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.
reduce or
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.
shutoff the charging currents if the temperature gets
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.)
-- --Larry Brasfield email: donotspam_larry_brasfield@hotmail.com Above views may belong only to me.
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
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.
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?
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?
Just design the battery compartment to have a 0.1 mm diameter leak to atmosphere.
-- Regards, John Woodgate, OOO - Own Opinions Only. There are two sides to every question, except \'What is a Moebius strip?\' http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
I read in sci.electronics.design that snipped-for-privacy@larwe.com wrote (in ) about 'sealed (air tight) rechargable battery technology', on Sat, 16 Apr 2005:
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.
-- Regards, John Woodgate, OOO - Own Opinions Only. There are two sides to every question, except \'What is a Moebius strip?\' http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
shutoff the charging currents if the temperature gets
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.
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.
-- Keith
In article , Mac wrote: [....]
They have a vent. They also have to positioned with the vent up during charging.
-- -- kensmith@rahul.net forging knowledge
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.
-- -- kensmith@rahul.net forging knowledge
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).
Well, that's your mistake. I'd never use SLACs in an "extereme" environment. They're wonderfull in class-A though.
-- Keith
keith wrote: (snip)
Roger that. I have some 5AH Cyclons that I acquired in '75 or '76. They are still healthy.
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).
That's the problem with Litiums. They don't take kindly to abuse and tend to return it.
SLACs sound like the right solution. Maybe Peltiers with one side in your arm pit? ;-)
-- Keith
In article , keith wrote: [... SLA batteries ..]
Remember this comment for later discussion (Below next section) [...]
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.
-- -- kensmith@rahul.net forging knowledge
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.In article , keith wrote: [...]
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.
[...]Good price, if you can get it :)
[...]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-- -- kensmith@rahul.net forging knowledge
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