sealed (air tight) rechargable battery technology

[Mook Johnson]

I have a small design that would require the batteries to the injection
molded (RTV) into a water tight package (cigarette box sized or smaller).
The outside world interface would be a small sealed connector for external
sensors and to rechange the batteries.

Battery capacity would be roughly equivilent to 2 ~AAA alkalines and take up
about the same amount of space. Other voltgae levels can be converted to
the 3.3V necessary for this application. Current levels from the 3.3V
regulator output will be 50mS bursts of 100mA with 10mA the rest of the
time.

It is my understanding that NiCad and NiMH require vent holes during
charging and operating and that sealed environment would damage them
(corrosion?). Is that correct?

Is there any other commercial (read not over $50) battery technology that
could be sealed air tight and still meet the applicaiton?

 

[[email protected]]

It is my understanding that NiCad and NiMH require vent holes during
charging and operating and that sealed environment would damage them
(corrosion?). Is that correct?

Vents are not a corrosion issue, they're a safety issue. Gases can be
generated. Rechargeable batteries have safety valves for this reason.
All the available rechargeable technologies I'm aware of in your market
(NiCd, NiMH, Li-Ion, LiPoly) specifically warn against charging in a
sealed container because of the risk that this container will become a
bomb.

 

[Mac]

I have a small design that would require the batteries to the injection
molded (RTV) into a water tight package (cigarette box sized or smaller).
The outside world interface would be a small sealed connector for external
sensors and to rechange the batteries.

Battery capacity would be roughly equivilent to 2 ~AAA alkalines and take up
about the same amount of space. Other voltgae levels can be converted to
the 3.3V necessary for this application. Current levels from the 3.3V
regulator output will be 50mS bursts of 100mA with 10mA the rest of the
time.

It is my understanding that NiCad and NiMH require vent holes during
charging and operating and that sealed environment would damage them
(corrosion?). Is that correct?

Is there any other commercial (read not over $50) battery technology that
could be sealed air tight and still meet the applicaiton?

Maybe lead batteries with a gel electrolyte? So-called gel-cells. I'm not
sure whether they have a vent or not.

There is a battery newsgroup somewhere. I think it is
sci.chem.electrochem.battery or something similar.

--Mac

 

[Ian Stirling]

I have a small design that would require the batteries to the injection
molded (RTV) into a water tight package (cigarette box sized or smaller).
The outside world interface would be a small sealed connector for external
sensors and to rechange the batteries.

As others have said, it's a safety issue.
Can you ensure that if the battery you've chosen starts outgassing
(in practice, NiMH/NiCd won't unless overcharged, or faulty) that
the case will rupture in a controlled manner.
Adding a pressure vent may be required.

 

[John Woodgate]

I read in sci.electronics.design that Ian Stirling

As others have said, it's a safety issue.
Can you ensure that if the battery you've chosen starts outgassing
(in practice, NiMH/NiCd won't unless overcharged, or faulty) that
the case will rupture in a controlled manner.
Adding a pressure vent may be required.

Just design the battery compartment to have a 0.1 mm diameter leak to
atmosphere.

 

[Mook Johnson]

Yes, these will be in a RTV type of encapsulant. Around hte batteries it
should be no thicker than .1". I'm it would blow a pretty goos sized bubble
them pop if overcharged. POP not expolde.

If it does blow a bubble but not big enough to pop the RTV, would the gas
cause problems with the electronics?

 

[Mook Johnson]

Also,

I can monitor the battery temperature while they are charging and reduce or
shutoff the charging currents if the temperature gets too high.

Would that be sufficient to prevent venting in nimh or nicad batteries?

 

[[email protected]]

I can monitor the battery temperature while they are charging and reduce or
shutoff the charging currents if the temperature gets too high.

The purpose of the safety vents is to provide backup when the
electronic controls of the type you've described fail. Sometimes gases
are vented because the electrolyte is boiling (high temperature). But
sometimes it's an unwanted electroltyic process generating, for
example, hydrogen. Study the datasheets for your cells and ask your
vendor for information on battery enclosure design, it will include
detailed discussion of what gases can be generated and why it is
mandatory to have an emergency vent in your enclosure.

 

[Larry Brasfield]

Also,

I can monitor the battery temperature while they are charging and reduce or shutoff the charging currents if the temperature gets
too high.

Would that be sufficient to prevent venting in nimh or nicad batteries?

The battery will not vent if you can avoid overcharging
it. The would include responding to the terminal voltage.
Keep in mind that if a cell fails shorted, the battery voltage
will still not suffice to help avoid overcharging. Venting is
due to a reaction other than the normal charging reaction
which results in simple rearrangement of the species. Gas
production to the point of venting depletes something and
should be strictly avoided for longevity.

Making the "fully charged" voltage depend on the
average cell temperature is a refinement that may or
may not be useful depending on your ambient range.
You might also use the temperature difference between
the cells and the outside as a means of detecting the
onset of overcharging. When that happens, "charging"
current result in more heat than when the battery is
actually accepting charge. (Some of this heat arises
from the evolved gas recombining instead of venting.)

You should also be aware that hydrogen can be
evolved during overcharge. That gas can collect in
explosive concentrations depending on the space
in which the battery sits. (This is a big concern in
airplanes for engine starting batteries.)

 

[John Woodgate]

I read in sci.electronics.design that [email protected] wrote (in

Study the datasheets for your cells and ask your vendor for information
on battery enclosure design, it will include detailed discussion of
what gases can be generated and why it is mandatory to have an
emergency vent in your enclosure.

On an occasion when this point determined non-compliance, I found it
extremely difficult to get the battery manufacturer to be specific about
how much venting is necessary (this for VRLA PB-acid rechargeables).
Eventually, it turned out to be a tiny thing. Hence my post about a 0.1
mm diameter vent.

 

[Ian Stirling]

The battery will not vent if you can avoid overcharging
it. The would include responding to the terminal voltage.
Keep in mind that if a cell fails shorted, the battery voltage
will still not suffice to help avoid overcharging. Venting is
due to a reaction other than the normal charging reaction
which results in simple rearrangement of the species. Gas
production to the point of venting depletes something and
should be strictly avoided for longevity.

In some cases, it can be possible to add redundancy fairly cheaply.
For example, isolate the batteries with two sets of different diodes in
series, and two overrated resistors in series, and add a fuse, polyfuse
(overtemp protection) and crowbar to the power input.

If the batteries simply cannot be charged at a rate of over 0.1C through the
combination of resistors and supply voltage, then the risk goes down
dramatically.
This can raise the number of independant failures to about 3 or more
in order to cause enough current to flow through the battery to cause gassing.

 

[Ken Smith]

Maybe lead batteries with a gel electrolyte? So-called gel-cells. I'm not
sure whether they have a vent or not.[/QUOTE]

They have a vent. They also have to positioned with the vent up during
charging.

 

[Keith Williams]

Maybe lead batteries with a gel electrolyte? So-called gel-cells. I'm not
sure whether they have a vent or not.

They have a vent. They also have to positioned with the vent up during
charging.
[/QUOTE]
There are two types of gelled electrolyte lead-acid cells. SLACs
(originally from Gates Energy, now Hawker, under the "Cyclon" brand)
are starved electrolyte so don't outgas under normal circumstances.
There is a vent, but it's for safety reasons and is only there in case
of a fault.

 

[Ken Smith]

They have a vent. They also have to positioned with the vent up during
charging.

There are two types of gelled electrolyte lead-acid cells. SLACs
(originally from Gates Energy, now Hawker, under the "Cyclon" brand)
are starved electrolyte so don't outgas under normal circumstances.
There is a vent, but it's for safety reasons and is only there in case
of a fault.[/QUOTE]

Even for the Gates cells, I still suggest that the vents be up. The
"normal circumstances" part is the key here. Eventually an abnormal
circumstance will happen. It is much easier to arrange things so that the
vent is up and thus produces only H2 than deal with the mix of acid and H2
you get when tey are faced down.

I learned this the hard way on an application using SLA batteries in a
fairly extreme environment.

 

[keith]

[....]
Maybe lead batteries with a gel electrolyte? So-called gel-cells. I'm not
sure whether they have a vent or not.

They have a vent. They also have to positioned with the vent up during
charging.

There are two types of gelled electrolyte lead-acid cells. SLACs
(originally from Gates Energy, now Hawker, under the "Cyclon" brand)
are starved electrolyte so don't outgas under normal circumstances.
There is a vent, but it's for safety reasons and is only there in case
of a fault.

Even for the Gates cells, I still suggest that the vents be up. The
"normal circumstances" part is the key here. Eventually an abnormal
circumstance will happen. It is much easier to arrange things so that the
vent is up and thus produces only H2 than deal with the mix of acid and H2
you get when tey are faced down.

I tend to agree, but it's not so clear-cut. The abnormal circumstances
are failure modes, which even upright aren't always pretty. The
Gates/Hawker SLACs are quite tollerant of abuse and will last ten years
(or better) if taken care of. Sure, they're going to be nasty if abused,
but less so than most alternatives (think LiIon).

I learned this the hard way on an application using SLA batteries in a
fairly extreme environment.

Well, that's your mistake. I'd never use SLACs in an "extereme"
environment. They're wonderfull in class-A though.

 

[Michael]

keith wrote:
(snip)

The Gates/Hawker SLACs are quite tollerant of abuse and will last ten years
(or better) if taken care of.

Roger that. I have some 5AH Cyclons that I acquired in '75 or '76. They are
still healthy.

 

[Barry Lennox]

keith wrote:
(snip)

Roger that. I have some 5AH Cyclons that I acquired in '75 or '76. They are
still healthy.

Me too, I have a 2.5 AH Cyclon that is still in reliable use, since
1979. I have never done anything special, just a periodic c/10 charge
for about 12-24 hours.

 

[Ken Smith]

I tend to agree, but it's not so clear-cut. The abnormal circumstances
are failure modes, which even upright aren't always pretty. The
Gates/Hawker SLACs are quite tollerant of abuse and will last ten years
(or better) if taken care of. Sure, they're going to be nasty if abused,
but less so than most alternatives (think LiIon).

Remember this comment for later discussion (Below next section)
[...]

Well, that's your mistake. I'd never use SLACs in an "extereme"
environment. They're wonderfull in class-A though.

Here's my environment:

Imagine all the garden spots of the world such as the senic Turan Basin,
the wild and breath taking Artic tundra, and the wonderous Antartic. Now
imagine that "normal use" is to tie the product to a ATV with a long rope
and go motoring over the land all day long. In the evenings, the
batteries may get recharged, or maybe not or maybe only partly depending
on the realities of the situation. The next day you do it again.

I think for this environment, the SLAs were the best of the awful options.
NiCad and NiMH batteries hate partial drain and recharging. All types of
Lithiums do too and protest more explosively. Lithiums also have the
proble of cost. You know you are going to ruin what ever battery you
select in very short order so you want it cheap and to have a highish
energy density.

Oh yes, and it has to be water tight during operation too.

 

[keith]

I tend to agree, but it's not so clear-cut. The abnormal circumstances
are failure modes, which even upright aren't always pretty. The
Gates/Hawker SLACs are quite tollerant of abuse and will last ten years
(or better) if taken care of. Sure, they're going to be nasty if abused,
but less so than most alternatives (think LiIon).

Remember this comment for later discussion (Below next section)
[...]

Well, that's your mistake. I'd never use SLACs in an "extereme"
environment. They're wonderfull in class-A though.

Here's my environment:

Imagine all the garden spots of the world such as the senic Turan Basin,
the wild and breath taking Artic tundra, and the wonderous Antartic. Now
imagine that "normal use" is to tie the product to a ATV with a long rope
and go motoring over the land all day long. In the evenings, the
batteries may get recharged, or maybe not or maybe only partly depending
on the realities of the situation. The next day you do it again.

Ok, your environment is a *little* more extreme than my Class-A computer
room. I don't think your batteries are going to last 10 years either. ;-)

If I were designing for that environment (or space, or even automotive),
I'd have a whole different list of concerns. That said, the (three
"D") Cyclons we used were mounted vertically, in a "bullet-proof",
nickel-plated, vented case that went for $1500 (blush).

I think for this environment, the SLAs were the best of the awful
options. NiCad and NiMH batteries hate partial drain and recharging. All
types of Lithiums do too and protest more explosively. Lithiums also
have the proble of cost. You know you are going to ruin what ever
battery you select in very short order so you want it cheap and to have
a highish energy density.

That's the problem with Litiums. They don't take kindly to abuse and tend
to return it.

Oh yes, and it has to be water tight during operation too.

SLACs sound like the right solution. Maybe Peltiers with one side in your
arm pit? ;-)

 

[Ken Smith]

Ok, your environment is a *little* more extreme than my Class-A computer
room. I don't think your batteries are going to last 10 years either. ;-)

If they make it through more than a year, customers will be somewhere in
the happy-delighted span of the scale so I was not too bothered by the
shortened life I expected.

[...]

"D") Cyclons we used were mounted vertically, in a "bullet-proof",
nickel-plated, vented case that went for $1500 (blush).

Good price, if you can get it


[...]

SLACs sound like the right solution. Maybe Peltiers with one side in your
arm pit? ;-)

There were several ideas like that rejected for reasons of cost and not
making enough current. The best second option we had was the "hamster
wheel" generator. PETA[1] got in a complete uproar over the abuse of the
poor animal being forced to work in the same environment as the human.


[1] PETA does not stand for People Eating Tasty Animals