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Old 05-21-2016, 06:02 PM   #1
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UPS for the PC, how about combining UPS and boat chargers?

I have a Minuteman 320 Pro UPS I use for the boat PC and some other things.
I actually have an on-off-on switch so this UPS can power all boat outlets if needed. I will leave that circuit as is, but thinking I can run the UPS as a UPS for the PC.

I modified the UPS by adding an internal cooling fan and hooked up to the boat house batteries. It works fine, with the extra fan it puts out almost 400 watts.
This UPS sits next to my 3000 watt inverter, so just a short big wire paralleled off the 3000 watt inverters lugs gives it DC power.

My question, I would like to run it as a true UPS, so connect the minuteman 320 AC input into my boats 120vac grid. Would be more convenient and seamless transparent, no turning the UPS on as it would always be on.

That would mean the UPS will attempt to charge the boat's big batteries.
I also have the onboard boat charger. So they would then be joined together, both outputting into the boat batteries, would that burn out the UPS? Or would the DC charge voltage off the UPS just fall to the level of the big charger ? I had them joined earlier like this, but don't know long term what would happen. The PDF says it is fully circuit protected.

"Overcurrent and short circuit protected, latching shutdown on overload"
Which I think it would just turn off if it got too hot, it would think it's 12vdc battery had an internal short idea.

http://www.minutemanups.com/products/pdf/pro_eng_ds.pdf

Pics of my mods. I made a remote on - off switch too.
https://photos.google.com/share/AF1Q...dFbWd2eGtqNVB3

Another reason to do this, it has the beep noise every few seconds when the AC input is off and it is using the battery. I could break the beeper. under the kitchen cabinet, it is not too loud of a beep.
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Old 05-21-2016, 06:56 PM   #2
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Sounds like a fire waiting to start.

Ted
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Old 05-21-2016, 08:47 PM   #3
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Sounds like a fire waiting to start.

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Sounds risky to me as well. I think you're getting too inventive.
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Old 05-21-2016, 11:27 PM   #4
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You already have an inverter, why would you be messing with the UPS?
Your really playing with fire here to no real end.
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Old 05-22-2016, 07:53 AM   #5
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Why mess with the UPS say for the PC?
Partly due to switching power sources.
If I run gen then turn it off to run the 3000 watt inverter, the PC may shutdown.
Anytime the power source gets switched, the way I run it now, PC may turn off if the switchover is too slow.
I have kept the smaller UPS to use as an emergency source of AC power, like a backup.

I could simply use it all the time for the PC, run a power cord to it directly and disable the beeper. That does mean at dock, have to run the UPS to run the PC. Doing that will prevent any USB charging issue.

Idea is use the UPS as a seamless power source as it is designed to be.

One way is get a power diode to block DC output flow from the UPS, that way it can not charge the battery. Such a diode would drop the voltage about 0.5vdc or if schottky diode, maybe 0.3 vdc.

A diode acts like a one way valve, power can go in, but no power can go out.
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Old 05-22-2016, 08:25 AM   #6
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just use a smaller local inverter.

Inverters R Us - Power Inverters and Accessories
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Old 05-22-2016, 09:14 AM   #7
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https://en.wikipedia.org/wiki/Uninte...iagram_SVG.svg

I wonder if disabling the battery charger of the UPS would be as easy as snipping a wire tap off the transformer, but which one goes to the battery charger. Maybe the voltage tells you which one. It does seem like it would be possible to disable the internal battery charger.

The wire taps from transformer may run to an easy to see charger circuit, or all run to the circuit board and the charger circuit would be harder to identify.
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Old 05-22-2016, 09:15 AM   #8
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just use a smaller local inverter.

Inverters R Us - Power Inverters and Accessories
Same thing as the UPS I have which works ok to power the PC. I don't gain anything by switching to an inverter, as the UPS is also an inverter.

For now, I will dedicate the PC to the UPS power and leave it unplugged to boats ac grid.

And need to disable the annoying beeper.
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Old 05-22-2016, 12:30 PM   #9
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So when your 3000W inverter switches from shore power to inverting, the PC crashes, or is it just something you are worried might happen? I ask because a lot of inverters can make the switch fast enough to not be a problem.

I don't think your original proposal in dangerous, assuming you fuse everything properly which you should do no matter what. But I do think it's convoluted and inefficient.

The worst case I think is when you are running on batteries with the inverter drawing DC to create AC, and the UPS running off that AC and creating DC to charge the same batteries that are powering it in the first place. The extra power drawn from the batteries by the inverter to power the UPS's charging circuit will always create a net drain on the batteries. So you are creating an artificial load that will only drain your batteries down faster than if you didn't have the cascaded Inverter + UPS.

In this situation, my preferred solutions would be:

1) Just use the inverter to provide UPS to the PC. If your inverter can't switch fast enough, consider upgrading the inverter.

2) Use the UPS as you are, and plug it into the Inverter AC output as you plan, however attach a dedicated battery to the UPS as its design expects. This will get rid of your UPS beeping, power everything off shore/gen power when available, provide UPS capability in the event of an inverter or main battery failure, and not incur any gross inefficiencies.
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Old 05-22-2016, 05:23 PM   #10
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My battery charger is separate from my inverter.

I have 4 potential AC sources, the fourth being the UPS which is not too significant, so that is out of the main AC system automatic etc...switching I designed originally for the boat.

I have two 4 pole 35 amp 4PDT relays to control my AC power from 3 sources, shore grid, generator, inverter. I use 4 poles since the boat is a twin 30 amp AC power design. Otherwise 2PDT relays could be used. You must switch both hots and neutrals together. The 4PDT relays are cheap and easy to buy.

This effectively isolates each source one from the other. I control the relay coils by switches or automatically by when the device comes online.

However I added some more millisecond time to send power from the 3000 watt inverter by use of another 35 amp 4PDT next to the inverter to make real sure the various AC sources can never actually interconnect with the 3000 watt inverter. Because if it did, it would fry the thing in an instant. Shore power and gen is much more robust, so I don't worry about an extremely rare momentary interconnection as the relay switches over. Arcs under a heavy load switching can do such a thing. For example if the coil power fell it might actually turn off an on real fast. I eliminated that possibility by driving the coils using DC 12 V power.

This schematic shows the basic idea. It controls 3 distinct power sources, grid, generator, inverter, and outputs them safely to the ship AC power.

I made a steel box and these are the relays before all the interconnecting wires are hooked up.

Since the gen has a start on demand feature, with these 2 relays solenoids deenergized, the power detection passes through no interruption.

For shore-grid power selection a switch activates one solenoid passing AC power out to ship.
For 3000 watt selection, when inverter power appears at the solenoid, it passes inverter power to ship, but only if shore-grid is off, either by turning off the switch or unplugging from dock power.

If everything is off, of course generator is passively selected.

I added some more safety for the inverter. If you turn on the AC generator, and your getting power from the 3000 watt inverter, inside the gen, the 12 vdc circuits are powered up first since the thing is not spinning yet. So using a 40 amp cube relay, powered from the generator, I break the 3000 watt inverter solenoid coil power open by cutting its power, so till gen spins up no Ac power. That is where my PC will shut itself off. I could do the same with shore-grid power, but since they are robust power sources why bother.

I also switched to driving the coils using DC power. Our marina AC power was cruddy, so sometimes the relays would buzz. I obtained the DC power from AC wall warts and using a voltage divider circuit using big resistors, drive the coils at about 15 to 16 vdc. !2vdc is not enough power to activate a 120vac coil.

Alternatively buy 12vdc coils in this relay type. But I already had these two.
Walwarts put out too much dc voltage if the load is too small. Running with too much voltage will heat up the solenoid coils. The best wall warts to use are the lightweight switching power supply type with no transformer. They maintain rated voltage much better if the incoming line voltage changes. Also I wanted this setup to work independent of the 12vdc on the boat.

This schematic shows the basic idea. Here with the inverter passively selected.
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Old 05-22-2016, 05:42 PM   #11
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OK, much more complicated (and sophisticated) than I imagined. I was assuming an inverter with internal bypass relay - most of those switch quickly after qualifying the AC power source.

Based on what you have, just hooking up a dedicated battery to the UPS seems like the most straight forward and robust approach.
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Old 05-22-2016, 05:57 PM   #12
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Well it would be, but not worth the expense. I will forgo the UPS idea and continue to use as an inverter only.
Really is no advantage to plug UPS into boat AC unless the charge circuit can be disabled.

Which I will examine when taking out that beeper from the circuit board.
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