assembling a 11.1v battery using 3 Lipo 3.7v with a protection circuit

[valerio_sperati]

Hi all,
I'm working on a project on autism therapy, the +ME experimental device,
http://www.plusme.it/last-prototype-to-experiment-with-children/
which is powered by a commercial 11.1V Lipo battery, 500mAh.

I realised right now that our commercial battery lacks of the protection circuitry preventing the cell to discharge below the safety voltage (3.0v). As  far as I know, this could be very very dangerous. I tought it was a standard safety component, but it is not!

https://learn.adafruit.com/li-ion-and-lipoly-batteries/protection-circuitry

Adafruit suggested me to  add this protection circuit to the pack battery

http://www.all-battery.com/PCBfor11.1VLIPO_Li-Ion_Li-MnNiBatteryPack-32071.aspx

but obviously this means I need to disassemble the original pack battery, de-sold and put together again in the right way: it sounds to me not a trivial work, even because I've not experience about it.
In your opinion:

1) is it feasible for an hobbyst like me to perform such operation in safety? If yes, can you suggest some tutorial about assembling 3 lipo cells + this protection circuit?

2) I looked on google for 11.1v lipo battery 500 mAh already endowed with this protection circuit, but   at the moment I found only Adafruit selling this protected battery, unfortunately only 3.7 battery. Do you know some site selling these products? It would solve my problem!

Thanks a lot. Please help me because the project is really important!

PS: some (hopefully) useful info:
-the power consumption of the device is about 110 mAh (device on, lights and sounds off), and reaches 200 mAh when all lights and sounds are on (the worst situation!). In average, it request about 150 mAh (device on and some light or sound on). We need the device to work for 1 hour, no more.   

[raschemmel]

I don't have time to read your post in detail right now but I can tell you for a FACT that a 3S Lipo battery is 12.54V , fully charged. (11.1V would be almost discharged. 9.6V is fully discharged)

[allanhurst]

I realised right now that our commercial battery lacks of the protection circuitry preventing the cell to discharge below the safety voltage (3.0v). As  far as I know, this could be very very dangerous.

Well it won't do the battery much good, but apart from that no great problem

regards

Allan

[valerio_sperati]

Well it won't do the battery much good, but apart from that no great problem

regards

Allan

Mmmm... not sure I understood. Are you telling this is not dangerous? As far as I know, when a pack is below 3.0 V it becomes "fat", and can break itself! Is it true? Or not?

[jremington]

It is a good idea, and relatively simple to add the protection circuit. If you are not comfortable with soldering, find someone who is, to help.

The battery won't explode or catch fire if it is discharged below about 2.8 V/cell, but it can be damaged to the extent that it won't take a charge (and attempting to charge such a battery could be dangerous).

[valerio_sperati]

The battery won't explode or catch fire if it is discharged below about 2.8 V/cell, but it can be damaged to the extent that it won't take a charge (and attempting to charge such a battery could be dangerous).

Ok, this reassure me. So, in your opinion, it is not dangerous to reach the threshold of 2.8v/cell, but rather the attempt to charge again the (damaged) battery. Is it right?   

It is a good idea, and relatively simple to add the protection circuit. If you are not comfortable with soldering, find someone who is, to help.

Good! But I need some tutorial I'm very comfortable with soldering , but I need some schema to follow explaining to me how to assemble 3 cells in 1 pack, so that: 1) it is protected by the circuit 2) it is well assembled so that i can recharge the pack in safety with a commercial recharger. Can you suggest one? Thanks a lot!

[ad2049q]

Don't do that !
Taking apart a three cell Lithium battery is unlikely to improve its reliability.

Lithiums really don't like being heavily run down or left empty and once full they don't pass through "overflow" if the next cell is not full yet.

To get a more predictable lifetime, will it work with a six cell "12V" Pb H2SO4 motorcycle battery or smaller ?
Expected maintenance cycle is for complete replacement in 1000 cycles or 3 years whichever is soonest.
Lead batteries might seem to need less cell to cell balancing than Lithiums, at the expense of being heavy .

[valerio_sperati]

To get a more predictable lifetime, will it work with a six cell "12V" Pb H2SO4 motorcycle battery or smaller ?
Expected maintenance cycle is for complete replacement in 1000 cycles or 3 years whichever is soonest.
Lead batteries might seem to need less cell to cell balancing than Lithiums, at the expense of being heavy .

Unfortunately, this is unfeasible for my project! The battery I'm using right now is 49gr weight, 56x30x19 mm in dimensions. A motorcycle battery is huge!

Don't do that !
Taking apart a three cell Lithium battery is unlikely to improve its reliability

It seems to me very strange that adding a protection circuit like this
http://www.all-battery.com/PCBfor11.1VLIPO_Li-Ion_Li-MnNiBatteryPack-32071.aspx
to the Lipo Pack is so difficult! I assume that in order to add such circuit I need to disassemble the pack. Am I wrong?  

               

[raschemmel]

Disassembling a Lipo pack is dangerous and likely to end in it bursting into flames.
In ten years of RC aircraft flying , most of the Lipos that"puffed up" , did so after discharging them to less than 3.2 V/per cell, not 2.8 V/per cell. Only a few of them "puffed up" during charging.

[Chagrin]

all-battery.com sells round cells with built-in protection and you can get them with or without the wires. Very simple to work with. Just connect three in series and you're good to go.

If you already have a battery pack you could use a BPC that doesn't have connections to the individual cells in the pack and eliminate the need to disassemble your pack. For the most part, assuming your project doesn't involve high current drain or require fast charging, the cells in the pack will stay balanced in voltage and the individual cell connections aren't required. If the cells aren't staying balanced then you need a new battery pack or cell anyway.

[valerio_sperati]

In ten years of RC aircraft flying , most of the Lipos that"puffed up" , did so after discharging them to less than 3.2 V/per cell, not 2.8 V/per cell.

Ok, so the safe threshold is 3.2v. Anyway, how can you know that your battery is approaching this threshold? Does your drone warn you in some way (with light or sound)? Another question: do you agree with the other user Allanhurts (see previous replies) that a "puffed" battery is not dangerous by itself, but only if I try to charge it again? Thanks for your opinion   

[raschemmel]

Ok, so the safe threshold is 3.2v. Anyway, how can you know that your battery is approaching this threshold? Does your drone warn you in some way (with light or sound)? Another question: do you agree with the other user Allanhurts (see previous replies) that a "puffed" battery is not dangerous by itself, but only if I try to charge it again? Thanks for your opinion     

To my knowledge, ALL RC ESCs (Electronic Speed Controls) have a configuration menu that is typically entered by setting some specific transmitter control sequence , like poweing on ESC with throttle at full , then moving to midpoint , then back to full, in conjunction with tones emitted by the ESC. The "trigger" sequence required to enter programming mode is different for each brand of ESC. The ESC instructions have all the details. One of the programming settings is "Soft-cut/Hard-cut voltage threshold". where you can choose the default 3.2 V or something else.

[valerio_sperati]

all-battery.com sells round cells with built-in protection and you can get them with or without the wires. Very simple to work with. Just connect three in series and you're good to go.

yep! I find this site some minute ago   I found a lipo 11.1v 800mAh  with protection circuit (more than 30 dollars!). About the round battery you suggest, this is not feasible for my project, because an individual round cell is about 50gr, and I need three of them! Too much weight!

If you already have a battery pack you could use a BPC that doesn't have connections to the individual cells in the pack and eliminate the need to disassemble your pack.

mmm.. interesting but I still miss how to connect this circuit to the battery pack. Just to know, the battery I'm using now is a turnigy 11.1v 500mAh

http://www.hobbyking.com/hobbyking/store/__9275__Turnigy_500mAh_3S_20C_Lipo_Pack.html

As you can see, there are four wires coming from the battery pack:red, yellow, blue. black. Should I connect the red blue yellow cables to cell1 cell2 cell3 pad  and black wire to cell- pad?
Thanks!

[raschemmel]

Never do anything with a lipo without measuring the voltages on the connector to be sure.
The voltages will make it obvious where the wires are coming from.

[jremington]

In ten years of RC aircraft flying , most of the Lipos that"puffed up" , did so after discharging them to less than 3.2 V/per cell, not 2.8 V/per cell.

Ok, so the safe threshold is 3.2v. Anyway, how can you know that your battery is approaching this threshold?

A theoretical discussion only. The battery protection circuits usually cut out at 2.8 V or less. The one pointed to by the OP has this cutout:

Over discharge protection Over discharge detection voltage (Cell) 2.5±0.080V

mmm.. interesting but I still miss how to connect this circuit to the battery pack. Just to know, the battery I'm using now is a turnigy 11.1v 500mAh

The turnigy battery you are using is three cells in series.

You can wire three "protected" cells in series by yourself!