Li-Ion remaining capacity vs. voltage?

[JanCPF]

Is it possible to determine the remaining capacity of a Li-Ion cell just by measuring the voltage? Does anyone have a capacity (in percent) vs. voltage curve and would it apply to all Li-ion cells regardless of size?

Jan

 

[andrewwynn]

from what i know 'not really'.. in my lights.. the battery is almost completely dead at 3.5V.. they have a great little device that is a 'fuel gauge' for LiON batteries it gets hooked up to the battery and can tell you the MaH or WH that went in and comes out of the battery.. they are very accurate I read.

-awr

 

[SilverFox]

Hello Jan,

Here is a rough guideline developed by the RC people:

4.2 volts 100%
4.1 about 90%
4.0 about 80%
3.9 about 60%
3.8 about 40%
3.7 about 20%
3.6 empty for practical purposes

This is "resting" voltage taken 5-10 minutes after removing the load. These are not absoulte numbers, just guidelines.

Since over discharging quickly destroys cells, they use a conservative 3.0 volt shut off under load (and are experimenting with raising it to 3.3 volts). This results in a bounce back to very close to 3.6 volts. It is also important to realize that the area below 3.6 volts yields very little additional capacity.

Tom

 

[VidPro]

seems like you would have to apply a specific load, with a lite load (meaning little current draw) mine read the same voltage for a long time.
4.0 under load just fully charged
3.6 under load for eons
3.0 under load Fricking dead

the graph would look like this for a lightweight load
\-----------------\
under a heavy load it certannly fluxuates more.
if you were going to try and put a indicator in that showed that the battery was almost dead (based on voltage), then it would depend on the unit it was in. and the load.

Like silver said, read it rested.

or microcontroller time, like andrew said

just totally depends on the load, with my 100A bank, it has sat at (about) 3.6 for Days and in that time at least 40% of it has run out.

look at Silvers graphs for the ni-mhys at light loads, same basic thing, high for a few minutes, then 1.2 for the majority of its discharge, then fast dropoff.

 

[JanCPF]

Thanks all. I did suspect that it was a bit more difficult than just measuring the voltage.

Jan

 

[SilverFox]

Hello Jan,

You might find this article interesting.

Tom

 

[VidPro]

the current sure goes away in an unregulated type of thing as soon as the batteries should be pulled.
so if your running a load on them, and the output drops be it even just one cell in the packs current dropping, then stop the load quickly.

possible stupid ways to automate it (other than protection).
how about a photoresister in the light /ubbthreads/images/graemlins/smile.gif when the photoresister isnt being pulverized, then the load should stop. that could be partly calibrated.
and that is basically what the user should do quickly after it happens.

Leds in Direct drive, are so sensitive , compared to incadescents, when the voltage/current drops its pretty visable, even if its when it goes from 3.5 - 2.7
a WHITE wont even trigger the gate at a lower voltage.
(so it basically goes OFF by itself in dd)
BUT
in the concideration of that, the current is so low before the gate non-trigger, the battery is certannly completly drained before it does stop. and after a rest it can be fired up again.

Once you go to PACKS in series, and DC-DC or boost or unboost curcuits all that CHANGES 100%

so parellel 1-1 white led & Li-Ion cell is pure dummy stuff almost, perfect for a dummy like me

 

[angerdan]

Is it possible to determine the remaining capacity of a Li-Ion cell just by measuring the voltage? Does anyone have a capacity (in percent) vs. voltage curve and would it apply to all Li-ion cells regardless of size?

It is possible, the onyl question is how precise measuring/estimating would be. Temperature differences and cell health/cycles also have influence.
An easy way to measure is the Nitecore UM20 charger with 2 digits %-display LCD.
It display 70% for a 18650 cell (Nitecore NL188 @27°C) with 3,83V (measured with LiitoKala Engineer Lii-500).

 

[magellan]

from what i know 'not really'.. in my lights.. the battery is almost completely dead at 3.5V.. they have a great little device that is a 'fuel gauge' for LiON batteries it gets hooked up to the battery and can tell you the MaH or WH that went in and comes out of the battery.. they are very accurate I read.

-awr

That's my understanding also, unless you have something like HKJ's battery review of that particular battery, with all the performance graphs so you can look up your situation to see if it matches one of his graphs.

 

[archimedes]

There are lots of other threads addressing these issues, that were started in the current decade.

Let's use those instead.