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Old 12-08-2003, 03:04 PM   #1
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If you want a cheap inverter/charger/transfer switch buy a used UPS (uninteruptable power supply) off of eBay that has a bad battery. These units are made to power computer equipment when the power goes out. They have a 12 V battery inside of them and most people get rid of them when the battery quits.



Basically they are the same thing as what you would buy for an RV. You can hook them directly up to any 12V battery and they have a good battery management system in them so you don't have to worry about overcharging. When you turn them on they will start producing 120V and if you plug it into a 120V source it will automatically switch over to that source and start charging your batteries. I should add that these units have the best transfer switches you can find anywhere. You will never see a flicker when it switches.



At the local surplus sale they are selling 1000W UPS systems with dead batteries for around $20.
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Old 12-08-2003, 03:06 PM   #2
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Since they are so cheap it's pretty cost effective to use a different UPS for each circuit in your bus eliminating the need for a circuit breaker panel. You can run 12V dc wire to any location in your bus you want one of these things, and then just use one of them near your battery as the transfer switch/charger.



Also, if you get one I would recommend getting an APC brand UPS.
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Old 12-17-2003, 02:46 PM   #3
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I bought a UPS from the local university surplus sale today. I got a 1000W for $10. I'll post pictures and describe how well it works soon.



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Old 12-17-2003, 11:16 PM   #4
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When I opened up the case I found two batteries connected in series.







The batteries are 12V 17.2Ah each. They were connected to the ups by two 10 AWG wires to each terminal. So I will be selected 4 AWG wire to connect the UPS to my battery bank.







It would seem that the UPS systems above 600 Watts run on 24 V which would be something you will want to consider if you try doing this. Also note that this unit has a fan to keep it from overheating.
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Old 01-19-2005, 02:09 AM   #5
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So how is it working out?.....I know of a used ups here I can get if this worked out for you.
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Old 01-19-2005, 04:47 PM   #6
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The annoying beep problem is easy, you can just remove the buzzer then with your multimeter measure the resistance across the buzzer itself and replace it with the same size resistor. ( Or just leaving nothing there should work in most situtations. )

To take care of the charging situation, why would you want to disable, it would work like a perfect transfer switch/charger. If you are plugged into shore power or generator why not let it charge the batteries.
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Old 01-20-2005, 11:23 AM   #7
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If I was going to use multiple UPS I would have given each their own battery, That way you can match each bank to the appliances that it will be running, plus you wont have to be running high current wires around everywhere.

I understand the efficient thing but if your plugged into shore power why care.
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Old 01-20-2005, 06:32 PM   #8
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Hmmm..so Steve is going to use his to control a large bank of batts, while Soul is looking at a group of apc's each hooked to the same batts????..you two are confuseing me *S* I was looking at breaking the elec. into groups based on when/where they would be used and what they need (power wise)...Right now I'm only trying to get my laptop and its base station into the bus as its going to be my atlas/sound system for my move home.
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Old 01-20-2005, 09:32 PM   #9
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hmmm, oddly I started that last year before I had it (working on it)..but thats for a page on the bus and not here *S* guess I'll go ahead and snag that ups and hope its not toast on the circets (sp?) ... all I need right now is enough to power my laptop..after that I have a camper van thats giveing up it goodies to the bus waiting in CO.
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Old 01-20-2005, 10:32 PM   #10
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I feel I am a creative electrical wizard when it comes to making things work both 12 and 110 volt, but I decided to use the KISS theory in my bus. My entire bus runs off of one 15 amp circuit. That circuit enters the bus through a GFI, then branches out to a series of plain 110 volt light switches that control the individual circuits ie: fridge, outlets, aux, and lights. I have a male plug that feeds power to the bus, and three electrical outlets that i can tap into: Shore power, Inverter, Generator. I have to manually move the plug from one outlet to another when switching power sources, but this ensures that i won't fry my inverter, or generator by accidentally plugging one into the other.

My electrical system is cheap, simple, and reliable.
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Old 01-21-2005, 07:31 PM   #11
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well I went by and picked up that ups..specs are: SL Waber Powerhouse 250
250VA back-up power supply....max output current is 12 amps..it h as four plug ins with two being on the back-up and two as surge protected only. Only has a single 12V, 4Ah batt in it though (ok, I was hopeing for two but it did feel light)..I'll plug it in a see if the batt chares at all..the case has been broken (of 4 screws holding it together only one does), so that may be the only thing wrong with it.
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Old 01-27-2005, 11:08 PM   #12
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I plan on doing the "UPS-as-Inverter" fairly soon.

(i've 3-4 ups's laying around)

I've already used a "car battery" with one.. increasing my 6 minutes of runtime to almost 1 hour of active computer use.

I do want to take advantage of the auto-transfer switch capability.
but i also worry about the charging.

I do plan to run the ups off of my house bank, and will be getting rid of my current converter for a (ghetto) 3-stage charger.

I cannot see how the UPS could be quick and/or efficient in charging a large bank (well someday a large one). and I'm worried that it's "trickle charge" will confuse my other charger into either shutting off or at the least going into float mode too early with a false recharged signal..

I've contacted APC's tech support.. they were happy to help.. but said help boiled down to "unplug it" D'oh!

I wont be doing this mod until spring, but i'll post what i learn, and wil check back for more from you guys

robert
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Old 04-18-2005, 03:54 PM   #13
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Has anyone found and documentation on the efficiency of UPS inverters? I have a large server UPS that seemed like a good idea at first to use as an inverter but that was a bust because it didn't support cold starts (starting up off batteries). Glad it was free. I also wondered how much extra power it used on communications & monitoring? The one I have looks like this. Since they are typically used only long enough to shut down your computer then I wonder if they try to make them efficient. No load draw could also be high. The one I have uses a large transformer which I suspect could hurt efficiency.

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Old 04-19-2005, 08:43 AM   #14
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Yeah, we verified with the manufacturer that it does not support cold starts. Anyone want to buy a nice UPS? I did gain a couple nice batteries for my riding lawn mower.
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