Battery run time calculator?

[Quattro Pete]

Can you point me to one, or how to calculate it by hand?

Say, I'm looking at this battery backup (as an example).

It says:

Runtime at Full Load (510 Watt): 1 min
Runtime at Half Load: 7 min

So, is there a way to calculate how many minutes it would run if I had a 70 Watt load? What's the formula? Or is this battery-dependent?

 

[tom slick]

you could do regression through calculus.

 

[jldcol]

Even with zero load I bet there is a limit. I guess 20-30 min at 70 watts. 1-2 hrs. no load but turned on.

 

[brianl703]

Based on those numbers, it sounds like the UPS has a 7-amp hour battery. See this data sheet. (You can assume that the specs for other manufacturer's 7-amp hour batteries are close):

http://www.csb-battery.com/upfiles/dow01231494624.pdf

Since they're specifying a 7 minute runtime with a 255 watt load, and the UPS is probably around 80% efficient(the two that I tested are), it's pulling close to 300W from the battery, which corresponds with the 1.6 volts-per-cell row and the 7-minute column in the Constant Power Discharge Characteristics table.

According to that table, for 30 minutes the battery could supply 104 watts and for 60 minutes the battery could supply 60.5 watts.

Since you want to power a 70 watt load, at 80% efficiency it will pull about 85 watts out of the battery which is roughly midway between the 30 minute and 60 minute ratings. So I would guess it would power that load for about 45 minutes give or take a few.

 

[chevrofreak]

Buy yourself a used UPS with a dead battery and modify it to run on a car battery. You'll get a ton of runtime that way.

 

[brianl703]

I recommend SmartUPSs for that. They have rather powerful chargers in them...10 amps for the SmartUPS 1500 that I have. I would also use a marine deep cycle battery (actually you'll need two in series for the SmartUPS).

It's funny that the charger in the SmartUPS is way oversized for the little SLAs that they come with...maybe this explains why they kill those batteries.

 

[kc8adu]

cyberpower units are made too cheap to take extra batteries.
they have just enough heatsinking on the mosfets to stay cool for the runtime of the stock battery.
they have a timer in software too.iirc its 15 min.
and the charger in the apc is so it can recover quickly after an outage.the reason these fry batteries is the float regulation with a max current.if you loose a cell shorted the charger will hold at approx 14v a battery forever.since it can deliver a good bit of current the rest of the cells hit thermal runaway.
just replace the batteries with good quality ones every 3 years.
i do this for my corp.customers and no failures.
all the melty batts were 6+ yeards old.at 3 they are still good but get replaced anyway.the pulls get tested and any good ones which means most go to my amateur radio friends.
and i have several apc and best power units running on external batts here.i even put a fan in several that get warm under heavy load.
used a switching transistor with its base on the control logic so it runs only in inverter mode.
this is easy on 12 and 24v units.48v units have the fan stock.

 

[Quattro Pete]

Actually, I just want something basic that will do AVR (automatic voltage regulation), surge protection, and provide about 1 hour of run time for my cable modem (about 20 Watt), router (about 20 Watt), and a gigabit switch (about 30 Watt). Here's another option, but the price is almost the same as that previous one that I posted:
http://www.cyberpowersystems.com/product...s/CP800AVR.html

Not sure if the battery run time is any longer.

 

[brianl703]

Originally Posted By: kc8adu

this is easy on 12 and 24v units.48v units have the fan stock.


The larger 24v Smart UPSs already have fans.

 

[chevrofreak]

Originally Posted By: Quattro Pete
Actually, I just want something basic that will do AVR (automatic voltage regulation), surge protection, and provide about 1 hour of run time for my cable modem (about 20 Watt), router (about 20 Watt), and a gigabit switch (about 30 Watt). Here's another option, but the price is almost the same as that previous one that I posted:
http://www.cyberpowersystems.com/product...s/CP800AVR.html

Not sure if the battery run time is any longer.


Do you have a Costco membership? I bought a couple of 1000 VA TrippLite UPS's there for $100 each and they've performed very well.