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Author Topic: 12V li-ion now 7.11V  (Read 1099 times)
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Bought one of those cheap chinese li-ion batteries. The charger stopped working the half way through the first time.. Used a similar charger and worked ok. And battery was ok.

Either I shorted the battery or one of the cells inside broke? At least I put it under somewhat strain (connected a pc to it).

Anyway, assuming there really isn't any easy way to fix this, so any point in opening it? Can I use the presumably two good cells inside for something?

I have another one and these batteries are about the same size as the one in my laptop (6800 mAh), but it looks like the battery wasn't able to provide power for more than like 10 minutes (voltage went down and then back up again after a while). Any ideas? Works fine with AC-DC adapter (psu is supposed to handle 8-36V or something..) -- is it just too much for it? Wikipedia says the intel atom d525 is supposed to use 12W (1A?)... didn't manage to get freq scaling working yet. and some other stuff at perhaps (keyboard, usb, etc) max 1A.. This would by my calculations mean it should hold for several hours??

Will try to connect my arduino with current sensor later today, but anyone have some comments to all this?
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Can you measure the voltage across individual cells?
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Can you measure the voltage across individual cells?

Would have to open it.. Guess I could do that, not much point in the battery now.. Just cut away the plastic?

Anyway.. Tested the current drawn from the PC and it's around 800-900 mAh on 18V DC, on 12V battery it's about 1300-1500 mAh... :/ Will leave it to see if it lasts longer now.. At any rate that _should_ equal at least a few hours..
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BTW you're confusing mA with mAh, mAh is a unit of electric charge, not current.
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I suppose.. It's quite some time since I had physics in school, so correct me where I'm wrong:

6800mAh / 1500 mA ~= 4.5h?

6.8Ah * 12V = 81Wh (???)

Now.. it's 1.5A at around 10.5V..

10.5V * 1.5A  = 15.75W

81/15.75 == 5.1h

Any of these correct?

I figured out why it shut down. The stupid undocumented PSU (have to stop buying stuff from china without usermanuals/datasheets) is for cars and it needed to think the car was on which ment connecting IN+ to something called ACC or it would shut down when voltage was less than 10.5 or something..

Now the Li-Ion battery is at 10V.. Will be interesting to see how long it can stay on.. And there we have the answer.. Another 30 minutes.. These numbers are a far cry from my calculations above... Comments?


Will cut open the first battery now to see what's inside..

edit: W not kW
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I should have probably started two topics.. but...

I have now ripped the battery apart and first off I found some cardboard inside to support everything.. is this normal?

Anway, have now measured the three individual cells and they are around 3.9-4V..!? Probably some bad connection inside then.. So now what?

And anyone have any ideas about my calculations above?
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  Your math is correct as you have shown it. The thing that is not always predictable is the performance of your battery. While it is rated at 6800mah it can produce more or less mah than it is rated for. However, I think it should be within 80% of it's rating. I am not sure what the standards are for battery claims.

 That being said, if your 12v battery only charges to 10v you might have a bad battery or you have not charged it fully.

 I have seen car 12v batteries "Lead-Acid" go bad were they would measure 9v while sitting. Then, during charge they might show 16v+ but, the voltage would immediately fall back to 9v when charging was discontinued.
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A little backround of li-ion type batteries and their terminal voltage.

Li-ion battery packs of any voltage are made up of single cells. A single cell has a range of voltages depending on it's state of charge. At 100% charge it will have a 4.2vdc terminal voltage, attempting to charge it to a higher voltage will destroy the cell and possibly cause it to catch fire. As the cell discharges into it's load it will decrease voltage in a fairly linear value which can be used as an indication of it's remaining charge. As the cell decreases to around 3.0vdc the cell is said to be completely discharged and should be immediately disconnect from any load. Discharging to below 3vdc is likely to cause permanent damage to the cell, however maybe without the fire and drama of over charging, but rather just to the point where it will not be able to accept a recharge, thus rendering the cell useless.

So a single cell should only be used in a range of 3-4.2vdc.

A two series connected li-ion cells therefore should only be used in a range of 6-8.4 vdc.

A three series connect li-ion cells (what you own) therefore should only be used in a range of 9-12.6 vdc.

So your three cell battery that is now measuring 7.11vdc, that means it's been discharged below
it's safety margin and may have in fact been damaged from over discharging.

Them are the facts as best as I can recall. You may get luck and attempt to recharge the battery with a proper li-ion charger and see what happens and if the pack returns to 12.6vdc when charge is complete. But watch the battery closely for overheating and remove the charger. Many R/C hobbyist recommend recharging li-ion and li-po batteries in a metal container in case of sudden fire.

Lefty

PS: just saw your respose about cells being 3.9 to 4vdc. That's good for you as it means they haven't yet been over discharged below their safe ratings. Do be careful with li-ion batterys, they do have wonderful charge density specs but do require special care in their discharge and charging process.

« Last Edit: February 18, 2012, 11:08:23 am by retrolefty » Logged

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Now a little about battery mAH ratings. A manufacture will state a nominal mAH rating for it's battery at a specific discharge rate, usually at a rather small discharge rate like C/10. So a battery rated at 6800 mAh may only supply the full 6800 total maH rating only if discharged at 680ma or lower. At higher discharge rates the total mAH rating will be less. How much less? Sometimes the manufacturer will show graphs of mAh capacity at various discharge rates in their datasheet for the battery. If you have no access to the datasheet then trying to calculate real world battery duration time at current draw rates higher then C/10 are bound to be in gross error. Only by testing and measurements with different discharge rates will you be able to get a more accurate idea of how long your battery will power your actual circuit.

Lefty
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Thank you for feedback, appreciate it. I should probably mention that the charger says 12.6VDC 500mAh, but outputs 12.3V.. Which should mean that full charge would take

6800mAh/500mA == 13h.. It doesn't.. More like 3-4 hours (charger led goes from red->orange->green)

Could this be the reason for "low charge"? A few months ago my cell charger stopped working and I changed it with the one for a tablet.. Now the battery on my phone lasts twice as long! 

I'm planning to build my own charger (LT1505) for the 12V batteries, anyone think this would help? Think I will select the 4.2V pin on the chip despite the cheap charger accompanying the batteries outputting 12.3V..

@lefty Thanks. Planning to hook two batteries in parallell but they will also power servos, but then it should be nicer to the batteries I suppose..
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@lefty Thanks. Planning to hook two batteries in parallell but they will also power servos, but then it should be nicer to the batteries I suppose..

Paralleling li-ion battery packs in NOT a recommended practice unless all the cells have been matched for proper balance by the manufacture. I do recommend you do some reading and research into li-ion batteries on their characteristics and proper handling. They cannot be treated like other kinds of batteries, and I'm sure I haven't covered all the issues you should be aware of. Mistreatment of this kind of battery can lead to damage and injury.

Be safe.

Lefty
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@lefty Thanks. Planning to hook two batteries in parallell but they will also power servos, but then it should be nicer to the batteries I suppose..

Paralleling li-ion battery packs in NOT a recommended practice unless all the cells have been matched for proper balance by the manufacture. I do recommend you do some reading and research into li-ion batteries on their characteristics and proper handling. They cannot be treated like other kinds of batteries, and I'm sure I haven't covered all the issues you should be aware of. Mistreatment of this kind of battery can lead to damage and injury.

Be safe.

Lefty

I am aware of that, but I have chosen to go that way anyway. I was actually planning to ask here on the forum how dangerous that was... ?

It will be two batteries ordered from the same place so hopefully from the same lot. It will all be monitored by an atmega (battery voltages, temparatures, current flowing back an forth) which will be able to shut down the charger (and now I'm thinking at least able to shut down one of the batteries) and buzz if something is wrong..

The batteries will also sit in lipo charging bags..
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but I have chosen to go that way anyway. I was actually planning to ask here on the forum how dangerous that was..
Well very dangerous.
This is because you have no control over what current is going into and out of each battery. One ends up charging the other and supplying all the current. So I hope you have a good cheap supply of batteries as they won't last long.
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That wasn't the answer I was hoping for.. I'm struggling to find some suitable larger capacity batteries, can find 9800mAh but that's not much better..
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Hmmmmmm... After having a couple of glasses of wine and trying to watch a B movie I got to thinking.. If the only problem is current flowing between the batteries, wouldn't it be possible to use one at a time? Have arduino switch between them?

The point is these 6800mAh batteries are more than big enough (physical size) for the project, but wouldn't make it last long enough with pc and x number of atmegas and probably 20 servos and cameras and 3g etc.. The option then would be to move up to 9800, which would probably occupy too much space and still provide only ~45% more than the 6800 battery.. And I would really like to have two batteries placed at two different places for symmetry.

So.. Could I switch one battery on and the other off using relays or mosfets (I don't really know what mosfets are, but think they might apply here) every ten minutes or so?
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