question

kokaruu avatar image
kokaruu asked

Newbie seeking help with "simple" battery backup for sump pump.

Hi, I'm hoping there might be some bright people out there willing to help me out with my project.

I have a 550w sump pump in the basement, which runs for ~40sec every ~10 minutes or so.
It's crucial that we get a battery backup for this system in case of grid failure, which are rare but do happen, I also need a backup system in case of mechanical failure of the main pump.

I thought I had everything figured out when I purchased all the necessary equipment, but just before connecting it all, it dawned on me that the inverter in got only shuts off at a battery voltage of 10v, which would ruin my battery.

I know my way around the basics, but lack experience and knowledge regarding all the more complicated aspects concerning off grid power.

The setup is as follows: ( I already purchased these items)

  • Additional* 550w, ~220v sump pump. Rated at 1000w max peak.
    (* The main pump is on the grid, so this one is purely for backup and only powered by the inverter, and will be mounted in way so that it only kicks on if the main pump fails)

  • Varta dual purpose 12v 240ah sealed lead acid battery.

  • Ctek M15 charger. 15A max, supposedly rated for lead acid batteries up to 300ah.


Questions and concerns:
How would all this play along with the 12v Victron Phoenix 1200VA inverter?
I hope that the adjustable low voltage shut down would be able to protect my battery from dipping much below a ~50% charge?
Furthermore I trust the dynamic cut off functionality would prevent false shut downs from loads etc?
I do plan to get the VE.Direct to Bluetooth accessory to be able to use this functionality.

I'm also concerned about running into some trouble by having the charger connected at all times, specifically in regards to the protection against high voltage and "DC ripple too high?" The latter being something I briefly read about in another post, so to be honest, I'm not really sure what exactly this is all about.

Edit/addition:
I must have missed it, but I read the input range on the phoenix inverter as up to 17 volts, which would be within the output of the charger at 15.8 volts? So maybe no reason to worry?
I also just found a section in the specification for the ctek m15 charger, claiming less than 4% ripple, but I'm still clueless as to figuring out if this is within the inverters "safe range"?

Thank you in advance, much love from Denmark.

Phoenix Inverteroffgridlow battery warning
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1 Answer
Kevin Windrem avatar image
Kevin Windrem answered ·

If your battery charger is a 3-stage charger (bulk, absorption, float) then it can remain connected at all times. I couldn't tell from the documentation I found if that is the case for the Ctek M15.

A 1200 VA inverter might not be sufficient to start your motor.

To avoid running the inverter all the time, I suggest you connect the water level sensor to the inverter on/off switch. That shouldn't be difficult with a little work if you know your way around circuitry.

You might consider a Multiplus as it bundles everything together. Then you could run both pumps off the Multi's output and either would get mains power if present or inverted battery power if mains not present. If the main pump starts first the backup pump should never run unless the main one fails.

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Victron Offgrid page

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