Ok, who fancys a go at explaining the science behind this...

I came home today from school and entered my house. I noticed a strong smell which smelt like a mixture of chlorine and sulphur (although im not sure what it was).

Further investigation showed the cause to be my APC Smart UPS 1000XL, which had two 24v 17Ah batteries attached to it. The battery pack was very very hot. By this time the smell was getting a bit strong, so I unplugged the UPS, evacuated the house, and rung the fire brigade (because I was scared of it setting something on fire).

They came and moved the UPS into my garden where it continued to give out gas and remained hot for several hours.

I assumed it was a shorted cell that was causing the UPS controller to continuously try and top-up the battery, creating heat, etc.

But when I measured the batteries they both read 33V, which is rather more than the 24v they are supposed to run at. Only one of the batteries was hot, the other was cool.

I also have a server monitoring the UPS, and it reported about 20 "power outages" of 1-2 seconds each, every 30 minutes to an hour, from 8:00 AM to 15:30 PM that day. There were no actual power cuts, so the UPS was switching to battery for some other reason.

My theory is the UPS tried to charge the batterys after one of it's routine battery checks, and the heat produced by the charging caused a themal runaway, which then caused all the horrible things to happen.

If any mad scientists can help my investigation, please do so!

Cheers

I have a APC 1000 XS , whew. I'd say the batteries just shorted out causing spike voltages which caused it to start - restart over and over. With large dc batteries the cells break down and cause current spikes or sparks inside it. Thus a lot of heat which the sulphuric acid boils during the short giving off hydrogen gas which will explode. I've seen several car batteries and large 128 volt coal mine batteries just rip apart from this type of explosion. Your truly blessed to have found it when you did.

Hi Cold Steel,

I wouldnt be worried about it because I have found no other cases of small UPSes such as this one doing things like this.

Your theory is one I first had, but I dont understand how that caused the batteries to be overcharged to 33V.

I am fairly sure it was due to overcharging, because I know these sealed batteries can be prone to the thermal runaway effect. Much more so then conventional lead-acid vented batteries.

However, I can see no justifiable reason for the controller to be overcharging them.

They were in a small space with limited ventilation at the time, so I am inclined to think that overheating caused whatever happened to happen.

Thanks

I agree that shorted cells was my first theory also until I saw that they were overcharged to 33V. The charge controllers in UPS's are some of the most complex out there and can be easily damaged to line conditions despite how well they're designed to handle it. Throwing multiple surges and spikes at a microcontrolled device will eventually cause it to fail. Which is what sounds like what happened in my opinion. The electric companies are notorious for bad line conditions which is why I have two Linksys WAP routers stuck in initialization loops.

Coincidentally I had a netgear router that decided to stop booting up, once.

Obviously I must have dodgy power....

Thanks for your ideas guys.

Little off topic but along the same lines of power surges.

There was one month several years ago where we had tons of thunderstorms. Of course we have a surge protector to protect the pc equipment. Well lighting struck and blew the surge protector. No big deal, lifetime warantees, And that is what they are made for. We didn't bother replacing it real quickly. Well later that week, we get hit again and it blows the modem(before cable internet) in the computer. So we went to the store and got another modem and mailed the surge protector in for a replacement. Before we got the new surge protector, lighting hit once more and blew the modem again (think we would of learned to unplug everything). We got the new surge protector and yet another modem and haven't had a problem since.

After those couple weeks we had noticed that the metal thing on top the chimey was bent up and kind of melted looking. My friend a couple blocks over had got hit by lightning and caused some wiring behind his tv to smoke.

I think our neighborhood is built on a mineral deposit of some sort because I dont think a storm comes through with out stiking someones tree or house in the neighborhood.

Apparently, this is more common than one might think. I have a small UPS and about a year and a half ago they did a recall of the battery, citing possible combustion. Naturayy they didn't go into detail as to why it might occur. They shipped me a new one free. I was wondering if it was true but...