low voltage cutoff?



## AilSnail (Nov 13, 2008)

How deeply should they be discharged?
I am thinking of implementing it in various variable converters, but a max of 2C.


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## steve6690 (Nov 13, 2008)

My protection pcb cuts out at 2.75v per cell if that helps you


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## linterno (Nov 13, 2008)

At 3.5 volts the battery is almost empty (~4% charge left). After this point the battery voltage will drop very fast.

I think 3.5 volts is the best low voltage cutoff.


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## qwertyydude (Nov 13, 2008)

There's lots of power left at 3.5 volts. Li-ion's have a linear discharge curve, it doesn't drop off quickly. In led's it seems like a steep drop off because below 3.5 it dims fast because the voltage goes below the vf of the led. If you use incans or in my case motors with li-ion then you know that they'll provide power even below their cutoff. In general the rule for lithiums is that you don't want to go below 3.0 volts per cell under load. If you're using a high discharge, for li-ion it's anything over 1C, you would be doing the cycle life a good favor by setting the cutoff for about 3.2 volts just to make up for voltage sag and give yourself some wiggle room.


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## rantanplan (Nov 13, 2008)

qwertyydude said:


> There's lots of power left at 3.5 volts.



True, if measuered under adequate load ...
False, if measured idle ... 



qwertyydude said:


> Li-ion's have a linear discharge curve, it doesn't drop off quickly.



LiIons have a linear discharge curve over almost the whole time, but there is a steep drop-off near the end. At least, if you apply a constant current for discharge. This dropoff shifts with higher loads to lower voltages. That "voltage sag" is the reason, why protection circuits are shutting off at such low voltages. Otherwise you won´t be able to use the full charge with high loads. Negative point of that is, that a low discharge doesn´t trigger protection early enough.



qwertyydude said:


> If you're using a high discharge, for li-ion it's anything over 1C, you would be doing the cycle life a good favor by setting the cutoff for about 3.2 volts just to make up for voltage sag and give yourself some wiggle room.



... which reduces the usable capacity highly. 

This whole problem concludes to one question which I couldn´t find an clear answer for during all the years here : How big is the impact of voltage sag onto the critical voltage levels and therefore battery health. You hear all the time that you shouldn´t go below 3.0 volts ... ok, but how bad is it, if I undercut it under load while the battery (idle voltage = state of charge) isn´t nearly discharged, thus would be way above 3.0 volts. According to the LiPo guys at RC every "millisecond" being off the "safe zone" has influence ... at least during loading. But is that also true for LiIon chemistry under load ... and how big is the influence?


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## qwertyydude (Nov 14, 2008)

It is especially true for li-ion to not go below the cut-off voltage. Li-po's have almost no voltage sag and are pretty safe to say idle voltage=state of charge=running voltage (at least at a drain of less than about 2-3C). But with a larger sag in voltage of the li-ion you can get small areas inside the battery to destabilize and on the subsequent recharge you can get a puffing, in li-ion's case venting. I've seen as little as 3 cycles below 3 volts/cell puff a lipo, so it's best to not go below 3 volts. 2.75 is just too close to pushing it over the edge where a recharge can vent.


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## SilverFox (Nov 14, 2008)

Hello AilSnail,

There are some "general rules" about depth of discharge with Li-Ion cells. These "general rules" are designed to maximize cycle life.

At the end of the discharge, when the load is removed from the cell, you will get better cycle life if the voltage rebounds to around 3.5 volts or higher.

The cut off voltage to make this happen depends on the load on the cell. Normal loads may use a cut off of around 3.2 volts per cell. High loads may be able to get away with a slightly lower cut off voltage. Light loads need a higher cut off voltage.

Charging efficiency with Li-Ion chemistry is nearly 100%. If you have a charger that tells you how much capacity has been put into the cell, you can adjust your usage so that you are using 80% of the cells total capacity. This will keep you from over discharging your cells and will give you excellent cycle life.

Tom


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## AilSnail (Nov 14, 2008)

great. thanks.


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## LEDite (Nov 14, 2008)

AilSnail;

The quality Li-Ion cells like Sanyo, Sony and Panasonic are not harmed by 3.0 volt discharge.

Most of the Chinese cells are questionable, even when only discharged to 3.5 volts.

I've used hundreds of the quality cells over many years.

Larry


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## linterno (Nov 14, 2008)

LEDite said:


> Most of the Chinese cells are questionable, even when only discharged to 3.5 volts.


I know these Chinese batteries have poor quality control, that can be of poor quality but you are paying 1/4 of the price just because of that. Less quality control means lower manufacturing costs and lower prices. It can be perfectly expected that some batteries have quality issues, but that is the price that many members here are willing to pay n terms of cost and quality.

AW the most reputable batteries here are Chinese and that doesn't make them automatically bad quality. Those are good quality but costs almost twice the price of UltraFire/Trustfire ones (including S&H).

I have two Ultrafire and two Trustfire batteries and I have never had an issue with them. I also have some Panasonic unprotected batteries and no issues. Considering my experiences I couldn't tell what the difference in quality is. Even if I know that Panasonic batteries are much much better ones with much much better quality control.

Respectfully, If you have never had a trustfire or ultrafire one then I guess you are just speculating based on someone else reported "experience".

Sorry, nothing against you.


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