converting 12 Volts to AC with a Used UPS ? - Fiberglass RV

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Old 07-16-2009, 09:32 PM   #1
Marty Smiltneek's Avatar
Name: Martin
Trailer: 1972 Boler American
Posts: 44
Hi everybody

Being the cheap (read that as fiscally conservative) guy on the block I wonder. . . .
Has anyone tried taking the 12 volt battery out of an old computer UPS and connecting it to the trailer battery via an isolation switch? I have seen several used UPS systems with a bad battery for less than $20.00 that will provide 1000 Watts of computer grade 120 volt power. The system is all set to recharge the battery when on shore power but would automatically be on when not connected to the AC power grid - thus the isolation switch.
any comments?

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Old 07-17-2009, 06:59 AM   #2
terryjeffers's Avatar
Trailer: 1978 Trek 1300 (Boler/Scamp Clone) / PT Cruiser
Posts: 39
Hi Marty,

Yes, I use an old 660VA Best Power unit (used to be made up there in Necedah, WI) connected to 2 33AH batteries in series (a 24v unit) for those times when I want AC power. I installed a dedicated AC outlet for output power near the sink.

May I suggest that you keep the UPS battery and UPS AC output power completely isolated from the campers battery and AC outlets? Since it is unknown as to the relationship between the internals of the UPS's battery (+ and -) and the UPS's AC output (especially grounding) you would not want to create some voltage differences or feedback loops to or from the UPS and the camper circuitry. The UPS AC input can be connected to the campers AC so the UPS battery can be recharged

Remember, that for the UPS to output 8 amps at 120VAC is going to require you to pull 80 amps at 12VDC from the battery, You will need some pretty hefty cables and fusing from the UPS battery.

It is really nice to have real sine wave power available if needed. I'm not much of a fan of the cheapie inverters.

Hope this helped to answer your question.


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Old 07-17-2009, 09:01 AM   #3
Trailer: 2008 Casita Liberty
Posts: 42
What about the noise!?! All of the backups Ive had made a terible shreik when there was a draw of power.
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Old 07-17-2009, 02:38 PM   #4
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Name: Pete
Trailer: 17 ft 1986 Burro
Posts: 881
What about the noise!?! All of the backups Ive had made a terible shreik when there was a draw of power.
wire cutters, cut the sonalert line.
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