Charging SLA batteries...

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starhawk

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I'm in the middle of designing a home-made UPS for a computer. I know how I want to build it.

I want to have an inverter that automatically cuts on when the power cuts off. (I think I can do that with an SPDT mains-rated relay. Easy.) At night, I electrically disconnect the inverter with a big ol' switch that also electrically connects the charger end of things. (Think two great honking DPDT switches ganged together.) Easy.

Worth noting is that I sleep for like 10-12 hours a night. I really do get my beauty sleep!

So a 14hr trickle-charger is about right here. I seem to remember, some time ago, finding a website with a schematic and all kinds of math for a charger design that I ended up building. I used that to test a 6v 4.5ah battery (which turned out to be both-feet-in-the-grave-and-covered-with-dirt dead). I still have the parts, battery included. It was made from a 7.5v wall wart, a resistor, and the battery itself. The resistor was in series with the battery, and those really were the only components involved (!). I remember the resistor was made from a length of pencil lead because I needed a really inexpensive 3ohm resistor that could handle a few metric tons of current going through it.

Now I'm looking for the website with that circuit diagram, and I can't bloody find it -- which is annoying because it had all the math I need to work out this charger. Of course this section of the forum has threads that don't display anymore after a while, so I can't just browse the section and find it. (...and, naturally, I was too stupid to bookmark the website as well.)

So (boy, I've never asked this one before) can someone throw some math at me for this? I need to know what the resistor and the wall wart need to be rated for.

The battery this time is a 12v SLA, rated 22ah. I want to charge it in 12-14 hours. (Specifically, it is this battery here -- http://www.ebay.com/itm/281023281189 -- I've bought from these guys before, and I know that they're good folks.)
 
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Sheesh, I should check in here more often :)

Just take care that you don't overcharge your battery--it's not safe to have hydrogen floating around your house on its own!!
 
Thanks for the advice.

I thought SLA batteries didn't vent because they're sealed...?

Also, if someone knows how to calculate charging current for this -- a 14hr charge (voltage AND current) for a 12v 22ah SLA battery -- I'd be quite grateful. I'm waiting on the company to get back to me with that info, but if I had it sooner I'd be delighted.
 
You'll need a charge curve for the battery in order to properly calculate the charge time. A lot of times the batteries will be labeled with one voltage to use while charging and another to float charge it. The idea is to charge it to full capacity at the first voltage until the battery hit its capacity, then switch to the lower float voltage.

An SLA will still vent the hydrogen (it's gotta go somewhere--the case on the battery isn't designed to hold pressure)--being sealed, though, you don't have to worry about liquid electrolyte (sulfuric acid FTW!) spilling out if you happen to drop the battery or tip it over.

EDIT: Oh, for reference--a "12V" lead acid battery has a nominal voltage of 12.6V, with a charging voltage of between 13.8 and 14.4V (depending on the battery) and a float charge voltage at 13-13.5V. If you can get a datasheet for your battery, it'll help tremendously.
 
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It's from Chrome Battery, and they don't seem to want people getting ahold of datasheets for their stuff :( so I'll have to wait for their email I guess. (No datasheet on their webpage for that battery.)
 
Here's the datasheet for Chrome Battery's 12v 20ah offering, which I guess I could use instead... it's about $2 cheaper, which helps as well...

http://www1.datafilehost.com/d/76b761eb

Note -- with this file host, uncheck the little box that says "use our download manager" and click the gray DOWNLOAD button. Do NOT click "DOWNLOAD NOW" -- that's got stuff in it that's bad for you ;)
 
I could be reading the datasheet wrong, but I think you could just hook up a 13.5V float charger to it and call it good.

Or, you know, you could shell out a few bucks and get a proper SLA charger :)
 
How many bucks is "a few"...? I might be able to do it if it was $20 or under...
 
Found this one...

http://www.ebay.com/itm/170958454559

...some of these chargers say "don't use with AGM batteries" of course the batt I want is an AGM battery... so I've sent an eBay message to that seller to make sure things will work out OK.

Mohonri (or anyone else), can you tell me the current limit I'll want if I can't get an off-the-shelf float charger that will work here?
 
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Datasheet says max charging current is 6A. The ebay listing you linked doesn't show the current rating of the charger, so I'm not sure if it charges slowly enough. I'm guessing it's a 1A charger, which would probably be ok for this application.

Out of curiosity, what is your application that has you running on UPS power during the day and mains power at night?
 
It wouldn't be battery power all day every day! I'd get about an hour and a half out of this setup.

Mom and I live in a fairly nice house surrounded by a tiny bit of pine forest, and our power line makes three turns through that forest before it reaches our house. Any time a particularly overweight squirrel bounds off the line on its way to an acorn, we get a brownout. Also ice storms and the occasional hurricane -- I'm smack in the center of NC (well, I'm pretty close, anyhow) but some nasty stuff has come our way. Hugo and Fran come to mind... although I'm too young to remember Hugo myself, Mom will probably never forget it.

My usual way of dealing with the power issue is to own a laptop -- but I'm moving to a desktop soon for various reasons and I need a way to deal with this. I do NOT want to lose all my work suddenly because a squirrel went exploring and bounced off the wires!
 
I gotta ask as well--why do you want a manually-switched charger/inverter setup? What's wrong with taking an old UPS without batteries, and adding your own oversized battery to it, like this?
 
That was quite an entertaining read you linked to :D

I don't have an old UPS. Well, I do, but it's the only one we have and it's attached to Mom's desktop right now so that she doesn't have problems if the power blinks. It's a nice one, and we recently replaced the battery to keep it going.

That particular UPS was bought before the designs were changed for those things. Used to be you could actually buy a UPS that would last 2 hours with a desktop PC -- my understanding is, now you can only get ones that last ~15min unless you want to shell out multiple hundreds of dollars.

I want something simple, cheap, and effective -- it only needs to run a Pico-ITX computer and a screen, and it really only needs to run for an hour. That's about 100WH. (Although I need to recalculate since I just traded the 15" Viewsonic LCD that I did my figuring with, for a 17" Viewsonic LCD that's apt to pull a watt or two more.) My calculations were that I could pull 100W from that battery for about 1:30 to 1:45 (h:mm) which is plenty of time indeed.

I should probably throw in some sort of voltmeter with this thing, though, so I know when it's time to cut the computer off without having to go get the timer from the kitchen (lol).

As for the "manually switched" bit -- only in regards to charging vs ready-to-work modes -- I don't need it to run my computer when I'm asleep. There will be a mains-rated switching relay which will tell the homebrew UPS when to cut on. Not as fast as a *real* UPS but it'll do.

Actually... come to think of it, I have a friend with an old UPS... ah, but it wasn't lightning-resistant enough and now it's got more to worry about than just a bad battery. It's one of those that's got a mains-only surge suppressor side and a battery-backup side (just like the one on Mom's desktop)... and only the mains-only side works now ;) I'd call that one burnt toast myself. Besides, he's not able to easily drive it to me (intra-family politics at work, his folks don't like me even tho he does, and we've never quite figured out why) and I can't pay him to ship it even without the battery in it... I just don't have the dough right now.
 
...and the new LCD screen says "100-240V 1.5A" instead of "100-240V 0.8A". CRAP!!

:(
 
I've been a fan of dan's data (and his accompanying blog, How to Spot a Psychopath) for something approaching 15 years now. His writeup on how to be a pest in Tribes 2 makes me chuckle even today, and even though I've barely played the game.

That power rating on that monitor is a maximum, not a continuous, rating. Actual power usage is guaranteed to be lower. Especially if you shut it off when the power goes out :) If you get a kill-a-watt, you can find the actual power draw. Note that turning down the brightness will also reduce the power consumption.

If you *really* want to squeeze every watt out, you could replace the CCFLs/inverter with LEDs. It's been done, but I can tell you from first-hand experience that it's no picnic. But a good learning experience.

If you're near a well-populated area, I'd recommend hitting up thrift stores or looking for a computer salvage place. You can find all sorts of goodies for pennies on the dollar, including working-but-no-battery UPSes for a song.
 
I live 1-1/2 miles from a town of ~8k (Mom & I joke that the count has to include the cows, chickens, and pigs...) and I'm friends with the owner of the local tech shop, but I've been asking a bit much from him recently -- that's where I got the last two monitors... and the newest one is going back tomorrow because it won't work with the Pico-ITX mobo :(
 
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