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Old 05-12-2010, 11:11 AM   #1
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APC Battery Backup - Extending Run Time with BIG Battery Bank


I just bought an APC BN1250 UPS battery backup from craigslist for $40 bucks. It's a 1250 VA / 780 Watt system that has an advertised runtime of 90 minutes when the power goes out.

This is technically an "electrical" thread rather than a PC thread, but since it's supposed to be a PC component, I'll post it here.

I plan on using this battery backup for my home theater system that includes a 55" LG LCD TV, Comcast Cable Box, a Media Center PC, and a pair of end table lamps with compact florescent bulbs in them.

The guy I bought this backup from says that the batteries are good as new, but I'm not sure if I believe him. Regardless, $40 isn't a bad price, even if it needs batteries. Here's why:

I plan on extending the run time using two, perhaps four deep cycle marine batteries. Since this particular inverter runs on 24 volts, I'll need at least two batteries wired in series to get the 24 volts. I was even thinking of running four batteries in a massive series-parallel configuration.

The unit comes with two 12V 9aH batteries for a total of 18 amp-hours. An Optima Yellow Top battery has 55 aH, or two for 110 aH. That's 5.1 times (510%) more capacity than the battery that comes in the UPS. Using this equation, assuming 90 minutes of runtime, I could theoretically get 459.9 minutes of runtime with two Optima Yellow Tops.

While I'm not going to get Optima Yellow Tops for this project, the "cheapest nice generic" batteries for a boat or RV would have similar output. I'm going to try to find sealed batteries so they don't emit hydrogen gas when charging.

I'm going to test the system as-is for a baseline. The specs claim 90 minutes of runtime with half load, so I'll see if I can push it for an hour running the TV and Cable Box. If it goes longer than an hour, I'll be happy. I won't risk it by running it completely out of juice.

There have been some issues with increasing the battery capacity in these APC battery backups in the past. Mainly with the inverter. Some of the smaller ones are only designed to be run for 10 minutes, and after that they start to heat up. Seems like this one is designed to be run at least 90 minutes, and it has a cooling fan built in. I could certainly add extra cooling fans inside with the extra space cleared by getting rid of the internal batteries.

Another issue is with the internal computer and the capacity it "thinks" it has. Some have reported that they won't run more than the time they originally ran with the old batteries because they are used to having smaller capacity, and you have to go in and change some of the parameters.

We'll see about that.

Has anyone done this before? Good results?

Homer
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Old 05-12-2010, 11:21 AM   #2
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APC Battery Backup - Extending Run Time with BIG Battery Bank


The only kink may be, the apparent battery capacity depends on how fast it's drained.
For 9 Ah the 'C' rate is 9A, and it will drain to some agreed-upon cutoff voltage.
At C/100 you will pretty much get full capacity. A lot of batteries are tested at C/20.
And, a constant current load is not the same as a constant resistance load. Your batteries may be drained by all this electronics at a constant power rate.


Last edited by Yoyizit; 05-12-2010 at 11:24 AM.
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Old 05-12-2010, 11:53 AM   #3
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APC Battery Backup - Extending Run Time with BIG Battery Bank


Why try to extend this to run a TV ?
You would be better off putting the $$ towards a generator setup
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Old 05-12-2010, 12:11 PM   #4
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APC Battery Backup - Extending Run Time with BIG Battery Bank


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Originally Posted by Scuba_Dave View Post
Why try to extend this to run a TV ?
You would be better off putting the $$ towards a generator setup
Good question.

I have a Generac 7500W / 13,500 W portable generator that runs the whole house except for the A/C, Furnace, and Electric Stove. I bought that generator at a pawn shop for $250 bucks. Sweet!

But that thing is LOUD! I don't want to be running it late at night. It'd be cool to have a battery backup on the TV, Cable Box, and living room lights (or whatever) until I can go get the generator started and hooked up, or for after I have shut off the generator. The cable box is most important to keep running, because as soon as power gets cut to that, it takes a good 5-10 minutes to start back up.

The APC backup will work just fine as-is, but this forum isn't called "Leave Things The Way They Came Chatroom", it's called "DIYChatroom". With that being said, I want to make it run as long as possible, and if I can do that with some cheap car batteries, that'd be awesome.

Plus, when the generator is getting warmed up, it sometimes revs up and down which creates a very inconsistent voltage. The cable box with the hard drive doesn't like outages and brownouts very much.

It might also be important to stay low-key and keep some lights on without attracting the type of attention that a loud generator creates. I could run a few extension cords from the UPS to run a few lights in other rooms too.

But when you think about it, I don't "need" any of this. None of us "need any of this". I could do just fine during an outage with some candles and flashlights and no creature comforts. An outage is a good excuse for me not to have to check all the annoying work emails anyway.

But it's fun to do. I like projects like this. I like having a few options.

Homer
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Old 05-12-2010, 12:38 PM   #5
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APC Battery Backup - Extending Run Time with BIG Battery Bank


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one of us "need any of this". I could do just fine during an outage with some candles and flashlights and no creature comforts. An outage is a good excuse for me not to have to check all the annoying work emails anyway.
I flip a coin.
Heads, you go without power for a day. Tails, you have a backup.

How much would I have to pay you, lump sum, for you to be completely indifferent as to which way the coin lands?

That's what the backup is worth to you, today.
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Old 05-19-2010, 01:13 PM   #6
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APC Battery Backup - Extending Run Time with BIG Battery Bank


I've used this unit before and it's great. The extended battery life is worth it but you still have to have the computer set up for automated shut down in the event of a power failure. APC has this software that interacts with their units and will do it the right way. It is not a replacement for a backup. Backup and UPS's are really like casino odds in reverse. It's possible but unlikely you'll lose everything and more likely something less terrible will happen. But with a UPS and backup, it is super unlikely you'll experience any of the problems.
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Old 05-19-2010, 03:33 PM   #7
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APC Battery Backup - Extending Run Time with BIG Battery Bank


Forgot to mention that in general paralleling batteries is tricky.
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Old 05-19-2010, 03:41 PM   #8
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Forgot to mention that in general paralleling batteries is tricky.
Thanks for the disclaimer. I'll take that into consideration.

Even if you have a basic knowledge of how electricity works, you'd be able to wire batteries in parallel or series. Or series-parallel.
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Old 05-19-2010, 03:51 PM   #9
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APC Battery Backup - Extending Run Time with BIG Battery Bank


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Thanks for the disclaimer. I'll take that into consideration.

Even if you have a basic knowledge of how electricity works, you'd be able to wire batteries in parallel or series. Or series-parallel.
Yes, but will the batteries object, sometimes explosively?
Also, charging vs discharging brings up different problems.

School buses have a 24 battery for cranking the engine and a tap @ 12v for accessories. Charging/discharging this arrangement requires a complex interface.
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Old 05-19-2010, 03:56 PM   #10
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APC Battery Backup - Extending Run Time with BIG Battery Bank


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Yes, but will the batteries object, sometimes explosively?
Also, charging vs discharging brings up different problems.

School buses have a 24 battery for cranking the engine and a tap @ 12v for accessories. Charging/discharging this arrangement requires a complex interface.
I see what you're saying. You're right, it is important to make sure you've got the batteries sized correctly, and to make sure they are wired properly for the application.
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Old 05-19-2010, 06:57 PM   #11
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I see what you're saying. You're right, it is important to make sure you've got the batteries sized correctly, and to make sure they are wired properly for the application.
No, there's more to it.
You don't ever want heavy currents to flow between batteries, and it's hard to say all the ways that this can happen.
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Old 05-19-2010, 07:45 PM   #12
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No, there's more to it.
You don't ever want heavy currents to flow between batteries, and it's hard to say all the ways that this can happen.
I know there's more to it. I just don't feel like explaining it because it's beyond the scope of this thread.

I'm not a novice when it comes to wiring up batteries and doing these types of projects. I worked in the 12 volt industry for six years and I'm an MECP master installer. I wouldn't even consider doing these types of DIY projects if I weren't so experienced with electronics, I'd just go out and spend the extra money on something with the capabilities I'm looking for instead of using my knowledge to save money.

I've seen lots of threads in this forum where people are constantly challenging other people's intelligence and knowledge about the topics that are being discussed. Before the original topic is even discussed in sufficient detail, there's always someone going off on a tangent about the risks and dangers of the topic at hand because they think the people discussing it don't know enough to consider the risks. It makes people not want to read the thread anymore, and it discourages people from starting their own projects.

Safety first, but let's all do ourselves a favor and instead of assuming that other users of the site don't know what they are doing, assume that they know what they are doing and that they have enough skills and intelligence to consider the safety precautions. There are always people that screw things up, but that doesn't mean every thread needs a disclaimer. There's already a disclaimer on this site somewhere. I'm sure the administrators of the site could set up a "disclaimer" bot for each thread if they wanted to.

I don't mean any disrespect to you or anyone, (and I thank you for being so kind as to post on my topic and share your knowledge with me) but I would appreciate it if people assumed I knew what I was doing. Of course, I don't know everything and that's why I ask for advice. But if I wanted someone's opinion about the safety precautions, I'd ask for it. I think I speak for most of us in the forum. If anyone disagrees with me, please feel free to explain why you think I'm wrong for what I just discussed.

Thanks.

Now, can we please get back to the topic at hand? Have you ever "hot wired" an APC battery backup?

Homer
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Old 05-20-2010, 06:10 PM   #13
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APC Battery Backup - Extending Run Time with BIG Battery Bank


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Now, can we please get back to the topic at hand? Have you ever "hot wired" an APC battery backup?
Do you mean by hot wire just that you don't have the correct connections? Or that you plan to make batter connections from home made designs? APC has wiring diagrams that are not difficult so I don't think it's a problem at all. When APC sells case shells they are just that - shells that have the batter connectors. You could also alternatively buy a shell with a wiring harness.
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Old 05-20-2010, 06:22 PM   #14
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Do you mean by hot wire just that you don't have the correct connections? Or that you plan to make batter connections from home made designs? APC has wiring diagrams that are not difficult so I don't think it's a problem at all. When APC sells case shells they are just that - shells that have the batter connectors. You could also alternatively buy a shell with a wiring harness.
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Using the proper connections, heavy gauge wiring, and using larger capacity batteries in the proper sequence and voltage in order to dramatically increase the run time of a used battery backup way beyond what APC had originally intended in order to save money over using an APC designed battery pack.

A decent APC battery backup system might have battery capacities of anywhere from 7-21 amp-hours. Enterprise class APC battery backups might have battery capacities of 50-100 amp-hours in stock trim and have connectors for external batteries in either 12, 24 or 48 volt configurations.

One of the enterprise class APC battery backups would be the best to start with because they have built in cooling fans, sine wave output, large capacity charging circuits, etc.

You can get a "good cheap" deep cycle marine battery with 100 aH capacity for $50-$60 bucks.

You could extend the runtime of a 750 watt APC backup with a couple large capacity deep cycle batteries to several hours, perhaps days depending on what's plugged in, and how many batteries you have.
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Old 05-24-2010, 06:11 PM   #15
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APC Battery Backup - Extending Run Time with BIG Battery Bank


You mean they aren't really worth the hundreds of dollars they charge? That's gonna make the UPS snake charmers upset... (ha ha)

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