Discussion - Lithium battery (A123) testing: voltage question (Full Version)

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FabioCode -> Discussion - Lithium battery (A123) testing: voltage question (8/15/2007 5:10:49 PM)

Hi to everyone, I'm new at this forum but I think you can teach me a lot about batteries.

I'm currently working on a lithium batteries pack (specifically a pack build by A123 cells). Before making the pack I'm going to characterize the A123 cell, but I have a doubt about that.

I want to start from the charged cell and make some current pulses in order to discharge it. The discharge process has to be make as follow:
1. charge completelly the cell
2. make a 2.3A discharge pulse for 10min (the battery should be at 90%)
3. make a 2.3A charge pulse for 5min (the battery should be at 95%)
4. restart from point 2 until the battery is fully discharged.

Now the problem I'm trying to resolve is the following: the datasheet says that the voltage cannot go upper than 3.6V and lower than 2V...when the battery if enought charged I cannot make a 2.3A charge pulse because the voltage goes upper 3.6V (but I know looking at the charge status that I can). The same thing happen when the battery is enought discharged: I cannot make a discharge pulse because the voltage goes under 2.0V (but, as before, I know I can).

So what I can safetly do?!? Is it correct to follow the voltage specification or is it better to look at the charge status.

Related to that I want to underline that the datasheet says to do not go upper than 3.6V but it doesn't say at which current: the battery has a custom internal resistance...if I charge it at 10A the voltage I will read will be upper than the one I will read charging it at 1A...so what voltage I have to look at??!?

Do you have any ideas?!?

Thanks for all
Fabio



guver -> RE: Discussion - Lithium battery (A123) testing: voltage question (8/15/2007 11:20:27 PM)

Let's see if I understand this.

1 Chargeing the cell at first can be done using a cc/cv routine so that the resting (full) voltage is 3.6
2 will be fine for many times
3 will be fine even if the the first few times the voltage goes over 3.6 during the 2.3 current, it should be fine to do the remainder of times as required.

Problem begins at point 2 when the battery gets nearly discharged and I think the same rule would apply here as I applied to the chargeing voltage going too high. During the 10 minute discharge the voltage can be pulled below 2 volts until a resting voltage of 2 is attained.

One question is the length of delays (to read resting voltage) between c and d cycles. Very curious test indeed, what device are you using to do the cycling?



FabioCode -> RE: Discussion - Lithium battery (A123) testing: voltage question (9/3/2007 7:29:40 PM)

Hi.

Thanks for the answer.

I have not already dediced the resting time, but it will be probably between 5min and 10min.
I'am using a system able to perform some profiles on the battery, one of them is the one I have described: it is based on a power supply, a loader and a PC which is able to command them.

I want to make this kind of test to look at the battery resting behaviour.

Maybe you can answer also to this question: what to do if I'll not use only a 2.3A current for the pulse but an higher one...for example 5C but for a smaller lenght of time? The voltage will increase over 3.6V but I don't think it is a problem (the battery should have enought SOC)...what do you think about?

Thank you
Fabio



guver -> RE: Discussion - Lithium battery (A123) testing: voltage question (9/3/2007 10:03:03 PM)

Yes, the same thing will happen at the early stage of your test. I would still not worry about the overvoltage if you started out with a discharge of (twice the capacity as the charge is going to be) capacities will still be as you stated earlier 95 and 90%

The problem is again going to show up at the lower end of the test. I think that at the small currents you are talking about that even a 1-2 minute rest is going to give a very accurate "resting voltage"

ps it sounds like a set up I'd like to see (the pc controlling things sounds pretty cool)



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