there are two ways to do this. the 1st is http://icrontic.com/articles/super_ups, a great artical on home UPS where the guy just bought an inverter and a bunch of batteries and tested how long it took for the batteries to die.
My way, not perfected yet as I am still learning, is based on numbers. After writing this, I have decided that maybe its a better way without numbers. just do it. read the web site. but i will explain a few things here.
So in the previous article I was saying kettels should not be run off the UPS. The reason being is that a home UPS is made up of deep cycle batteries, not your normal car battery. Deep cycle batteries a typically used for a long release with no spikes in power consumption, as in golf carts. The reason golf carts have a limiter too, that way there there is a constant current, stopping starting and accelerating will decrease the life of your battery. A car battery on the other hand is is used for a power start, quick high current for a short period of time. It should never drop below 80% voltage.
Basically we need to find out the amount of watts we will be using in the period the power will be off. I will be using lights, microwave, a computer, adsl modem, wireless router, satellite, dvd player, tv, decoder and a few plugs for charging laptops and mobile phones. i will add their consumptions together, multiplied by the minutes per hour i will be using them divided by 60. it would look like this... a little (25W*8 lights) *60/60, on all the time, 850W * 15min cooking time / 60, ......
This will give us a max wattage. Ill come back to this in another article.
Another thing you should know, store your batteries in a garage. hydrogen is expelled into the atmosphere when using wet batteries. the cheaper of the lot (gell cell and dry cell although better for safety and deep cycle) and make sure there is some way of venting. a little fan would be nice, but potential sparks can be hazardous. being in a garage there is enough movement to disperse the hydrogen sufficiently.
Sometimes this gas rusts and corrodes metals, watch out for this. it can save your life to catch it before it catches you.
these are two threads from the website above. valuable info! very valuable
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.
This is a great artical, thanks for the write up mike, I got a great deal on ups's last year i bought a pallet of them from a conpany here in portland when it went out of business and got 14 for 50 bucks, none of whice had good batteries, after wards i bought 7 12V 55 Amp/Hour batteries that were designed for telephone backup systems for 40 Dollars each, i run two mac G4's with 3 14 inch lcd flat screens, each item has its own ups and they all have there own batterys, i get around 12 hours of run time, also dont give up on solar, it works grteat, i have one batteries hooked up to 2 ups that are charged only by a 45watt solar panel setup, and it runs a gaming computer just fine, iv never ran this setup more then a couple hours but its always ready to go when i want it, BTW i live in oregon, USA
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.
