Which kind of batteries are you talking about? There is a type of lead- acid battery sometimes called a "gel cell" which uses a gel electrolyte and which does not leak any kind of gas at all under normal circumstances. At least that is my understanding. It is possible for the gel cell to vent hydrogen gas if it is overcharged, but the vent does not normally open unless there is pressure inside the battery container, and such overcharging wrecks the battery anyway, so has to be avoided.
There is another lead-acid battery technology called absorbed glass mat (AGM) or something like that. These are similar to gel cells, but the electrolyte is absorbed in a glass mat, as I understand it.
Both of these battery types can be mounted in almost any orientation, and don't leak AT ALL.
I don't think this spewing of sulphuric acid mist applies to gel cells and AGM batteries. If you think it does, please provide more specific references.
You could use some other battery technology. Li ion, or NiCad or Nickle metal hydride.
I'm sure there is some type of conformal coating which will render your board immune to corrosive atmospheres.
This is silly. I'm sure you don't need to do this.
The problem is not just for lead acid type batteries. ALL batteries emit corrosives. In battery powered appliances that run at low power, the effect can be observed on the metal plating of the clips that maintain electrical continuity between the battery and the electronics.
We see this on our battery powered wall clocks and on our electronic thermostat that runs our heating system. Every 1-2 years, these fail to operate even though the battery shows full voltage under load. Clean up the contacts, reinstall the battery and they operate agian for 1-2 years before failing again.
I just noticed it happening to a laser poiter I have, same pattern although I can't examine the internal battery contacts as well as I'd like to. So, can't be absolutely sure it's the same problem.
The sealed lead acid batteries are not totally sealed and do vent some mist of sulphuric acid.
The manufacturers say not to charge in a confined space, but the biggest problem as I can see is that this is quite corrosive to electronic components, with several reports of it eating pcb tracks, attacking a crystal in a very high quality oscillator so much it could not be put onto frequency etc.
Has anyone found an answer to this problem? Sometimes you want battery backup in a case with electronic components, but it seems they don't exist too well with each other.
I've been thinking whether there is any checmical solution, such that you place a sacrificial chemical that will be more reachive with the gases, leaving less to attack your components. Of course, any byproducts would have to be non-corrostive.
I've also thought about sealing the batteries in a plastic bag. The bag and expand/contract as and when the batteies vent cases, but they are kept inside the bag. I'm not sure if the batteries would like this though. Any you still have the problem of sealing around the leads.
You don't mention the application or other requirements of the battery supply so it is difficult to be specific.
If the application has low power requirement you can use NiMH or NiCd cells.
If the power requirement is high such that it demands a lead acid type battery for heavy duty then you shoulds look at one which is capable of external venting. Such a battery is OPTIMA D31A
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If you don't want to or can't vent externally then you have a problem.
If your application is not one requiring deep cycle, or deep charge/discharge operation, then you shouldn't be charging to the extent where gassing occurs. Gassing generally only takes place when a heavy charge current is applied such as when boost charging. If normal float operation is all that is required then you should select a battery type suited to this type of operation and gassing shouldn't occur if you are using the correct charger.
Protecting pcb's and mounted components for use in volatile atmospheres is generally accomplished by proper housing of the electronics components in conjunction with the application of conformal coatings.
You are mostly wrong with your assumptions. Sulphuric acid mist is only a byproduct when gassing occurs. What gets produced is hydrogen and oxygen. Together in a bag or in a small room this is a potentially lethal condition. Get Gel-cells if you charge in a confined space, like the hull of a boat or a small room.
Far less accurate and stable than your rubidium oscillator. Any reason why you need several sources?
That rubidium oscillator shuoldn't vary more than 10^-10 from
-20 C to +65 C. Are you sure you need an oven?
Like this? [
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The kind of batteries used in commercial alarm boxes put out the kind of power you have been talking about, and they do a quick discharge when the bells start ringing and the lights start flashing. I have never seen any commercial alarm box with external venting.
OK, to be more specific, is to generate an accurate frequency standard, for a home project, using a combination of
1) A 10MHz oven controlled crystal oscillator - which has an oven using more current during startup, but less when running. Needing 24V
2) A 10MHz rubidium atomic source - again with an oven, taking more during startup, and less during use. Needing 24V.
3) A GPS recieiver to lock both of them to 10MHz, which will accept 24V.
I want to keep this on 24 hours per day, as that way the frequency stays more accurate.
To keep these within the current of my power supply (2.3A), I will sequence the starting up of these.
Once running, it total current consumption will be about *900mA*. I have no need to start this on batteries, so the batteries will only supply a little under 1A @ 24V during mains power failure.
Not reallly economic (for a home project anyway) at 24W of power consumption, with a design time of 7 hours.
I have bought two "sealed lead acid" but not the gel type 12V 7Ahr batteries. Now I am begginging to wonder if that was the right decision or not - I think the answer is not.
I'm in a no mans land here. The power is too much for economic use of NiCd/NMiH, but too small for using the large lead acids that allow external venting.
I don't mind venting externally, and hence have been thinking about possibly making some sort of gas tight seal around the batteries. However, its only a 3U high box, so some of that will probably get sucked back in via a fan. It's not going into a rack (will sit on its own), so there should not be too much
There is no reason to fast charge, but under load, I will be taking about 0.9A from a 7AHr battery, so a little over 0.1C.
Will gassing occur during discharge?
I have selected one for normal float operation and can design the charger for it myself. It only needs a constant voltage, which must be temperature compensated.
I will be limited to supplying about 2.2A of current, due to the capacity of my power supply. (Yes, I know that is less than the 3A startup current of this, but I will ensure the heaters in the two devices that have heaters are not both starting together)
These are OEM units - a Stnaford PRS10 rubidium source, an old HP ovened oscillator, but none are hermetically sealed.
A company made a similar unit, and it is well known for basically destroying itself. I don't know if it was bad design of the charger, or just that you can't avoid this. Apparently the batteries and electronics were sepparated by a metal
Gas Recombinant type batteries with AGM construction are used in critical medical backup systems and vent 0 gas unless overcharged. I have used these in several applications, including a rubidium backup and GPS system based on BALL/Efratom oem OSCILLATORS (Rb) for long term remotely located applications.. The critical step is a well designed power supply to strictly control charge rates. Since these cells are a lead acid technology your power density sounds realistically obtainable.
ISTR that you can get the battery caps with a piece of platinum wire fixed in them. This was said to force the H and 0 to recombine to water. The main idea was to prevent water loss, in remotely located systems
One other alternative, is to look at 'manifold vented' batteries. These have a chamber over the cells, with a seperate vent pipe. You then connect this to a rubber hose, and have this fed outside the casing. You can even feed the vent pipe to a seperate chamber with alkali filter elements.
It's not quite as simple as that. The rubidium standard itself has a crystal:
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as you see it says "10 MHz rubidium-disciplined crystal oscillator"
There seems no way if having getting around having a decent crystal if you want a decent frequency standard, as the short term stability of a crystal is better than anything else. So that rubidium has an oven - I am not putting it in any oven.
Also, the phase noise performance of the HP10811A oscillator I will be using has a spec that is 100x better than the phase noise on that rubidium disciplined crystal.
I will get someone with a cesium source or hydrogen maser to compare them for me.
The rubidium has its own internal oven. I'm not putting any external one.
Yes, pretty much like that. Just building it myself, although with the Stanford PRS10 locking to GPS is a doodle, as the PLL is built into the box. For the crystal, I will build a PLL, based on this design.
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I guess it depends on the design life. The alarm is probably not designed to last 20 years, but that rubidium standard has a design life of 20 years. Hence I suspect I need to be a bit more careful not to destroy the thing.
Dave, from bitter experience, I can agree that AGM "sealed" batteries do leak acid vapour. We have a product with AGM battery and temp compensated charger set for a fairly low charge voltage and we get leakage.
If there is very slight leakage and we find it in time, then we can wash the board and put it back in service. Once the acid gets through the board coating, the the board must be replaced due to electrical leakage.
I heard on this newsgroup that batteries sitting with valves at the top are less troublesome than batteries at other orientations, especially inverted batteries, because liquid is less likely to issue forth. Unfortunately, our product has a battery on its side.
In future, we will place batteries - of whatever type - in a separate compartment with an external door, and seal the wires or terminals off from the innards with silicone rubber. NiCads and other types can leak too.
Leakage depends on brand, age and charge-discharge history. We also suspect the practice of charging multiple new batteries in parallel at our factory - we think a more discharged battery gets a lot of current from the better charged batteries, exceeding the battery's max charging rate.
Gell batteries may be better. Others may know if and why.
If you prise off the top cover of an AGM battery, you find puny little rubber valves, which look as though they vent at low pressure.
There are other lead acid batteries - eg the Cyclon type, which are much better sealed. There are alternative chemistries also, which we now prefer.
Thanks for the compehensive description. I would definitely use gel cells for your project. Given the quite modest power requirements you won't have any problems with venting of any volatile gases since they won't exist. I would just use polyurethane conformal coating on boards.
I think my approach is going to be to b) Remove the plastic cover on the top of the battery (it just unclips, exposing the 6 vents.
2) Drill a hole in the cover and attach some form of pipe fitting, so I can vent the fumes out.
3) Seal the plastic cover back, using an adhesive - quite what I do not know, as clearly any mist attacking the adhesive could cause a problem.
As long as I don't cover the vents in adhesive, this only leaves one place for the fumes to escape, and that is outside the unit.
Ross, this just doesn't fit our experience with thousands of AGM cells of about 3 major brands. Our product has a good temperature compensated voltage regulated charger system which sits voltage at the low extreme of the manufacturer's float charge voltage vs temperature graphs. This is not a mickey mouse piece of engineering and product quality is good. Some units come back with acid mist damage and the charger in those units checks out perfectly.
Recently, we carefully voltage charged new batteries from stock, removed the vent cover plate and observed moisture around the vents on 10% of that particular battery stock.
The school of reality has forced me to accept that AGM batteries *do* vent sometimes. Often its a tiny amont of gas which you would never notice unless you enclosed the battery.
I suspect the factors like:
manufacturer and batch temperature and rate of temperature variation, especially during charge depth of discharge rate of discharge max available charge current manufacturing defects battery ageing period of shelf storage cell differences wearout characteristics charger algorithm for programmed charger cycles
I bow to your superior experience and I do agree that AGM batteries can vent a small amount of gas during charging, particularly at higher temperatures. I haven't any direct experience using AGM batteries in enclosed spaces, my experience has been on larger 1000Ah and up, flooded cell batteries operated in float conditions, and these batteries don't issue gas except when being boost charged, which is a rare occurrence.
I was considering the fact that the OP was using quite small capacity batteries in a float situation and surmised that if he used a good quality 3 stage charger then gassing shouldn't occur.
While it is beyond the scope of the project being developed by the OP, the US defence forces specification requires AGM batteries which do not gas even with heavy charge and discharge currents at elevated temperatures. According to one particular manufacturer, Lifeline advanced AGM batteries are the only ones which currently meet this requirement, so hopefully their technology may eventually be applied to smaller capacity batteries suitable for rack mounted applications.
I'll have to double-check the fact, but it did not look like my vents had released, but I must admit I did not look too carefully. The cover went back on without (at least as I noticed), it then being obstructed by the vents, which looked to be in place.
But whether or not they had raised up 0.5mm or so I am unable to say.
I'll try to clarify this with the manufacturer. If nothing else, it might make them consider the issue for small batteries, which never seems to be done.
I am now in a position to know where I could drill without removing the cover, so could in future do it without even removing the cover, if removing the cover automatically releases the vents. But I don't think it does in my case.
I hope so, but I will ask the manufacturer.
I was going to look around the web to try to find what is most likely. The had thought about silicon rubber and epoxy myself.
Vent construction varies - some vents are rubber plugs held in place by the welded on cover, which release when you remove the cover. Once you prise off the cover, these batteries aren't really good for serious work, even if you glue or tape the cover back down.
Looks like you have a battery with a better type of vent where the cover is not doing work.
Silicone rubber (window - bathroom stuff from hardware store) handles acid, holds quite well over an area and gives assurance of a seal. Also polystyrene cement gets a bite on that battery plastic. Epoxy ??
Reinforces the idea that some brands of small AGM will leak a less often than others -even if none of them are up to the Lifeline standard. We have just recently dumped a particular brand - lets hope the Century-Yuasa ones we are trying will do better. A very useful insight for me. Thanks.
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