Battery Backup Design

[archangle]

While I don't count on wikipedia as an authoritative source, the article here does seem to give a pretty good explanation.

(still having posting problems)

[archangle]

If you're still afraid of lead acid batteries, don't assume the alternatives are safe.

The main competitor to lead acid batteries is lithium batteries. They have a risk of fire and may release toxic chemicals if they burn.

Reputable manufacturers do a lot of things to reduce the risk of fire in lithium battery packs, but there's probably still some risk.

Scary youtube videos for lithium battery fires exist.

[moresleep]

As Archangel's figures show (see the revised one), the amount of hydrogen produced is miniscule, and unlikely to be a problem. Possibly, in terms of an explosion, the "wet" deep cycle battery is actually safer than a "sealed" battery, as it has more copious venting of gas, with vents probably less likely to get plugged. But, remember, "sealed" AGM and Gel batteries are widely used with all kinds of home and hospital medical equipment, with little indication that explosions are a real worry. In any case, a battery box (there are cheap plastic boxes used for trolling batteries) with adequate venting should contain any explosion. I still favor the AGM battery, as I would worry about a kid exploring my bedroom somehow tipping over or otherwise coming into contact with sulpheric acid.

Sure, batteries have to carry some risk. But, with lead acid batteries, we're dealing with a fairly mature technology, with few surprises. It would be different if this were lithium ion technology, where manufacturing or design flaws still sometimes produce rather spectacular surprises for users.

[archangle]

As Archangel's figures show (see the revised one), the amount of hydrogen produced is miniscule, and unlikely to be a problem.

Miniscule outside of the battery case. I won't dispute the possibility of a small explosion inside the battery case.

[archangle]

I found out that, in theory, hydrogen can explode at a 4% by volume mixture with air.

1 ounce of hydrogen will take up the same space as about 7 ounces of air.

That means that 1 ounce of hydrogen can explode if mixed with 7/4% lbs of air, or 175 ounces of air.

Electrolyzing water with 10 amps can produce 0.1 ounce of H2 per day. This would be non-explosive if mixed with more than 17.5 ounces of air. Air weighs 0.08 lbs or 1.28 ounces per cubic foot. 17.5 ounces of air is 13 cubic feet. A 3x3 foot closet would be 72 cubic feet, so that wouldn't be explosive.

With 100 amp charging current running 24 hours, you get 1 ounce per day. This would be non-explosive if mixed with 130 cubic feet of air. A 3x6 foot closet would be slightly too large to explode.

Remember the above numbers assume the largest theoretically possible hydrogen production. They assume that there is no mixing of air in the room. It seems unlikely that even a closet won't exchange the air more than once per day. As I said before, a 100 amp current is probably going to destroy the battery in a fairly short order without generating that much hydrogen.

Now, if you somehow convert all the water in the battery to hydrogen, you get about 1 lb of hydrogen. That would be about 2000 square feet. That's a little larger than a small house.

I don't see any way for a battery to quickly convert all its water to hydrogen, but if it does, then yes, you have an explosion hazard in the air in your house.

And, once again, yes, the small amount of hydrogen inside the battery case can explode. It could squirt and spill sulfuric acid. That would be bad if you're nearby. I don't think it's going to knock down any walls or break any bones unless you're sitting on the battery.

[moresleep]

Thanks for the further number-crunching, Archangel, which seems to confirm that a hydrogen explosion, at least outside the battery box, is extremely unlikely, if not physically impossible, in any reasonable real-world situation. There also apparently isn't much risk of explosion of the battery or battery box from trapped gas, assuming venting. That still leaves the danger of runaway charging causing a battery "melt-down" with similar nasty side effects; but, that probably isn't much of a danger, either, with a small float charger.

[JohnBFisher]

... That still leaves the danger of runaway charging causing a battery "melt-down" with similar nasty side effects; but, that probably isn't much of a danger, either, with a small float charger. ...

Which is why I advocate for use of the smallest charger that will work. When I use my battery backup system, it takes a couple days to fully recharge the battery. I rarely have power outages that last for more than three days. Then when power is restored, it stays stable for at least several days. Thus, a design that can last two or three nights and then take two or three days to recharge (1) meets my needs, and (2) is no more powerful than necessary. As a result, even if this charger went into a "run away" state, it simply does not have the "UMPF" to cause a big problem.

As Archangle noted, the lead acid battery technology is well understood and mature. This helps reduce the chance of problems. The battery box helps contain the risks. Proper ventilation helps contain the risks. Proper sizing of the charger helps contain the risks. In essence, as with Archangle, I also sleep every night with this setup near my bed. But ... even then ... I place it on the other side of the side table beside my bed. Distance and a side table helps further minimize risks.

[mstevens]

[quote="archangle"That means that 1 ounce of hydrogen can explode if mixed with 7/4% lbs of air, or 175 ounces of air. [/quote]

I had never even considered the possibility of solving stoichiometry problems using Imperial measurements. Isn't this part of why SI units were invented in the first place?

[wolftracker]

HiYa,

Does anyone know where to find the power requirement for the PR ONE with AFlex.

I have looked thru all the manuals and googled the web
I must be missing it somewhere.

Also any thoughts on using the Optima deep cell battery on this build?

thanks for any help

wolf

[GumbyCT]

HiYa,

Does anyone know where to find the power requirement for the PR ONE with AFlex.

I have looked thru all the manuals and googled the web
I must be missing it somewhere.

Also any thoughts on using the Optima deep cell battery on this build?

thanks for any help

wolf

Check the label on the bottom of the machine.

[wolftracker]

i see

12v -> 5A

i found that in the book
but I thought power useage was based on pressure set, not just the number
on the bottom of the machine or the book.

am i not understanding something correctly here?

confused

[GumbyCT]

i see

12v -> 5A

i found that in the book
but I thought power useage was based on pressure set, not just the number
on the bottom of the machine or the book.

am i not understanding something correctly here?

confused

It does vary according to your pressure and other things but 5A would be the Max. or worst case. Are you planning for worst worse case?
HTH

[JohnBFisher]

i see "12v -> 5A" .. I thought power useage was based on pressure set ..

It does vary according to your pressure and other things but 5A would be the Max. or worst case. Are you planning for worst worse case?

I agree with GumbyCT on this. The manual notes the amperage needed for AC is only 2.1. The amperage for DC should actually be a little less. Sounds as if an engineer said "Well, 5 should be high enough for anything!".

CPAP.com has a battery calculator for the PR System One REMstar Auto unit. It says that at a pressure of 10 the 14.4 Amp-Hour (AH) Respironics battery should last 19 hours. The Respironics battery can be drained lower than what you would drain a deep cycle battery. Normal lead deep cycle batteries should not be drained below 1/2 of the charge they can hold. Draining them more than degrades their ability to hold a charge. So, if you want a deep cycle battery to last about as long, then you would need a 30 AH deep cycle battery. The deep cycle battery does not have to be more than just a deep cycle battery. Sometimes you find deep cycle / starting batteries (for marine usage). Those cost more. You just need a 12V DC deep cycle battery.

Hope that helps.

[archangle]

ResMed publishes a current vs. pressure settings chart for some of their machines. I haven't seen one from Respironics, but it might be out there.

[Breathe Jimbo]

JohnB, how long have you had your battery backup system, and has your battery ever exploded?