DIY Double Conversion UPS

[GREG3f]

I know this is not a SOLAR question, but I think it is close enough that someone here should be able to help me out.

I am wanting to create a Double conversion UPS. The unit will use 2 x 6v 235 amp/hr batteries connected in series to provide 12v. There will be a 400 watt AC inverter connected to the battery that I expect will draw between 200 and 400 watts ac power 24/7. I would then like to use a WFCO WF9855 WF-9855 55 Amp Converter Charger to maintain the battery and provide power to the inverter while AC is available. Then if AC fails, the inverter will continue to draw from the battery until the battery is considered dead (inverter has a low voltage protection). I then need to be sure when AC returns that the converter/charger has adequate power to run the inverter immediately and start charging the battery in preparation of the next failure.

So my concerns are...

1. Is the WF-9855 adequate for the task?
2. How low can the batteries be drained without causing serious damage to it? Could I use an inverter with MPPT that claims to operate with 10.5 v or is that too low?
3. Is there a better way to build this kind of UPS? The goal is to have long runtime and constant power. A standard APC UPS tends to have short runtimes and tends to not come back on without intervention if power has been off for an extended period of time.

Thanks to any and all suggestions!

[Sunking]

So my concerns are...

1. Is the WF-9855 adequate for the task?
2. How low can the batteries be drained without causing serious damage to it? Could I use an inverter with MPPT that claims to operate with 10.5 v or is that too low?
3. Is there a better way to build this kind of UPS? The goal is to have long runtime and constant power. A standard APC UPS tends to have short runtimes and tends to not come back on without intervention if power has been off for an extended period of time.

I can help you out here.

I am not sure I would use an RV/Trailer rectifier to do this job because it is not intended or designed to run fully loaded 24/7. Granted on paper 55 amps is enough to do what you want, but I am not confident it can supply 55 amps for long periods of time.

A1. As stated on paper yes it is. At 400 watts will draw about 35 amps, leaving 20 amps to recharge battery.
A2. Difficult to answer without knowing exactly what the load amps will be. But as an example let's say the load is 30 amps, and you are using AGM batteries going to 80% DOD. 235 AH x .8 = 188 AH. 188 AH / 30 A = 6.26 Hours. So roughly 6 hours with a 30 amp load.
A3. If it were me I would look for a industrial grade rectifier the kind used in Telephone offices and operate at 24 volts. The rectifiers used in Telephone office battery plants are designed to run @ 110% of rated output indefinitely. There are a lot of decommissioned rectifiers out there you can pick up for a song in the 50 to 200 amp range. Some can operate at 120 volt, many are 208-240 volt AC input. Run time is determined by the battery size. If you were to operate at 24 volts the 24 volt equivalent of a 12 volt 235 AH battery is roughly 120 AH. Secondly for this kind of Float operation I would use AGM batteries rater than flooded.

[GREG3f]

I am not sure I would use an RV/Trailer rectifier to do this job because it is not intended or designed to run fully loaded 24/7. Granted on paper 55 amps is enough to do what you want, but I am not confident it can supply 55 amps for long periods of time.

With 55 amps, wouldn't the draw reduce from fully loaded (55 a) to the required (30 a) once the battery is fully charged?

My concern is more with the converter's smart charging rate changes... It is supposed to drop to float mode once the battery is fully charged, but if there is a constant draw, will it ever drop to float mode? Could this damage the battery?

I have also been looking into using a charge controller between the converter and the battery. It also has connectors for the load side, so I assume it can seperate the battery from the load and simply bypass the battery when ac is present?

[Naptown]

Why not use a relay to disconnect the batteries when AC is present. There are relays that will act within one cycle of thee AC

[Sunking]

With 55 amps, wouldn't the draw reduce from fully loaded (55 a) to the required (30 a) once the battery is fully charged?

My concern is more with the converter's smart charging rate changes... It is supposed to drop to float mode once the battery is fully charged, but if there is a constant draw, will it ever drop to float mode? Could this damage the battery?

I have also been looking into using a charge controller between the converter and the battery. It also has connectors for the load side, so I assume it can seperate the battery from the load and simply bypass the battery when ac is present?

No once the battery is charged up, all power comes from the rectifier. If there is no load, there is no current. For this type of operation you only want a Float/EQ charger. You do not want 3 or 4 stages. The battery is only there in the even there is a power failure. Otherwise the battery does noting but keep fully charged Floating. This is why I said look into a Telecom rectifier as it only has 2 modes of Float and EQ.

Do not use a charge controller to act as a LVD or bypass. Not going to work.