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Battery ID?

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studio45 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Topic: Battery ID?
    Posted: 06 May 2015 at 3:39pm
I have a lithium battery out of an e-bike with which I am experimenting. Initially I assumed it was a Li-Po type, and would have a per-cell voltage of about 4.2v fully charged. However, it won't hold that voltage - my charger will boost it up to 4.2 on its LiPo program, but it won't hold that voltage for more than a couple of minutes, and settles back to 3.6-3.65 per cell with no load.
It's currently happily delivering about 100 watts to some light bulbs, at about 12.8v for a pack of 4 cells. This would be the "flat" voltage for a Li-Po, and yet it carries on quite happily, the voltage and current have been stable for at least an hour now. If it was Li-Po I think they would be dropping like a stone at this point. It's not developing any heat at all, still stone cold.
Do you think it's safe to assume it is in fact a LiFe battery, and I can discharge it to about 12.0v (3v/cell) without damaging it?
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slaz View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote slaz Quote  Post ReplyReply Direct Link To This Post Posted: 06 May 2015 at 4:03pm
Would correlate with it being LIfePo I think ..... FCV is about 3.7/cell isn't it ?
You might be damaging the cells by trying to charge up to 4.2V/cell.

Is there a capacity marked on it ? If so (assuming your charger has a setting for LiFePO like my Accucall 6) ..... assume FCV of 3.7/cell and see if the capacity is reached .....


REMEMBER....POLITICIANS AND DIAPERS SHOULD BE CHANGED OFTEN AND FOR THE SAME REASON
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lutkeveld Quote  Post ReplyReply Direct Link To This Post Posted: 06 May 2015 at 4:04pm
If you look up some discharge curves of lipo cells you'll see that they drop to 3.7-3.8V per cell pretty quick.
Isn't there a ID number on the pack, so that you can look it up online? 


Edited by lutkeveld - 06 May 2015 at 4:04pm
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slaz View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote slaz Quote  Post ReplyReply Direct Link To This Post Posted: 06 May 2015 at 4:05pm
Originally posted by lutkeveld lutkeveld wrote:

If you look up some discharge curves of lipo cells you'll see that they drop to 3.7-3.8V per cell pretty quick.


Thats not my experience at all.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 06 May 2015 at 4:46pm
No markings on the pack that would identify it...If you saw it you would laugh. Originally it was an 8S pack, it's made of  quite large, flat pouch cells. The foil tabs from these are terminated onto small pieces of PCB in two groups of 4. The joint between these two PCB's was a piece of copper foil poorly spot-welded onto the aluminium foil tab protruding from the lowest cell of the 4-stack. The whole thing was held together with paper masking tape (I shit you not) then glued into a cardboard case, then a large piece of shrink-plastic over the outside. It had a balance/charge control board built into it. This would never seem to let the pack get above 27.5v, which I thought seemed low for an 8S LiPo pack. But then, the thing never worked reliably at all, due to the aforementioned crappy joint working loose whilst in motion all the time. Stated voltage is 24v, stated capacity (in the bumf for the original bike) is "8 Amps" which I take to mean 8Ah, though elsewhere I have seen the bike advertised as having a 10Ah battery. Unclear if it was exactly the same model or some later update.
I had it on the lightbulbs for about an hour, during which time it delivered a steady current of 7.5A at a voltage from 13.5 to 12.0v. Below 12v the voltage started dropping rather more rapidly, and I stopped the test at 11.6v (2.9v per cell) where the current was 7.3A. The pack was slightly warm to the touch at this point. Now it's on the charger, which counts the mAh's as the go in, so I shall be able to determine my actual usable capacity....My amp packs will just about start to have trouble at 11.6v as the 12v protective relays will un-click and mute the outputs, I suppose I could put them on a regulator but it acts as a rather "definitive" low-voltage cutoff system ;)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote lutkeveld Quote  Post ReplyReply Direct Link To This Post Posted: 06 May 2015 at 4:57pm
8S and 24V is LiFePO4. Hope you didn't damage the cells too much charging them with 4.2V/cell.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 06 May 2015 at 8:33pm
Hopefully not - they were only there for a few minutes. If this all works out I will have a pair of batteries that can run the rigs flat out for about 2 hours and only weigh about 1kg each. Much better than my current 15kg Yuasa NP-38's :)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote WesleyK Quote  Post ReplyReply Direct Link To This Post Posted: 06 May 2015 at 8:50pm
Originally posted by slaz slaz wrote:

Originally posted by lutkeveld lutkeveld wrote:

If you look up some discharge curves of lipo cells you'll see that they drop to 3.7-3.8V per cell pretty quick.


Thats not my experience at all.


Strange, after charging the voltage should drop to about 4.0-4.1V pretty quick. Some slight load later its should drop to about 3.9V, with most of the capacity around the 3.7-3.8V mark. 

@Studio45, can you tell us some more about your charger, charging current for example? And the age of your battery pack?

A few minutes shouldnt have damaged your batteries normally :)


Edited by WesleyK - 06 May 2015 at 8:53pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 06 May 2015 at 9:17pm
The charger's a Sigma EQ Mini, claims to be able to charge at 4.5A but only seems to want to do 2.2A into this pack. Has programs for Li-Po, Li-Fe and Li-Ion (strange as I thought all rechargeable lithiums were a form of Li-Ion??) as well as NiCd, NiMH and lead-acid.
Age wise I think the cells are about 3 years old - the original bike came from a mate who works for a large European sporting goods retailer, who tend to throw out perfectly good customer returns rather than try and fix them at all. So it sat in his cellar in bits for about a year, then he gave it to me and I bodged the electric system and motor wheel onto my bike frame, this was last year. I rode it around for about 9 months then it failed completely so I dismantled it. It's been sat on my bench unused since February. Now it shall ride again ;)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote WesleyK Quote  Post ReplyReply Direct Link To This Post Posted: 06 May 2015 at 10:48pm
In that case it might just be the age and worn state of the batteries. That aould also explain why you were onlu able to charge with about 2.2A. The internal resistance of the cells is high enough to have the cell voltage reach the constant voltage level of 4.2V, lowering the charging current. After disabling the charger the cells drop again to their actual voltage. I would discharge the cells to 3V using a moderate load and write down the battery voltage every now and then to see if the discharge curve matches either li-ion or lifepo.

Edit: hmm reading the start post again im not quite sure, according to your test with the 100W load the batteries seem to be fine. I would discharge them to about 3V/cell using the 100W load and see what that does to the discharge curve.

Edited by WesleyK - 06 May 2015 at 10:53pm
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