Hello to all,

I am looking into some guidance on the matter of air vents in rechargeable lithium batteries. For a product I'm working on, I am wondering what has to be done regarding the possibility of the battery needing to vent some gas, when it is located inside a product case that is fully enclosed, and the enclosure can probably stand a bit o pressure itself before breaking. The normal battery protections, like fuse, overvoltage/undervoltage/overcurrent, temperature when charging and such are included in the circuit, but I am unsure how this extreme case has to be accounted for. Since the battery itself includes this type of protection - the air vent, I would imagine one has to include a way to vent the gas in the enclosure. Does anyone have any experience with these design considerations?

Any info on this is greatly appreciated.

Thanks - Emilian

 

Which Lithium chemistry specifically are you referring to? Some chemistries will swell with age and use and you also need to account for this swelling in your battery compartment.

 

In a well designed circuit the lithium battery will not be subject to a charge/discharge voltage/current outside the normal operating specification of the battery, even under single fault condition of the circuit – and therefore the battery should never vent.

If lithium batteries are operated outside their operating specification, a much greater hazard than venting is fire/explosion – there are any number of youtube videos of exploding lithium batteries, should you want your design to contain such an occurrence.

 

Which Lithium chemistry specifically are you referring to? Some chemistries will swell with age and use and you also need to account for this swelling in your battery compartment.

Thank you for your answer, MrSoftware!

It's a Lithium-Ion battery, namely a Panasonic NRC18500A. I've added a link to it.
https://industrial.panasonic.com/ww/products/pt/lithium-ion/models/NCR18500A

 

In a well designed circuit the lithium battery will not be subject to a charge/discharge voltage/current outside the normal operating specification of the battery, even under single fault condition of the circuit – and therefore the battery should never vent.

If lithium batteries are operated outside their operating specification, a much greater hazard than venting is fire/explosion – there are any number of youtube videos of exploding lithium batteries, should you want your design to contain such an occurrence.

Thanks for your reply, and for the insights, Hymie! I hadn't considered adding such an explosive feature )

It is, of course, not my intention to have hazzards introduced by poor design, so I don't really intend to allow for something like that to happen. But I am worried that, should the battery need to vent due to some issue and the product case interferes with the process, it could worsen things and lead to the worst case which you have stated - a fire or explosion. So what I am wondering is if I need to, and how should I, account for that case in the enclosure design, so that it does not interfere with the batterie's last-resource safety measure. It's a bit on the line of industrial product requirements.

 

Hello to all,

I am looking into some guidance on the matter of air vents in rechargeable lithium batteries. For a product I'm working on, I am wondering what has to be done regarding the possibility of the battery needing to vent some gas, when it is located inside a product case that is fully enclosed, and the enclosure can probably stand a bit o pressure itself before breaking. The normal battery protections, like fuse, overvoltage/undervoltage/overcurrent, temperature when charging and such are included in the circuit, but I am unsure how this extreme case has to be accounted for. Since the battery itself includes this type of protection - the air vent, I would imagine one has to include a way to vent the gas in the enclosure. Does anyone have any experience with these design considerations?

Any info on this is greatly appreciated.

Thanks - Emilian

Hi,

This is a damn good question. There may be no good answer though because it's almost like building a bomb squad container.

First, as suggested in post #2 battery swelling is an issue. If you try to stop the battery from swelling it starts a fire and this has been found by Samsung (i think it was them) in the famous automobile fire some years ago. The problem was that the battery compartment was too small so when the battery tried to swell the compartment prevented that and thus it started on fire.

As to venting, the kind of venting we see with these batteries is not for gas it is for flame. The cell would start on fire but burn like a blow torch with flames coming out of any holes that happen to be in the device case. I don't think there is any way to help this situation except to completely contain the entire thing with a metal that can stand some heat. Im not even sure if that would work.
The 'professional' way to do this is to use a case style that has venting with tiny apertures that allow the pressure to escape in a gradual way. The only thing is, even they would have one side or top that cannot be obstructed with anything or anything in the way that could burn or that could catch fire. That would mean that part of the instructions for the product would have to include where the product can be placed, and that can impose a lot on the customer as to how and where they can use it, which probably places a big damper on sales.

Since it is so hard to do this, the best bet is to do everything you can electrically to prevent a problem. For one, measure the temperature of the cell and cut it out at any sign of a fast temperature rise or some upper temperature threshold.

If you want to see some designs do a search on the web for Li-ion containers that are used to hold cells either in storage or when charging. You can find videos of exploding cells that are placed inside various containers and how those containers hold up.

 

I'm not a battery expert so please consider that when reading this. At work we make water tight battery powered devices used by the military, the devices use custom designed lithium batteries. I don't know enough about it to give specifics, but I can tell you that analysis was done to predict how much the batteries might expand over the expected service life and the battery compartments were designed to allow at least that much room for swelling, the device enclosures are made from materials that will not burn and the battery compartments are sealed with o-ring gaskets to satisfy the water tight requirements of the product. I don't know what if-any consideration was made for the batteries venting themselves. My guess is lithium batteries should not vent during normal use, and if they start venting then something has already gone seriously sideways.

 

The venting of Li-Ion 18650 cells during thermal runaway includes hydrogen. If you are putting the cells inside a gas tight box, you are risking creating a literal bomb. You should include a pressure relief valve to allow the overpressure to vent.

Li-Ion 18650 cells can vent small amounts because of non-catastrophic overheating. The gas vented by the cell into your housing will have nowhere to go, and if the housing is actually gas tight it will be waiting for the next person to open it. This is mitigated by the fact that your housing will probably not contain hydrogen for extended periods in practice.

Non the less, this is a serious thing and a properly qualified consulting engineer should review the situation and make recommendation.

Personally, I would drop the Li-Ion chemistry in favor of LiFePO4 which, while is has a lower energy density is much safer and doesn't have the same problems with fire and explosion.

 

Hi,

This is a damn good question. There may be no good answer though because it's almost like building a bomb squad container.

First, as suggested in post #2 battery swelling is an issue. If you try to stop the battery from swelling it starts a fire and this has been found by Samsung (i think it was them) in the famous automobile fire some years ago. The problem was that the battery compartment was too small so when the battery tried to swell the compartment prevented that and thus it started on fire.

As to venting, the kind of venting we see with these batteries is not for gas it is for flame. The cell would start on fire but burn like a blow torch with flames coming out of any holes that happen to be in the device case. I don't think there is any way to help this situation except to completely contain the entire thing with a metal that can stand some heat. Im not even sure if that would work.
The 'professional' way to do this is to use a case style that has venting with tiny apertures that allow the pressure to escape in a gradual way. The only thing is, even they would have one side or top that cannot be obstructed with anything or anything in the way that could burn or that could catch fire. That would mean that part of the instructions for the product would have to include where the product can be placed, and that can impose a lot on the customer as to how and where they can use it, which probably places a big damper on sales.

Since it is so hard to do this, the best bet is to do everything you can electrically to prevent a problem. For one, measure the temperature of the cell and cut it out at any sign of a fast temperature rise or some upper temperature threshold.

If you want to see some designs do a search on the web for Li-ion containers that are used to hold cells either in storage or when charging. You can find videos of exploding cells that are placed inside various containers and how those containers hold up.

Many thanks for your detailed answer, MrAI!

After yours and Hymie's answer I did look more into the battery swelling issues, and have tried to find numerical values for what to expect, but haven't found them yet. Extra space has to be provided, but the percentage values elude me.

I have come across a discussion with some comments regarding designs that consider the venting, which point out at providing some failure points for the gasses to escape, which is pretty much what you have said. I've added it just for refference.

In a presentation from this link I've seen some numbers on vent vs burst pressure (page 28), so I would hope the venting can at least save the battery from exploding. Unless the pressure buildup is too fast, I would assume. But the provision of failure points in the case definitely seems like something that has to be considered.

It would not be feasible to have the product case whitstand the fire, that much I am sure of, but maybe the storage case could be designed to account for this risk. I have looked into some solutions that are used for transporting lithium batteries, which are basically metal cases with some form of materials to control the flame, some example here. The approach is to let them burn, but not let the fire escape. Not much else to do to them once the fire starts, I guess.

Since it is so hard to do this, the best bet is to do everything you can electrically to prevent a problem. For one, measure the temperature of the cell and cut it out at any sign of a fast temperature rise or some upper temperature threshold.

The electrical protections are definitely the way to go. Just to sum up, and maybe for a check of protections measures that have been considered/implemented:

1. the battery charge circuit monitors the battery temperature to stop the charge if the battery temperature exceeds about 45°C, and also different charge rates will be used when close to the lower and higher limits (0°C and 45 °C).
2. safety timer to stop charging if the trickle charging doesn't stop normally
3. overcurrent and overvoltage protections provided by the battery charger
4. additionally: a standalone battery protection circuit, and a fuse.

It might be interesting to monitor the battery swelling in some way, but I'm not sure of how difficult to implement vs beneficial that would prove to be.

 

To be clear, the battery doesn’t explode because of the gas build up, there is an explosion outside the battery because if the flammable gas, and a fire because of the flammable electrolyte.

Also, 18650 cells don’t swell, LiPo prismatic cells do.

 

The venting of Li-Ion 18650 cells during thermal runaway includes hydrogen. If you are putting the cells inside a gas tight box, you are risking creating a literal bomb. You should include a pressure relief valve to allow the overpressure to vent.

Li-Ion 18650 cells can vent small amounts because of non-catastrophic overheating. The gas vented by the cell into your housing will have nowhere to go, and if the housing is actually gas tight it will be waiting for the next person to open it. This is mitigated by the fact that your housing will probably not contain hydrogen for extended periods in practice.

Non the less, this is a serious thing and a properly qualified consulting engineer should review the situation and make recommendation.

Personally, I would drop the Li-Ion chemistry in favor of LiFePO4 which, while is has a lower energy density is much safer and doesn't have the same problems with fire and explosion.

Thanks for your reply, Ya'akov!

You've provided very interesting insights. I was wondering if that venting of small ammounts of gas could happen, and not just the extreme one, so it's good to have this confirmation. It's definitely something to consider, apart from the extreme case venting.

As for the battery chemistry, it is something to consider as well, but it would be a bit of a more complicated change, at this point, but I will look into it.

 

I'm not a battery expert so please consider that when reading this. At work we make water tight battery powered devices used by the military, the devices use custom designed lithium batteries. I don't know enough about it to give specifics, but I can tell you that analysis was done to predict how much the batteries might expand over the expected service life and the battery compartments were designed to allow at least that much room for swelling, the device enclosures are made from materials that will not burn and the battery compartments are sealed with o-ring gaskets to satisfy the water tight requirements of the product. I don't know what if-any consideration was made for the batteries venting themselves. My guess is lithium batteries should not vent during normal use, and if they start venting then something has already gone seriously sideways.

Thats is very interesting to know! Thank you, MrSoftware. I would expect the design to be pretty well thought, for such a field of application, so it is somewhat reassuring.

 

To be clear, the battery doesn’t explode because of the gas build up, there is an explosion outside the battery because if the flammable gas, and a fire because of the flammable electrolyte.

Also, 18650 cells don’t swell, LiPo prismatic cells do.

Thanks for the clarification, it is what I understood you meant.

As for calling it swelling, I might be mistaking the term - I am reffering to the battery expansion, due to temperature, charging, aging, or some fault.

 

Many thanks for your detailed answer, MrAI!

After yours and Hymie's answer I did look more into the battery swelling issues, and have tried to find numerical values for what to expect, but haven't found them yet. Extra space has to be provided, but the percentage values elude me.

I have come across a discussion with some comments regarding designs that consider the venting, which point out at providing some failure points for the gasses to escape, which is pretty much what you have said. I've added it just for refference.

In a presentation from this link I've seen some numbers on vent vs burst pressure (page 28), so I would hope the venting can at least save the battery from exploding. Unless the pressure buildup is too fast, I would assume. But the provision of failure points in the case definitely seems like something that has to be considered.

It would not be feasible to have the product case whitstand the fire, that much I am sure of, but maybe the storage case could be designed to account for this risk. I have looked into some solutions that are used for transporting lithium batteries, which are basically metal cases with some form of materials to control the flame, some example here. The approach is to let them burn, but not let the fire escape. Not much else to do to them once the fire starts, I guess.



The electrical protections are definitely the way to go. Just to sum up, and maybe for a check of protections measures that have been considered/implemented:

1. the battery charge circuit monitors the battery temperature to stop the charge if the battery temperature exceeds about 45°C, and also different charge rates will be used when close to the lower and higher limits (0°C and 45 °C).
2. safety timer to stop charging if the trickle charging doesn't stop normally
3. overcurrent and overvoltage protections provided by the battery charger
4. additionally: a standalone battery protection circuit, and a fuse.

It might be interesting to monitor the battery swelling in some way, but I'm not sure of how difficult to implement vs beneficial that would prove to be.

Hi,

Oh as to the swelling issues, yes i guess you are using 18650 cells?
The rectangular ones used in cell phones are the ones i have seen swell. I had one phone where the battery had swelled to two times (2x) it's normal thickness, and that pushed open the sealed-up case. The back pushed right up. That's how much force they can exert and that's how i knew the battery had swelled.. Lucky for me, that helped me get the back open so i could get out the old cell and replace it with a new one. They are not normally replaceable, but people do it anyway after prying open the back. The swell caused the thickness of the battery to increase but did not seem to change the other dimensions at all.

 

Do Li Ion cells vent gas in anything apart from a faliure mode?
i.e. over charging, under discharging ?

 

Hi,

Oh as to the swelling issues, yes i guess you are using 18650 cells?
The rectangular ones used in cell phones are the ones i have seen swell. I had one phone where the battery had swelled to two times (2x) it's normal thickness, and that pushed open the sealed-up case. The back pushed right up. That's how much force they can exert and that's how i knew the battery had swelled.. Lucky for me, that helped me get the back open so i could get out the old cell and replace it with a new one. They are not normally replaceable, but people do it anyway after prying open the back. The swell caused the thickness of the battery to increase but did not seem to change the other dimensions at all.

Hi, the battery is a 18500, just shorter than the 18650. I have seen prismatic batteries swelling up, I know the change in size is quite easy to notice in those ones, when it happens. I guess for the tube types the change is on a different scale.

 

Do Li Ion cells vent gas in anything apart from a faliure mode?
i.e. over charging, under discharging ?

The venting occurs on overpressure which requires the cell to heat up abnormally, but this can happen during charging if the charger or cell are bad in some way. The vent is on the positive terminal and the valve is below it.

Almost all cylindrical cells have a PTC device (Pressure, Temperature, Current which is ring built into th positive end that acts as a relief valve. a TCO (Thermal Cut Off/Out), and OCP (Over CUrrent Protection). This is supposed to reset but gets a little less good at its job each time it is activated.

It can also start to noticeably increase a cell’s ESR.

 

The venting occurs on overpressure which requires the cell to heat up abnormally, but this can happen during charging if the charger or cell are bad in some way. The vent is on the positive terminal and the valve is below it.

Almost all cylindrical cells have a PTC device (Pressure, Temperature, Current which is ring built into th positive end that acts as a relief valve. a TCO (Thermal Cut Off/Out), and OCP (Over CUrrent Protection). This is supposed to reset but gets a little less good at its job each time it is activated.

It can also start to noticeably increase a cell’s ESR.

Thanks
yep got that part
My question was more
old NiCad and such cells I remember used to have a out gas port on them
but i can't remember any of the LiIon ones having a port
The "bag" ones just seem to swell,the smaller ones ones just see to "change very very little"
until they destructively go ..

 

Thanks
yep got that part
My question was more
old NiCad and such cells I remember used to have a out gas port on them
but i can't remember any of the LiIon ones having a port
The "bag" ones just seem to swell,the smaller ones ones just see to "change very very little"
until they destructively go ..

It is not normal for a cylindrical cell to vent gas. It is an abnormal condition. That’s true even off the NiCad and NiMH cells. They only do it when they overheat. NiCads in particular have a strange behavior in that when they start to lose electrolyte as gas the cell’s chemistry converts it back to liquid.

That’s why NiCad was so popular (and in some cases still is) for things like emergency lighting where they can be allowed to constantly trickle charge at very low current without damage.

 

Regarding flammability of the device itself; it may not be feasible to build something that can 100% contain the fire, but plastics that don't burn (they melt, but won't catch fire) are very common and readily available. Even off-the-shelf enclosures from a place like Polycase can be made from non-flammable plastics.