how long does a a123 20ah hold 3.65v?

[izeman]

i've searched a lot but couldn't find an answer. not here not somewhere else.
i charge to 3.65v, and within 2 hours, cells are down to 3.4v.
i will monitor in 8h again, and see how much lower they are then. i know that those cells love to be around 3.3v, but why should i charge to 3.6v, when all this seems to vanish within hours? or do i need to charge that high to have that charge still available but at a lower voltage?
cells are victpower short tabs from different sources (as it seems) but they all behave the same.

 

[migueralliart]

I think typically with high power cells either lipo or lifepo4 there is always an initial charge top off and then it tapers down. Like when you charge a lipo pack to 4.2v per cell and then you connect it to the bike and do full throttle for a couple of secs all the voltages are now less than 4.2v. Lifepo4 cells tend to be in the 3.2-3.3 range the majority of the time.

 

[izeman]

i know that the first some percent of the top voltage vanish really fast when used, but did you charge those and measure voltage after some time? w/o using them? just letting them sit?

 

[dogman dan]

Ideally they'd hold 3.65v overnight when brand new. After some use, dropping to 3.5v would be fairly normal. After much much use, dropping to 3.4v would not be unusual.

This is how ping lifepo4 cells have behaved for me, so I can't say if A123 should hold it's surface charge better or not. Either brand, dropping to 3.4v when new is not ideal.

Charging to 3.65 is not really needed, but once you have, you know it's really at it's full capacity. Charging to 3.5v would be ok. Typical lifepo4 bms's will charge to a higher voltage, 3.6,3.65 or something like that. That way as much wh as possible gets into the lowest voltage cells before the charger shuts off. My ping bms for example, will show cells charged to 3.8v as the bms does it's work.

 

[neptronix]

Not long. No cell holds the top of it's charge for a long time.

With a RC Charger capable of producing discharge graphs, you can find this out.
A CBA II could also show you this as well.

 

[agniusm]

I can comment on this as well. I have replaced my cars battery with A123 4 cell battery for better performance in winter. After using it for 2+ months i took it out to check voltages and charge it to see how much capacity it has lost. Voltage was 3.43V

so i thought i will charge it. Hocked up to a charger at 5A rate and after few seconds it started dropping down and within 1.5 min it was at set voltage of 3.60V:

I still need to discharge it to check the capacity but it looks good minding i dont use BMS and it is being used at 0 to -16C. I am yet to find out what effect cells have from alternator charging the battery at temperatures below 0.
So i guess it is normal that cells settle at 3.4/5V

 

[999zip999]

My hyperion charges to 3.59v. and is prefect as the cells settle to around 3.5v. Till I twristed the throlle and 3.44v or something drop and slowly work down form there. Then i don't worry till I see under 3.1v. after that the battery feels as if going flat. No bms no worries. Maybe 17ah plus and no worries.

Edit: that's settle to 3.554v. - 3.531v.

 

[TMaster]

After charging to 3.6V per cell, my A123 3P packs are around 3.5V - 3.55V in a week if not used. I have a main key switch so there is no current draw sitting. The cells get a tad warm when charging, so when they cool off, Im sure thats when the voltage comes down a tad.

 

[izeman]

... Hocked up to a charger at 5A rate and after few seconds it started dropping down and within 1.5 min it was at set voltage of 3.60V...

so it was not dropping. it was raising from 3.43V to 3.60V i guess. i was 3.40V, and charging it to 3.60V @5A took <2min. correct?
i see it raising from 3.5V to 3.65V really fast.
i also had the 2nd 6s part of the 24s battery charge overnight with the bc168 set to 4A. it doesn't stop charging at 3.65V completely, and still trickle charges with 0-0.1A. this seems to make the cell really full. now it holds voltage longer as well. and they are 5mV balanced. will see what it looks like when i'm back home @10h.

 

[sendler2112]

There is no real power in an Amp20 cell above 3.3v or below 3.1v Anything above 3.3v is quickly used up after charging. Here are some charge/ discharge curves for an 8s battery at .5c .
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[dogman dan]

That makes sense. After a block of riding, likely you have dropped to 3.4v or less, and the meat of the discharge is around 3.3v. That's normal for lifepo4.

But I still say, for new or close to new cells, self discharge to 3.4v should not be normal. Newish, they should self discharge overnight to about 3.5 more or less. By self discharge, I mean plugged into nothing but a bms.

Not saying they are ruined if they do, but that they are displaying behaviour more like older cells with hundreds of cycles. Or that failed a QC check.

 

[izeman]

all i can say is that those cells DEFINATELY are FAR FROM NEW. but you can't say that just by looking at it. if a cell gets ripped out of a pack by a (maybe not well trained) chinese worker, than chances are big that this cell looks like sh*t afterwards.
those cells still deliver good 19Ah+, so that's ok. i could be, that the bc168 stopped charging a bit too early, as this charger likes to push high amps until the very end of the charge. so the cell may have reached 3.65V for a second, and the charger stopped.
those connected over night seems to be more in balance, and seem to hold charge longer.

@sendler2112: thanks for the graphs. this was what i was looking for for a long time. as a technical engineer i just never will understand why ppl make graphs w/o determining the measurement units of x/y axis. but as i'm very clever i figured [min] for x, and [V] for y *scnr*

 

[sendler2112]

His cells in the charts are Victpower cells and tested at 18.3 Ah at 0.5C They entered the discharge test at 3.48v and quickly dropped to 3.31v under load. The charge cycle also shows that the curve is near vertical at 3.55v and bounced down to 3.48 after the charger shut off.
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http://www.youtube.com/watch?v=Z7BMvU29k2g
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http://www.youtube.com/watch?v=RYOZUteqNEg
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[izeman]

looking at the date of the video they should be long tabs. afaik these where the only cells available at that time.
Vdelta of .17V @9A gives a high IR of 13mOhm. so my cells seem to be better
but this was very accurate testing and very well described description of measurement methods.

 

[izeman]

i now measured my over-night charged cells. they still hold 3.62V. so they dropped 0.03V after 12h. will see where they are tomorrow.
i will charge the 1st set tomorrow for some more hours, and see if they hold the voltage that long as well. then i'm happy

 

[SamTexas]

I have replaced my cars battery with A123 4 cell battery for better performance in winter.

Do you mean a 4s4p pack using A123 26650 2.3Ah cells? I am planning a 4s5p pack for my car. I'm in Houston and the average temperature of the coldest month is 7 celsius. Coldest ever was -13 celsius, but that only lasted a few hours. My car needs 250A max to start. I wonder if 5p is overkill? What do you think based on your last two months of experience?

 

[sendler2112]

Vdelta of .17V @9A

That's not really Vdelta is it? From no load fresh off the charger to 0.5C? Every battery will have a high surface charge when fresh off the charger. You would need to take the pack to %50 SOC then vary the load from medium to heavy to get an idea of the true sag voltage.

 

[izeman]

tested at 18.3 Ah at 0.5C They entered the discharge test at 3.48v and quickly dropped to 3.31v under load

3.48V is not fully charged. is it? but yes. almost full.
you are right. discharge 10Ah and then do the load test. add some 0.5C load, than maybe 2C and calculate difference. this will give accurate results. let's conclude that i can not measure IR with the given data

 

[SamTexas]

i know that the first some percent of the top voltage vanish really fast when used, but did you charge those and measure voltage after some time? w/o using them? just letting them sit?

My experience with LiFePO4 is some what limited at this time. But I observed about the same thing you did. I stop charging at 3.60V, let it rest overnight and the voltage comes down to 3.50V. Anything above 3.45V is fully charged for LiFePO4, I think. I have not, but will do a capacity test to see if there is any significant stored energy above 3.45V.

This behavior is only applicable to LiFePO4, as far as I know. Laptop LiCo, RC LiCo, LiMn all hold their full charge 4.20V for days.

 

[TMaster]

When i was doing RC, the lipo's I charged would drop to 4.15V - 4.17V within minutes.

 

[SamTexas]

When i was doing RC, the lipo's I charged would drop to 4.15V - 4.17V within minutes.

You mean from 4.20V? If so there are 2 possibilities:

1) You were charging at too high a rate. Because of the high rate you were seeing a reverse voltage sag. So when the charge is removed the voltage recovers.

2) If one is not true, then your RC LiCo (LiPo) has degraded.

I use a low current 1.5A CC/CV charger. The final cell voltage is 4.20V and stays there for 24hrs. In fact this is how I weed out degraded cells. Anything that drops more than 0.02V (from 4.20V) is eliminated.

 

[999zip999]

Izeman alot of us are running without a bms so 3.59v on my hyperion 1420. I know if charged to 3.65v. or 3.8v you can squeeze a .5 0r 1 ah more out of a pack. But it likes 3.59v and I'm happy. It lives below 3.45v.

 

[izeman]

i measured my pack this morning. the FULLY SLOWLY charged 6s pack still has 3.62V after 24h. so i had been loosing 0.03V which seems perfectly fine. atm i'm charging the first 6s pack which had this voltage decline, as i (as written before) expect it to not having been "fully" charged - just touched the 3.65V and then let of the charge.
so i'm ok with that, as it seems fine. i expect the voltage to drop with load attached to some 3.4V which is fine also. i'll do some discharge tests with two heaters this evening and monitor voltage.

 

[dogman dan]

Good to know they aren't self discharging much after all. You were just seeing the difference between what your charger was putting out, and what the cells were really charged to.

I see similar drop with my lipo, when you use the quick charge option on the chargers. Stopping a bit earlier, they aren't actually fully charged 100%.

We see this sometimes in battery troubleshoot threads. Guy will say, "I charged the pack" and we reply, "but did the pack actually get charged?" Sometimes communicating this way has it's problems.

Enjoy your A123's, they're good!!!

 

[megacycle]

I was looking for this info for a while, i think somewhere in this thread Doc bass also posted a chart of SOC/DOD expected cycle life, help doc can't find it http://www.endless-sphere.com/forums/viewtopic.php?f=14&t=37955&start=45 "For long life use, every lithium cells have some preferences. They don’t like heat so keep them between 10 and 25 celsius to prolong their life. They don’t like being too much time at full SOC and being depleted so keep them in a range between 80-20% SOC."Doc