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I was part of a ISP startup that ran on a shoestring budget and the UPS being one of the places that we saved the $$$ to put into good modems. We used an 800kva UPS with dead batteries and two group 27 Lifeline sealed Lead Acid batteries ( series, for 24V ). We removed the dead batteries from the UPS, cut a small hole in the side of the case and added our own 4ga. battery cable extensions.
After using it for a bit, it seemed that the UPS's internal battery charger got confused by the additional capacity of the larger batteries ( though it's possible that it had an issue the whole time, and that's why the original batteries were dead :rolleyes: ) and didn't fully recharge the batteries, so I added an external float charger to keep the batteries at the correct voltage.
It worked well, gave us something like 3+ hours of runtime ( enough for someone to run down and start the generator) and cost us about $425 ( $200 each for the batteries, $25 for the float charger- the UPS was free ).
After the ISP went under, it came to my house to run my entertainment center through hurricane Georges.
You're right about it being a bad idea to add more batteries in series, but actually, I think you'll get BETTER than three times the runtime by tripling the number of batteries. ( Pairs of batteries in parallel, that is two batteries in series for 24V, and those pairs each in parallel )
like this:
-[__UPS___]-
-[bat]-[bat]-
-[bat]-[bat]-
-[bat]-[bat]-
Battery or wiring resistance should only go down since the pairs ( two batteries in series ) are connected in parallel. The one part of the string where the current is flowing through single wires shouldn't have any more current than a single pair.
Besides, if you're getting significant resistance from the wiring, it's too small, at least at this scale.
Next, battery capacity goes UP when you lower the demand. ( I can't post a link, I'm too new. Look up "Peukert's Law" on Wikipedia to learn more and see the equations...)
The numbers below are made up, but they illustrate the principal. The actual equation will vary a lot with battery chemistry and construction. Flooded lead acid batteries are affected by this more than sealed lead acid batteries.
Batteries are normally rated for a 20 hour discharge. That is to say a 100Ah battery is rated to deliver 5A for 20 hours when tested to full discharge.
If you pull the power faster, you get less power out of the battery. If you drain the battery in 10 hours, you might only get 85Ah. If you did it in 5 hours, you might get just 70Ah. If you did it in 1 hour, you might get as little as 50Ah, or just half the energy you'd get in a 20 hour discharge.
Let's apply this to the UPS example. Let's say your UPS draws 50A from a battery to run your stuff.
If you use the 100Ah battery from my example above, a 50A draw will drain the battery in just an hour, since the high current draw will make the battery able to deliver just 50Ah.
However, if you triple the number of batteries ( thus tripling the rated capacity of the batteries ) you get more than 3 times the runtime. Since the drain rate is slower ( at least three times the single runtime...) each battery delivers it's power more slowly and you'll get more out of each battery, perhaps 65Ah. 65Ah time three ( the number of batteries ) is 205Ah. That's a runtime of a little over 4 hours.
So a tripling of the batteries might quadruple the runtime, more or less.
YMMV.
(I know, this is for a setup with only a few devices in need of backup, which is not what the article was about, but since we're on the topic of UPSs ...)
You guys must bear in mind that when I did the initial test after installing the UPS I had everything maxed out on 2 computers where the CPU's were running 100% the graphics cards were working hard, the wireless router was under load and I had a huge 3-in-1 lazer scanner/copier/printer spewing out pages of print. (Lazer printers and UPS don't mix so I wouldn't recommend anyone runs one off the UPS unless they rated to handle it)
With our recent power outage I shut down all the peripherals and only had 1 PC and monitor going. The CPU was still at max load and the graphics card was working hard for some of the time. I got over 5 hours out of. I'm quite sure that if I had another set of batteries wired in parallel I could get a straight 10 -12 hours out of it. That supports what airmon has said. (great post BTW)
The batteries I used were deep discharge and recommended for a UPS. However as someone suggested to me the other day, if you used a car battery, there's nothing stopping you from popping it into the car and going for a drive around the block to recharge it. That way if you had a spare, you could have an endless supply of power :D
Digg!
I've been playing around with photovoltaics the last few weeks (for glamping) and your write-up cleared up a few things I was wondering about.
One thing I noticed is that, in your diagram of multiple batteries in parallel, you have both leads connecting from the inverter/charger to the same battery. I recently read that in a battery bank the two leads should be connected to opposite ends of the bank; i.e. in your "Parallel battery connection" diagram the negative wire should run from the leftmost battery back to the inverter. Otherwise that right-hand battery gets charged and discharged much more than the others.
Why don't you register (It's quick and easy) head over to the general hardware section of the forum, post your request and we can take it from there.
It's in it's infancy but I'm sure that would make for an interesting follow up article :D
I updated the link.
Just need to find a few solar panels now... :)
Thanks for listening and thanks for writing such a great artical. feel free to e-mail me
Pockets
I am a auto/diesel/industrial master technician and have personally seen all above conditions and failure modes. Sealed,maint free,vrla,gel cell all have vents for emergy venting.
Almost forgot 600cca 12v batt exploding sounds like a shotgun and spray acid in all directions ask me how I know. No disrespect meant by this post, keep your dod below 50-60%, enjoy none of the above problems and battery life measured in years not months.