AW C-size li-ions: toast :(

[c0t0d0s0]

OK, so today I've been playing with the BigLeef kit in 2C configuration that I just got, only to discover that the runtimes on my AW C-size li-ions absolutely suck.

What I'm saying that I'm getting only 30 minutes of runtime with 2A load (which is like .6C discharge rate) before the protection circuit kicks in... which tells me that the batteries now deliver like a third of their stated capacity.

These cells have never been in active use. They only have a few cycles on them. They were purchased back in June, and back then, I didn't notice any problems, the runtimes were as expected. I must say that they were stored in a discharged state ( 3.5V) for a couple of months after the last use, I'm wondering if this contributed to the problem?

Anyway... I'm frustrated because: 1- I can't use my brand spanking new BigLeef system, 2-because those cells were $22 a pop, 3-they are no longer available, and 4-I'm not sure if I can trust them and will probably have to go the A123 route instead :/

Just wanted to share/vent.

 

[AW]

The 3.5V discharged state means the cells are bone dry and empty. Storing LiIon cells in such a state will kill them. Recommended storage charge is 40% ( around 3.85V ).

These cells have never been in active use. They only have a few cycles on them. They were purchased back in June, and back then, I didn't notice any problems, the runtimes were as expected. I must say that they were stored in a discharged state ( 3.5V) for a couple of months after the last use, I'm wondering if this contributed to the problem.

 

[c0t0d0s0]

This explains it then. Lesson learned.

So, when will those big boys be available again?

 

[AW]

Work in progress. Will post update as soon as I know.

 

[LuxLuthor]

FYI, I learned that same lesson with two Pilas which were $$$. AW's C cells with his protection are great for all the many <5A applications! :thumbsup:

It's a good reminder for everyone to check their stored Li-Ion voltages.

 

[donn_]

It's a good reminder for everyone to check their stored Li-Ion voltages.

What do you recommend for performing this test?

 

[DM51]

What do you recommend for performing this test?

All you need is a DMM to check the voltage. Voltage is an accurate indicator of the state of charge of Li-Ion cells. As AW has said, 3.85V is recommended.

As has been seen in this thread, storing them empty (3.5V or below) will kill them. Storing them full (4.2V) is also bad for them - there will be permanent loss of capacity. At 4.1V and below, the SD rate is low, but 3.85V is best for long-term storage.

 

[donn_]

All you need is a DMM to check the voltage.

Ok..recommended DMM?

 

[DM51]

Fluke 179... but there are plenty of far less expensive ones that will do perfectly well.

 

[donn_]

Is there a test tool which is especially made for batteries; that is, one which will test voltage, capacity, etc.?

 

[DM51]

You can get a ZTS tester, but to be honest a DMM is far more useful for Li-Ion cells.

 

[donn_]

The ZTS units appear to test capacity only.

 

[DM51]

... and for Li-Ion cells, a DMM will give you a better indication of that.

 

[donn_]

How does the DMM test capacity?

 

[DM51]

Approximate figures:​

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 practically empty ​

3.5 empty​

<3.5 = over-discharged​

 

[donn_]

Excellent! Thanks.

I'll order a DMM today.

 

[donn_]

Storing them full (4.2V) is also bad for them - there will be permanent loss of capacity.

How do you discharge freshly charged cells? Simply run them in a light for a while?

 

[LuxLuthor]

How do you discharge freshly charged cells? Simply run them in a light for a while?

That's the easy way. Hobby chargers like my Hyperion (Triton, Schultze, Bantam, etc.) have discharge modes...and able to set it to a particular voltage discharge target.

 

[Illum]

Approximate figures:
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 practically empty
3.5 empty
<3.5 = over-discharged​

come to think of it....my 17670s pretty much fubar
we're talking 3V on DMM and sitting here for days

 

[mudman cj]

Can anyone give a better idea of how long it takes to ruin a cell like that when stored at 3.5V? Every data point helps. How long was yours in storage c0t0d0s0?