Battery Backup Design

[Guest112011]

Let me clarify for those that do not understand. GEL cells and AGM batteries are a form of lead-acid battery, but they are completely sealed. No need to worry about them, even if dropped and broken... but really with a name like physicsbob, I would think that you should know better. If you can open it to check the water level, it belongs in your car, not your house. Just google AGM batteries to find links to the many online resources like Battery Stuff. Check out their Gel cell AGM comparisons. Good read for those of us that like that kind of thing. Solar panels, chargers, DC to AC inverters. Pure sine wave (about the same price as our hosts) Always a good thing to know that your supplier is in line with the people that sell these things as a main business. And remember, this battery backup thing is for our safety and comfort. Why should hose-heads not be able to go camping unless there's a 120VAC outlet handy.

[moresleep]

Yes, hydrogen can be explosive, and a "wet" lead acid battery both can spill dangerous sulferic acid if disturbed, and does generate significant amounts of hydrogen during charging. We've been using this type of battery in cars and other mobile applications for a long time, so obviously have ways of dealing with these two problems. If I had a "wet" deep-cycle battery (same as the car battery, but with thicker lead plates so that it can survive more and deeper discharge cycles) in my bedroom, I would keep it in a sturdy box of the type used to contain the battery for a trolling motor on a small boat, and would have a system for venting the hydrogen to the outside of the house. At least some batteries have a nipple for venting to which you can connect a hose; others vent straight out of the top of each cell, and you would have to collect the hydrogen from the box. Hydrogen is lighter than air, so not hard to deal with venting. The big advantage of wet deep-cycle batteries, of course, is that they are a lot cheaper (per AH) than the sealed lead acid batteries. But they are not usually a good choice in close quarters or anywhere they are likely to be disturbed.

An AGM battery is much safer for the house. The absorbed glass matt separator between the lead plates is "starved" in that it always wants to soak up more solution; so even if the battery is broken, it should not leak acid. However, both Gel and AGM batteries do still generate small amounts of hydrogen while charging, and with some designs it is possible for this to be vented outside the battery in some conditions, such as overcharging, I believe. Apparently, it's not usually a concern, as these batteries are widely used in medical equipment.

Jump starters generally use an AGM battery, and some models work surprisingly well as a cheap non-interruptible 12-volt power supply for a Cpap. But, it is important to understand the possible problems and to proceed with caution. Some jump start devices, regardless of their stated features, are also so poorly made and with such bad quality control as to be very iffy in use--it pays to read review and see what the professions are using. Something like the referenced PWRgate PG40S would be a safer, more predictable, and more robust solution, as it isolates the power supply and battery, but with it you have to supply both the battery and the power supply, so you are looking at a total of about $400.

[archangle]

OMG - - Most of these posts are great, but I missed where someone warned about using lead-acid batteries in the house. Or any confined space for that matter...and probably where the warning for using a charger while running an xPAP came from. Yes, I have an EE degree, but working on cars should be enough to heed this warning. LEAD-ACID batteries give off EXPLOSIVE HYDROGEN GAS WHEN THEY ARE charging, and is not breathable. That is why they come from the factory with that warning on them. Also why Battery Backups come only with sealed GEL cells. The hydrogen gas is not leaked into the air, but contained in the sealed GEL CELL unit. That was what they were designed to do. Oh and if your wondering, yes, I have had a small explosion occur because of a battery. No major damage, probably because it was in the garage, but I can just imagine the effects in a bedroom. Also, I am a CPAP user since Nov. 2001, eleven years and can not stand it without my humidifier. And believe me when I say, without heat it might as well not be there at all. PLEASE - PLEASE - PLEASE *****DO NOT USE LEAD ACID BATTERIES INSIDE THE HOME*****!!!!!!!!!!

Hydrogen gas IS potentially explosive.

AGM and gel cell batteries have gas vents in them. If the charging or other conditions are such that they are producing hydrogen, the hydrogen will be vented from the battery. AGM and gel cell batteries (and some car batteries) are designed to reduce hydrogen formation, but it can happen, especially if the charger is malfunctioning.

Let's forget the explosion hazard for a minute and consider the toxicity of hydrogen.

Hydrogen gas is entirely nontoxic. Like any other nontoxic gas, if you get enough hydrogen to displace enough oxygen, you can suffocate.

http://www.isocinfo.com/DocumentRoot/13/Hydrogen.pdf

There is no way you are going to get enough hydrogen from a lead acid backup battery to suffocate you in your house. (Assuming something about the size of a car battery.)

A normal car battery contains about 1 gallon of water. Hydrogen makes up 2/18 of the mass of water. (H2=2, O=16). 1 gallon of water weighs about 8.3 lbs. That means there are about 8.3*2/18 lbs of hydrogen in a car battery. That is about 1 lb.

If you could somehow quickly convert all of the water in a car battery to hydrogen, you would release 1 lb of hydrogen into your house.

Air weighs approximately 0.08 lbs per cubic foot. A 12x12x8 foot room contains about 92 lbs of air. 1 lb of H2 plus 92 lbs of air is not going to reduce the oxygen concentration enough to suffocate you.

This assumes that your battery "boils" (really electrolyzes) all the water in the battery into hydrogen quickly. Think of all the car batteries you've dealt with over the year. Have you every had a car battery that needed even a cup of water added in a 24 hour day?

Hydrogen is the lightest gas, even lighter than helium. It will quickly escape through any air holes and dissipate out of any room in your house. Unless the hydrogen is produced quickly, it will dissipate before the concentration builds up. Think of it like having a cigarette burning in the house all the time. It may stink up the house, but you're not going to consume all the oxygen in the room and fill the house with impenetrable smoke and deadly levels of CO2 no matter how long you keep chain smoking.

I sat down and did a quick calculation. I need to double check it, but here is my calculation. With a 10 amp charging current, a 12V lead acid battery could theoretically produce a maximum of 0.4 ounces of hydrogen per day.

--Edit--- Correction - 0.1 ounce of hydrogen per day.

[moresleep]

A normal car battery contains about 1 gallon of water. Hydrogen makes up 2/18 of the mass of water. (H2=2, O=16). 1 gallon of water weighs about 8.3 lbs. That means there are about 8.3*2/18 lbs of hydrogen in a car battery. That is about 1 lb.

If you could somehow quickly convert all of the water in a car battery to hydrogen, you would release 1 lb of hydrogen into your house.

Air weighs approximately 0.08 lbs per cubic foot. A 12x12x8 foot room contains about 92 lbs of air. 1 lb of H2 plus 92 lbs of air is not going to reduce the oxygen concentration enough to suffocate you.

This assumes that your battery "boils" (really electrolyzes) all the water in the battery into hydrogen quickly. Think of all the car batteries you've dealt with over the year. Have you every had a car battery that needed even a cup of water added in a 24 hour day?

Hydrogen is the lightest gas, even lighter than helium. It will quickly escape through any air holes and dissipate out of any room in your house. Unless the hydrogen is produced quickly, it will dissipate before the concentration builds up. Think of it like having a cigarette burning in the house all the time. It may stink up the house, but you're not going to consume all the oxygen in the room and fill the house with impenetrable smoke and deadly levels of CO2 no matter how long you keep chain smoking.

I sat down and did a quick calculation. I need to double check it, but here is my calculation. With a 10 amp charging current, a 12V lead acid battery could theoretically produce a maximum of 0.4 ounces of hydrogen per day.

Your figures provide some comfort. But, let's assume a large, runaway charger boils the battery dry over the course of the night, releasing that entire 1lb of hydrogen into a small hermetically-sealed bedroom, to rise to the ceiling where, eventually, it will be exposed to a spark, maybe as the ceiling light is turned on. Would there be enough of it to cause an explosion? How big an explosion?

[JohnBFisher]

... I sat down and did a quick calculation. I need to double check it, but here is my calculation. With a 10 amp charging current, a 12V lead acid battery could theoretically produce a maximum of 0.4 ounces of hydrogen per day. ...

archangle, you are spot on on this.

Folks, PLEASE read what I've noted. There are risks involved in this (or any backup battery) design. There is no way to eliminate the risks. There is a LOT of power stored in a battery - regardless of the type. That power can reach out and grab you, if you are not careful.

I do recommend AGM / GEL batteries. However, you can use a lead acid battery - under careful conditions. You *MUST* use a battery box that you includes vent holes. You *must* allow air to flow around the battery box. You *must* store the battery in normal indoor temperatures - no attics or garages. You *must* use a proper charger that will not overcharge the battery. Using an improper charger does produce more hydrogen gas. Using a proper charger reduces the production of the hydrogen gas. You *must* periodically test the unit and charge time for the battery. If either does not work as expected, then unplug it and investigate.

I have used and safely stored a lead acid battery for more than 12 years now. It works. Done properly - with proper respect for the danger it represents - it can be safe.

But if you do not *understand* the risks and are not willing to do it correctly, then do NOT use a lead acid battery. Besides the AGM / GEL batteries are just about the same price and safer.

And frankly, if you are not willing to use ANY tool safely, don't even pick it up. Always know the risks of any tool before you use them.

In a word:

D'oh!

[archangle]

Your figures provide some comfort. But, let's assume a large, runaway charger boils the battery dry over the course of the night, releasing that entire 1lb of hydrogen into a small hermetically-sealed bedroom, to rise to the ceiling where, eventually, it will be exposed to a spark, maybe as the ceiling light is turned on. Would there be enough of it to cause an explosion? How big an explosion?

Assume for a moment that my calculation on hydrogen/amp is correct.

Now assume the runaway charger produces 100A at 12V DC.

It would still only produce 4 ounces of hydrogen in 24 hours. In the course of 8 hours, it could only produce 1.3 ounces of hydrogen. I'll have to do a little further thinking, but gut feel is that 1.3 ounces of hydrogen is going to make much of an explosion. I also suspect that if it's distributed around the room, the concentration won't be high enough to burn.

-edit- Correction - It would still only produce 1 ounce of hydrogen in 24 hours. In the course of 8 hours, it could only produce 0.33 ounces of hydrogen. I'll have to do a little further thinking, but gut feel is that 0.33 ounces of hydrogen is going to make much of an explosion. I also suspect that if it's distributed around the room, the concentration won't be high enough to burn

Note: Hydrogen and oxygen might collect inside the air space inside the battery. That could easily be concentrated enough to explode. I'm only talking about the H2 in the room air at the moment.

Now, consider the 100A runaway charger. It's going to be dumping 1200 watts into the battery. That's as much as a space heater or a toaster. You're assuming it's going to run full blast for 8 hours. Some of the energy is going to go into electrolyzing the water, but I suspect most of it will go into heat. The battery is probably going to melt (or explode) in an hour or two if it's getting 100 amps.

A 100 Amp battery charger is a large beast, too. They're usually about the size of a 3 or 4 car batteries stacked on top of each other. A lesser model would probably self destruct pretty quickly if it developed a fault that made it put out 100A.

By the way, if someone else wants to check my numbers for hydrogen production rate, all I did was assume it takes two electrons to produce 1 H2 molecule by electrolysis. (2 H+ ions plus 2 electrons -> 1 H2 molecule.) Calculate how many electrons in an amp, etc.

[moresleep]

Sounds like the main danger to using a "wet" deep cycle battery inside (as opposed to a AGM battery) is not the hydrogen, then, assuming charging and venting are set up properly, but the danger of an acid spill. I would still be reluctant to have a "wet" battery in the bedroom, though, for fear that someone cleaning or some kid would somehow get to it and manage to tip it over or extract acid.

Apparently, there is some risk of a runaway charger causing even a sealed lead acid battery to get hot and explode. So, the battery box might still be a good idea.

[physicsbob]

Let me state that I am very familiar with the handling of hydrogen, at the age of 13 I made a "hydrogen bomb" which blew my parents garage off its foundation, didn't hurt it though. Since that time in my profession I have worked with it frequently and know its dangers and safety precautions. Being a former member of the "back to Earth" movement in the seventies, I have designed and built several "off the grid" power systems using 2000 Amp/hour lead acid batteries for friends. So to address safety concerns I have my battery in a standard battery box with a strapped down lid, which I then placed in a plastic dish pan. which I placed underneath an open night stand so no one can possibly trip over it. There are holes on both ends of the battery box so there can be no hydrogen build up. I have the battery fused at ten amps, the feed to the cpap is fused at six amps and the battery charger is fused at three amps. I use one of the better battery float chargers that has built in over voltage and current protection. I have volt meter hooked up to monitor battery voltage. I feel that my backup system is safer than any UPS, car starter, or portable battery pack system. I have repaired many of these systems and know how cheaply they are built.
To add to Archangle's argument he is assuming that the electrolysis is 100 percent efficient which it is not, I have found that it is only about 60 to 70 percent efficient under the best conditions, the rest is lost to heat. Also for even moderate over charging AGM and gel batteries will vent the same amount of hydrogen as wet batteries

[archangle]

Sounds like the main danger to using a "wet" deep cycle battery inside (as opposed to a AGM battery) is not the hydrogen, then, assuming charging and venting are set up properly, but the danger of an acid spill. I would still be reluctant to have a "wet" battery in the bedroom, though, for fear that someone cleaning or some kid would somehow get to it and manage to tip it over or extract acid.

Apparently, there is some risk of a runaway charger causing even a sealed lead acid battery to get hot and explode. So, the battery box might still be a good idea.

Acid spills are a problem if you turn a spillable battery over. You definitely need to make sure turning it over is unlikely. The case can also crack and leak acid. Sometimes a little acid will bubble out the top. If it does leak acid, it's not a hideous problem, although it will damage carpets and furniture. If you get it on yourself, wash it off immediately. Getting it in your eyes would be bad, but even then, you MIGHT be OK if you washed it out immediately.

A battery box is a good idea with any kind of lead acid battery. It doesn't necessarily have to be a special battery box, although those are cheap. It's mainly something to take care of leaks if they happen. You also want to be sure that the top of the battery is covered such that something can't fall over and short out the battery terminals. Whenever you move the battery, check that there isn't any acid in the bottom of the box, so you don't get it on you or spill it while moving. Also watch out that you don't get acid on you from the outside of the battery.

I mentioned runaway chargers mainly as an example of how you can't make much hydrogen even under extreme conditions. A low current charger is really unlikely to fail and turn into a runaway high current charger.

I wouldn't put the battery in an airtight container, but otherwise I don't think venting is a big problem for this kind of battery setup. If a hamster wouldn't suffocate in the box, don't worry about it.

I am talking about something like a single lead acid battery about the size of a car battery with a 10 amp charger system, not your forklift, a bank of batteries for your home solar power system, or a UPS system for your commercial server room.

I've had a deep cycle wet cell marine battery within about 10 feet of my sleeping or TV/computer position for the past 15 years or so. Yes, there are some risks. Obviously, I'm not that worried.

[archangle]

To add to Archangle's argument he is assuming that the electrolysis is 100 percent efficient which it is not, I have found that it is only about 60 to 70 percent efficient under the best conditions, the rest is lost to heat.

I believe your 60 to 70% number. Just to be clear, though, that means you get less hydrogen than my calculation, correct?

I was deliberately stating the worst case.

Actually, I'd be really surprised of you got anything near that high of an efficiency of electrolysis in a lead acid battery, since it's not intended to make hydrogen.

Did you check my calculation? I'd really be glad if someone would confirm that x amps can make a maximum of y grams of hydrogen by electrolysis.

[Gustave]

I use Sealed lead/acid Sealed Battery for more safety.

Get a look to "Optima Batteries"

"...Does an OPTIMA battery ever gas?
When used with a properly regulated constant voltage charging system (such as an alternator) the OPTIMA will usually not emit hydrogen gas. However, gassing can occur when charging at higher voltage levels or in extreme high temperature conditions. In automotive applications this typically will not happen if the alternator/regulator stay below 15 volts...."

Gustave

[archangle]

I double checked my numbers and found an error. My hydrogen calculation was 4x too high. (I multiplied by 2 when I should have divided.)

10 amps charge current will only give you 0.1 ounce of H2 per day.

[Guest112011]

Do me a favor and light an ounce of FFFF gun powder that is lying on a board out in the open. What's it do? It goes FFFSSSSTTTT with a flash of light just for good measure. Now put that same ounce of FFFF gun powder in a pipe sealed on one end, with a fuse in it. now jam a potato down the end of that pipe and light the fuse. Your neighbors might be wondering what the hell happened to their windows. Just poking the bear here. Didn't mean to PO anybody, you guys know it all...but like JohnBFisher said about little fingers and prying eyes. Well let's hope they keep those eyes, because we're not talking Hindenburg explosion here, we are talking an explosion just big enough to spray Sulfuric acid solution all over everything close to it. Hydrogen is not that dangerous in the open, but it will run an internal combustion engine, so it is explosive enough. Also, the only time a GEL cell or AGM vents gas is if the manufacturer recommends mounting only top up, or if it has been overcharged and ruptured (that rupture can be a small split not a gapping hole). Go run for your scientific calculators guys. I wasn't trying to upset anybody, but I have a way of doing that without even trying. And of coarse that is why I work alone, and don't join in most of the time. I wasn't trying to tell you know-it-all guys, hell, I'm one of those, I was trying to WARN people who don't have a clue..........

[archangle]

As I said, you might explode the gas inside the battery. This would be a bad thing. It might cause the battery to explode with enough force to rupture the case, scatter some acid around, and throw some shards of plastic around. I don't think it's going to blow the windows out of your house or knock down any walls.

It would be bad to get the acid on you. It would also be bad for whatever it sprayed on. Scattering lead oxide and lead around the room would be a bad thing as well.

Your original post warned "LEAD-ACID batteries give off EXPLOSIVE HYDROGEN GAS WHEN THEY ARE charging, and is not breathable." I'm arguing that the hydrogen can't build up enough in the room to be a breathing hazard or an explosion hazard (outside of the battery case) with the size and type of battery we are discussing. The only way to build up a high enough concentration outside the battery case would be to have the battery in a somewhat airtight container.

[archangle]

- Note for some reason, I couldn't post this as one post-

Now, as to gel cells and AGM cells not building up hydrogen.

AGM cell diagram showing the safety valve:
http://www.yuasabatteries.com/cutaways.php?action=cutaway&id=1
MSDS safety data sheet for AGM battery:
http://www.yuasabatteries.com/msds.php
http://www.yuasabatteries.com/pdfs/MSDS_VRLA_Non-Spillable_081308.pdf

Note the manufacturer's warning: "Highly flammable hydrogen gas is generated during charging and operation of batteries."

Gel cells work essentially the same way as AGM batteries in terms of hydrogen formation and venting. I can probably find a gel cell manufacturer's web site to verify this if necessary.

Gel cell and AGM batteries are designed to "recombine" any generated hydrogen back into water in normal use, but they do generate hydrogen gas and oxygen inside the battery case, and they will vent hydrogen gas under unusual situations, especially if overcharged. As far as I can tell all AGM/gel cell batteries have safety vents.

As I said, I sleep near a car battery sized wet cell deep cycle marine battery every night. I'm not that worried, but I realize there are risks and try to minimize the risks.