Charing Li-Poly Battery with voltage below 2.84V

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Hi All

 

I been using Li-Poly Batteries on a GSM based system for some time. My original spec of the battery was 2000mAh 4.2V, with protection circuit.

 

Recently i upgraded my battery to 5000mAh 4.2V, still with a protection circuit.

 

My Battery charger IC is stc4054, set to charge at 0.5A

 

To confirm that i was getting at least twice as much life, i charged the battery fully. And let me system drain the battery for few days to my system was reporting 0% battery capacity.

 

With the original(2000mAh) version , when the battery reached 0%, even after few days. If i plugged in the USB , the battery would get charged.

 

But with new 5000mAh, when the battery reached 0%, it would not charge the battery.

 

After some investigation it seemed the battery voltage reached under 2.8V. 

 

 

 I had to set my desktop power supply to 4V with current limited to 100mA. I then charged the battery directly for about few mins till it reached 3V, after which i then connected the battery back to the system to charger properly with Li-Poly Charger IC.

 

I know i should not charge the battery directly, but i limited my current to 100mA as a protection. Which seemed to recover the battery from this under voltage stage.

 

Is this a normal behaviour of Li-Poly battery? Or could the protection circuit not include an under voltage protection?

 

Thanks

 

 

 

 

Thanks

Regards

DJ

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I used some military LiIon battery packs that needed special care if they were discharged too much. 

 

You need to get the specs for the battery you are using to answer your questions.

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With the original(2000mAh) version , when the battery reached 0%, even after few days. If i plugged in the USB , the battery would get charged.

What is 0%...someone may say it is 2.885 V, someone else may say 3.06V.  How far down in voltage did both batteries get before recharging? 

Simply sounds like you let the new battery go too low.

 

your chip:

VPRE Pre-charge threshold RPROG =10KΩ VBAT falling 2.8 2.9 3.0 V Hysteresis RPROG =10KΩ 70 100 130 mV

Or could the protection circuit not include an under voltage protection

Those come in many many many flavors...I had to look into specing one recently....it's like its own industry. 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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I don't think that CHARING one of those battery is a good idea.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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He meant Charing Cross, don't you know? The battery was 'up the junction'.

 

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Lithium batteries of all kinds do NOT like to be discharged to too low of a voltage. For most kinds, that minimum voltage is in the range of 2.2V to 2.4V. Once it gets lower than that, you need to take special care to recharge them back to the minimum acceptable voltage. If the battery pack has a protection circuit, the circuit should disconnect the battery from the load just above its minimum voltage. When that happens, you may need to do something special with the  protection circuit so that it will accept charging current because it may still have the battery disconnected.

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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As already said you need to check the datasheet of the battery.

And the schematic of the protection circuit on the battery.

I find it interesting that you were able to measure a voltage at all.

If there is a low level protection, the cell should be giving you 0V when you measure the voltage after discharge beyond capacity.

 

How long did you leave the battery connected to the Charger?

I always use a MCP73832 to charge LiPo cells. I know that when the battery voltage is below a certain level it will start with trickle charging the amount depends on the set charging current I think ( we designed it a zillion years ago and now always just do copy paste as we always use the same battery and thus the same charger can be used. ).

When the voltage of the cell goes above a minimum threshold then and only then the charger will switch to the set charging current to do a normal full charge.

whet the 2000mAh battery that threshold will be reached much faster than with the 5000mAh battery so you will need to wait a lot longer.

 

Now the battery normally will have 2 protection circuits, 1 is over voltage protection and the other is under voltage protection. Their values should be mentioned in the datasheet as your charger settings depend on them.

when you go below under voltage, the battery will no longer seem to be connected. until you start charging and the battery goes above the set recovery threshold. Then the protection FET will be opened again and you can measure the battery voltage. It might be that at this voltage you are still only trickle charging the battery. With over voltage is the same. The FET will kick in and only be removed again when the voltage on the battery connections has dropped to below a certain value.

 

this is what we use:

 

Note that this circuit and the specification are part of the battery.......

 

 

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djoshi wrote:

 

 I had to set my desktop power supply to 4V with current limited to 100mA. I then charged the battery directly for about few mins till it reached 3V, after which i then connected the battery back to the system to charger properly with Li-Poly Charger IC.

 

I know i should not charge the battery directly, but i limited my current to 100mA as a protection. Which seemed to recover the battery from this under voltage stage.

 

Is this a normal behaviour of Li-Poly battery? Or could the protection circuit not include an under voltage protection?

 

 

I've done this several times with under-voltage and completed drained batteries, although I usually use a an even lower charging current - like 10mA. In fact, I've done it so many times I keep telling myself I need to create a "resurrection charger" just for this purpose.

 

I think you still want the under-voltage protection for normal use. My understanding is that discharging Li-ion batteries below 2.7V with a heavy discharge current is dangerous. However, perhaps if the discharge is slow (like the self-discharge that naturally occurs) they can be brought back to life with a trickle charge current. This seems to be my experience.

 

Last Edited: Mon. Sep 14, 2020 - 04:14 PM
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Most of the recent Li type batteries have a built in Battery Management System (BMS) to prevent over-charging/discharging by disconnecting the I/O when the condition occurs.

I'm not sure what the magic formula is to cause the BMS to reset, I would suggest you contact the battery maker to confirm it's existence and how to handle it. 

Best just not to rely on the BMS and correctly charge/discharge the battery per the makers spec's. 

 

Jim

 

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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When flying model aircraft I have, once in a while, forgotten to disconnect LiPo from ESC after flying and only realised subsequently (usually the following day) at which point the cells were well below the usual lower bound. On the whole if batteries get to this state and you try to recharge (even trickle) they will balloon with the potential to ignite/explode. I have a few packs that are complete balloons where I foolishly thought I might be able to recover them. When dealing with 250mAh / 370mAh packs there is some danger but if it does explode it may not be catastrophic. However, with 5000mAh I imagine the danger is very real indeed. I would only attempt any form of charging outdoor in a sandpit with a spade on hand to cover any ignition with piles of sand (pretty much the only way to extinguish a lithium fire).

 

if I were you, although I know the packs may have been fairly expensive, I'd write it off to experience. Get new packs and move on. (of course that then raises the question of how you discard a potential bomb but if it's discharged I guess it's not too dangerous).

 

Here is just 1100mAh just passing the balloon stage:

 

https://www.youtube.com/watch?v=...

 

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Hi Jim

 

I think maybe the disconnection voltage might have been set to 2.2V -2.4V. My charger requires a minimum of 2.8V, therefore i guess i need to request a battery with voltage protection of under voltage of about 3V.

Thanks

Regards

DJ

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The battery was left on the charger(desk top power supply) for about 5 mins to get it back to 3V, after which it was placed in a proper charger. 

 

It seems the protection circuits under voltage was less then the requirement of my charger, and maybe the reason why my charger could not charge the battery till it was about 3V. There can also be a possibility there is no under voltage protection. 

Thanks

Regards

DJ

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Another possibility just came into my mind.

 

Can it be possible that once the battery reaches 2.84V which is very low, that charger would go into trickle charge but my circuit would draw that small amount of current, causing the battery not get much of a charge?

 

 

Thanks

Regards

DJ

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Any of those is possible. Protection circuits are not equal. Only the battery supplier can tell you.

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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Hi Jim

Yes I learnt from this experience the battery protection are not equal. Il

Thanks

Thanks

Regards

DJ

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djoshi wrote:

Another possibility just came into my mind.

 

Can it be possible that once the battery reaches 2.84V which is very low, that charger would go into trickle charge but my circuit would draw that small amount of current, causing the battery not get much of a charge?

 

 

That for sure will be true.

If you check the datasheet on your charger IC you should be able to find at what current it is trickle charging. When you then know how much current your circuit draws you will know the actual charging current.

Note that 5 minutes of trickle charging might be rather short, specially on a 5Amp battery. Best connect it ( for a first time outside or in a safe environment as Cliff said ) and leave it there for 1 hour, and see what is happening then.

Charging a 5Amp battery with only 0.5Amp will ensure it will be charging for at least 12 Hours ( last part of the charge takes longer to get into the battery as the current will drop ) So you could even go sleeping over it, as long as you put it in a safe place were it is not a problem if it would balloon and catch fire.

I am still puzzled by the low voltage though. IIRC a Lipo should never go below 2,9V or it will get damaged so most cells will have been put at 3V cutoff....

 

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5 mins is charge is on standalone desktop power supply till battery get to 3v, after which my systems battery charger is able to charge battery while device is also powered.

In trickle mode it's 1/10 charge current.

But when using a 2000mah battery, I did not get this issue.

Thanks

Regards

DJ

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That might be because the 2Amp battery is at a higher voltage level much quicker than the 5Amp one. You need to add 2,5 times as much energy to get to the same charge percentage.

Also the cell structure might be slightly different and as such the charge voltage curve might be a bit different. A lot of variables.