UPS charging and mill charging

[domwild]

Friends,

Even if my UPS is switched off at the ON/OFF switch, it tries to float the SLAs (I use car batts at the moment) to 40.4 V, this is a 36V system. A mill will want to float car batteries at least to 3 times 14.5 or so, which is 43.5 V.

As my mill still needs to be finalised I cannot test the system. Am I right in suggesting the UPS may have an overvoltage cutout for just this situation, just like it switched itself off when I tried to get it to start an angle grinder (750 W)??

The unit is an Accupower Select 2000 for which there is no info available on the net or Emersons, so I would like to ask for educated guesses, pls!

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[whatsnext]

What are you trying to do with this system?

If the UPS is unplugged there will be no chance of it's float circuit ever kicking in and if it is plugged in there is no point in hooking your mill to that battery bank. Also, unless you have an addition charge controller between the mill and the batteries I'm confused as to why you think the mill will "float" the batteries at 43.5v.

[domwild]

Thanks, whatsnext. The problem with this UPS is that it is not a "cold start" model. Mains must be connected to it for the UPS to start on mains failure. Even with the manual ON/OFF switch set to OFF, it expects to float SLAs (not connected, car batts only). There is the grid trying to charge the batts to 40.4 and the mill trying to charge the batts up to 43.5 V. Conflict?

Where I would want to place the mill is in a windy area but we are grid-connected. Power fails regularly in Walpole (SW of Western Australia) and I want to become at least partially independent with the help of the mill + UPS.

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[whatsnext]

Well then, when the grid is "up" there is no point in having your mill connected. The grid will keep your batteries charged and everything your mill produces would need to be dumped anyway. I guess when the grid goes down and your UPS starts working you could somehow be there and connect the mill to keep the batteries charged but that seems like a hassle to me especially if there is no wind to do so. I would perhaps rethink my system and get equipment more suitable for whatever my goals are. If you've already built the mill maybe you could use it to reduce your hot water bill by direct connecting it to a water heating element or buy a more suitable inverter and then babysit your state of charge by using excess electricity whenever your mill produces some. Doing this would mean you are not grid connected with your wind power system and there would be no reason for it to be changed at all in the event of a grid going down.  I still have no idea why you think that the mill will auto float your batteries at 43.5v but maybe you have some sort of charge controller you haven't mentioned.

[whatsnext]

One other thing. Are you sure you know how your UPS functions? Maybe if you turn it "on" it will will cold start. I don't understand how your UPS would know the difference between being unplugged and a grid failure if it was turned on. It certainly can't be trying to charge the batteries if the "back up" function is functioning when the grid is down. Not that having a UPS that can't be turned off would make a very reliable inverter anyway.

[domwild]

Thanks again, whatsnext. My controller would limit the charge to 14.5 V (x 3 for 36V) and I would need to dump once that voltage is achieved.

If I observed my UPS correctly, then the charge function is ON all the time. The supply of AC can be switched ON/OFF. Of course, one can always disconnect the supply from the mains to the UPS via a relay. I was trying to avoid this step by asking the Forum if there is a problem with the mains charging the UPSs batts. to a lower float level while the mill is also charging right up to a higher voltage.    

Would a UPS stop floating once the designated float voltage is reached?

You will find that not all UPSs are "cold-start" versions. Found that out by contacting the Forum before.

I will experiment more and certainly keep your idea of heating water in mind.