Quoting Junkymagi

If you added more batteries in parellel (I think he mentions that in the article) it would increase the up time. 2 batteries last 2 hours, 4 batteries would last almost 4 hours, 6 batteries would last about 5.5 hours. You lose a small fraction each time you add batteries because of the added resistance of the extra wiring and the batteries' internal resistance.

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.